CS50P - Lecture 1 - Conditionals

so [Music] [Music] all right this is cs50s introduction to programming with python my name is david malin and this week we focus on conditionals conditionals or conditional statements in python and in other languages are this ability to ask questions and answer those questions or in order to decide do you want to execute this line of code or this line of code or this other line of code instead they allow you to take the proverbial forks in the road within your own code logically so how might we go about making some of these decisions well it

turns out that python comes with a lot of built-in syntax for instance here are just some of the symbols you can use in python to ask questions admittedly mathematical questions but we'll start there if only to keep the example simply simple early on this first symbol as you might know from math represents greater than the second symbol might not look too familiar because we usually write it all as one thing on a piece of paper but on a keyboard if you want to say greater than or equal to you'd use this symbol instead this of

course means less than this means less than or equal to and this one's a bit of a curiosity we've seen in our look at functions and variables how we were able to assign values to variables using a single equal sign but that equal sign didn't represent equality it represented assignment from right to left that's great because it solved that problem but it kind of left us in a bit of a bind because how do we now compare two things left and right well in python and in many languages you actually use two equal signs so

two equal signs represents equality comparing the thing on the left and the right one equal sign as always represents assignment copying the thing from the right to the left lastly this last symbol represents not equal to so the exclamation point or bang followed by an equal sign means not equal to some value next to it well to ask the questions using these symbols or any others we're going to need another keyword in python and that keyword quite simply as in english is if you can ask questions in python code along the lines of if the

answer to this question is true then go ahead and execute this code for me so let's go ahead and write some of these examples here i'm going to go over to vs code and let's go ahead and create a program first called compare.pi the goal of which is simply to write code that compares values and makes decisions based on those values let's go ahead and type code of compare.pi in order to create a brand new file called compare in which we'll start to express some of this logic all right well what do we want to

compare suppose we want to compare for the sake of discussion just a couple of integers but we'd like those integers to come from the user so that we can make decisions based on numbers we don't know the values of in advance well let's go ahead and do this as we've done in the past let's declare a variable like x let's assign it equal to the return value of the int function and pass to the in function the return value of the input function asking the user a question like what's x question mark as we've done

in the past let's do this one more time with y asking the user for the value of y and again converting that ultimately to an int as well so with this amount in the story we have two variables x and y each of which has values and ideally we should be able to now compare these values so suppose i want to make a decision based on the values of these variables i'm going to use the keyword if and i'm going to use some of those mathematical symbols to actually ask the question itself so how about

this if x is less than y then let's go ahead and just print as much out quote unquote x is less than y so this isn't a very interesting program yet i'm literally just stating the obvious based on the math but it's allowing me to now introduce some new syntax and exactly what is this syntax well it's this not just the keyword if which i've added here at the start of line four but then i ask my question here x less than y x is one variable on the left y is one variable on the

right and of course the less than sign is expressing the mathematical question i have what i've highlighted here is technically called a boolean expression a boolean expression named after a mathematician named bull is simply a question that has a yes or no answer or technically a true or false answer and that's nice because if there's only two possible answers it's very easy for me and in turn the computer to make a decision do this or don't do this thing now notice if you come from other languages you might notice that i have not typed any

parentheses they are not in fact necessary at least in this case in python but i have typed a colon at the end of the line and even more importantly at the next line i have begun my line with some indentation hitting the spacebar four times or just hitting tab once which will automatically be converted to the same that indentation is what tells python that line 5 should only be executed if the answer to line 4's question is in fact true so if x is less than y that phrase will be printed thereafter well let's add

a few more lines of code how about another question if x is greater than y then let's go ahead and print that x is greater than y and let's do one final question if x equals y then wait a minute what have i done wrong here right a good i here i don't want to assign y to x if x equals equals y is how i express equality let's go ahead and print out x is equal to y so i now have a three conditions if you will one question asking x less than y one

asking x greater than y one x king x equals equals y let's run the code well down here in my terminal window i'm going to run python of compare.pi and hit enter what's x let's go with one what's y let's go with two this should of course execute that first line of code and tell me indeed that x is less than y exactly as i would expect there well what just happened though in code well let's take a look perhaps at this same code visually particularly if you're a more visual learner this i dare say

is what just happened so what we're looking at here is a flow chart it's a diagram of this program's logic and more technically it shows the program's control flow that is the ability of you and code to control the flow of a program generally from top to bottom in fact let me go ahead and zoom in on the top of this flowchart and you'll see an oval at the very top that says quite literally start that is irrespective of what shape or layout the diagram is is where your own thinking and logic should start when

trying to wrap your mind around this program notice that there's an arrow from start to this diamond shape and inside of that diamond is a question a boolean expression x less than y and this shape just means based on the answer to that question go left or go right specifically go left if the answer is true or go right if the answer is false well the inputs i typed were 1 and 2 respectively for x and y so of course 1 is less than 2 so that's why my program printed out quote unquote x is

less than y but recall the code the code then proceeded to ask two more questions is x greater than y is x equal equal to y well the flowchart depicts those questions too notice that no matter whether the question had an answer of true or false the arrows both converge back down to this second diamond shape here and that second diamond shape asks the second question x greater than y that too has a true or false answer so we go one way or the other but if x is 1 and y is 2 then no

the answer is false 1 is not greater than y so logically in the flowchart you follow the false arrow this time and notice along that false arrow you don't print anything this time that's why we only saw one print out on the screen now there was still a third question and this flowchart captures that as well the third diamond asks x equals equals y now that two has a false answer in this case because one of course does not equal equal y and so we again follow the third false branch here and that leads us

of course to stop and stop just indicates that's it for the program so i think that's correct and that particular flowchart does happen to represent the actual code that i wrote so it's correct it does what it's supposed to do it answered the question correctly by printing on the screen x less than y but what is perhaps poorly designed about it let's make this first distinction it's not enough necessarily for the code that you write to be correct and do what you intend longer term especially as our programs get longer and more sophisticated more complicated

we're going to want them to be well designed too thoughts on in what way this program is arguably not well designed even though it's correct let's see here uh khalid if i'm saying that right your thoughts too many ifs i think is getting repetitive we can make our code more concise maybe yeah it seems a little repetitive i'm asking if this if this if this and yet logically i should know the answer to some of those later questions once i figure one out and in short if you look at this diagram here notice that no

matter whether i go left or i go right i'm always asking three questions no matter what all of those arrows lead to the first the second and the third diamond so i'm asking three questions no matter whether any of those answers are true or false well how might i go about improving this well let me propose that we introduce another keyword to our python vocabulary namely l if and this too is kind of a succinct one it's a conjunction of else if in english which allows us to ask a question that takes into account whether

or not a previous question had a true or false answer well what do i mean by that well let me go back to my code here and let me propose that we now improve upon this here by asking ourselves ultimately how can we ask fewer questions and let me go ahead here and propose that instead of asking if if if let's make these conditions potentially mutually exclusive that is to say don't keep answering questions once we get back a true answer so i'm going to change my code up here as follows instead of asking if

if if i'm going to say if x less than y l if x greater than y l if x equals equals y so i'm going to implicitly just like in english take into account that i'm only going to keep asking myself these questions if i haven't yet gotten a true response think about the logic here the english if x is less than y on line four print out x is less than y well if that's the case you're done logically because if the english is saying if x less than y else if x greater than

y those are going to be mutually exclusive if the answer to the first question is true you don't have to keep asking questions to which you already logically know the answer so let me go ahead now and run this program and i think the behavior is going to be the same python of compare.pi what's x let's do 1 what's y let's do 2 x is less than y now honestly i didn't really notice a difference when i ran the program and honestly my mac my pc my phone nowadays are so darn fast that these kinds

of improvements aren't going to necessarily feel any faster until we're writing bigger faster programs but it's laying the foundation for writing better code longer term now what is the improvement i've just made well if previously my diagram looked like this which was problematic in so far as i was asking three questions no matter what even if i already figured out what i want to print on the screen this new version of the program that says if l if l if might look a little something like this instead now it got a little wider that's just

because we drew the arrows to be a bit wider here but let's focus on just how many questions are getting asked let me zoom in at the top as before and let me propose that we note that the start oval is at the very top and it's asking us to ask one question first x less than y is one less than two but notice here let me zoom out if one is indeed less than two we follow this longer arrow down mark true we print out quote unquote x is less than y but then we

immediately follow this next arrow down to the icon that says stop so that's what's implied by doing if l if l if if we get back a true answer right away to that first if we're going to print out x is less than y and then stop we're logically at the end of the program so this picture is just representing graphically what the code is actually doing but suppose i typed in something else suppose that my code actually ran and i typed in 2 for x and 1 for y that is to say the answer

to the first question is now false but the answer to the second question is now true because of course 1 2 is greater than 1. well let's go back to the diagram same as before we start at the very top where it says start the very first question up here now x less than y is an answer of false because no 2 is not less than 1 so we follow this arrow to the next question this diamond is x greater than y well yes 2 is greater than 1 so now we follow this left arrow

which is true we print out quote unquote x is greater than y and then stop so what's the improvement well in the first case we got lucky and we only had to ask one question and boom we're done this time we had asked two questions but then boom we're done only if x happens to equal y do we actually find ourselves logically getting all the way down to this final l if in my code and pictorially only if x is equal to y do we find ourselves going all the way down to the third diamond

the third question asking is it equal to y or not now hopefully the answer at that point is not false we've included a false arrow just so that the program itself is well defined but logically we shouldn't actually be getting there anyway because it's got to be less than or greater than or equal to in this case well let me pause here to see if there's any questions now either on the code version thereof here or on this diagramming of that very same logic questions here on this control flow uh aren't we supposed to put

an else at the end ah a good question and yes so that's going to be my my third and final approach and if you don't mind let's pivot there right away identifying a third keyword that indeed exists in python that allows us to be even better at expressing this logic to design this program even better and that's going to solve a particular problem so if i take us back to our code here notice that what i've highlighted earlier l if x equals equals y it's not wrong to ask that question in fact if you're trying

to be especially thorough it makes perfect sense to check if x is less than y greater than y or equal to y but why don't i need to ask this third and final question we don't need to ask if x is equal to y anymore because logically if the two conditionals evaluate to false there is only one um conditional that will evaluate to true and that is x is equal to y exactly if we're all pretty comfortable with math and comparisons here of course x is either going to be less than y greater than y

or equal to y but once you rule out the first two scenarios logically it's got to be the case that x must equal y if it wasn't the case that it's less than or greater than so hope proposed that we use this other keyword else and how do we use this well exactly as we might in english let me go back to my code here and instead of bothering to ask the third and final question let's not ask a question at all let's just have this catch-all so to speak a final line of code that

says else just assume that x is equal to y therefore printing it as well so what's the upside of that my code is still going to work exactly the same and again my computer's so darn fast i don't even notice that it's working even faster than it was before but we would notice these kinds of things if we were doing a lot more work a lot bigger programs here but let me run python of compare.pi let's do for instance one and two it still works for that let's do two and one still works for that

let's do one and one and it indeed now works for that but in these cases now let's consider the path we just went down previously our diagram when we had if l if l if in place looked a little something like this and notice that again we might have asked one question or two or worst case three whole questions but we can do better than that using else as hope proposed we can whittle this diagram now down to this and even though it looks like the diagram is getting bigger notice that it's having fewer building

blocks inside of it there's fewer arrows and there's fewer nodes in this picture let's start at the top now start leads us to the first question still x less than y if the answer is true great we can say as much x is less than y and we can stop if it's not true if it's false we can ask the next question x is greater than y true or false if it is great we can print x is greater than y and stop else if it's not the case that x is greater than y the

answer is false we can just immediately logically say x is equal to y we don't have to add the third question at all we can just immediately conclude there so what's the implication here you can see with these pictures a relative decrease in the complexity of a program the first one was very long and stringy with lots and lots of questions unnecessarily ultimately the next one got a little shorter and this one's even shorter still and again the fewer lines of code you have the less likely you are arguably to make any mistakes the easier

it is for other people to read and so generally this readability this simplification is indeed a good thing well let's go ahead and add another piece of uh capability into python and that's this one here just like in english where you can ask this question or this other question you can say the same thing in python using literally this word or so let me go back to my python code here and let's propose how we might ask a couple of questions at once this time perhaps this time considering how we might ask not whether or

not it's greater than or equal to and caring about the precise answer let's take a coarser approach here and let's just try to determine is x greater is x equal to y or not well let me go ahead and delete some of this code and change the question we're asking let me do this well if i care about whether it's equal or not let's check the possible scenarios if x is less than y or x is greater than y let's go ahead and print out x is not equal to y now why is that no

pun intended if x is less than y well it's obviously not equal if x is greater than y it's obviously not equal so we can conclude x is not equal to y so if we instead want to make sure that it is equal to we can just use uh hopes else using print quote unquote x is equal to y and again why is this well if x is less than y or x is greater than y they're obviously not equal otherwise logically they must be equal in fact so let's run this let's go ahead and

run python of compare.pi what's x1 what's y2 okay x is not equal to y let's do it again but 2 for x 1 for y x is not equal to y and one third time how about x is 1 and y is 1 x is now equal to y now if we want to compare that visually to let me propose that the picture looks a little something like this and again this is the exact same thing logically but it's a pictorial representation thereof what's the first question well if x is less than y well then

we follow the true arrow and we say quote unquote x is not equal to y and then we stop but what if x is not less than y what if it's greater than y what if it's 2 and 1 respectively then the answer to x less than y's first question is false so we go here we ask the second question because of the or and that asks is x greater than y if so notice this we can kind of reuse some of the same parts of this picture and just say x is not equal to

y we don't need to add arrows and add boxes unnecessarily we can reuse lines of code picture parts of the picture just as we have lines of code and then we stop lastly we have the following if we know that x is not less than y we know that x is not greater than y it must be the case that x equals y we don't need to ask a third question another diamond we can just immediately print as much and then say stop as well well what could i do here i bet i could improve

this code slightly and if we really want to be nitpicky i would argue that this is now really just a minor refinement but it's a good habit to get into thinking about could my code be better could my code be simpler could i improve this code further it's subtle but could i improve the design could i ask fewer questions could i tighten it up so to speak what do folks think you can ask if x is just equal to y then if we print x is equal to y else x is not equal to y

perfect recall one of the other symbols we saw in the available list earlier we can check not just less than or greater than or equal to we can literally ask the question is it not equal to why are we wasting time asking if it's less than or if it's greater than well if all you care about is is it not equal i think we can do exactly that let's just ask the one simple question we do care about and so let me go back up here and let me just say not both of these questions

let's get rid of the or let's just say if x is not equal to y then go ahead and print x is not equal to y and that too i think is going to work exactly the same but the picture now looks a little bit different notice that this was our flow chart earlier that represented that same logic and there's a bit of complexity you got to go left you got to go right based on the answer to these couple of questions if we now take into account what this version of the program looks like

it's even simpler perhaps the simplest one we've seen yet when we start off the program we ask just one and only one question is x not equal to y and if so true we go ahead and print out x not equal to y if the answer is false then of course it must be equal to y so we say that instead and if we really want we could invert this if i go back here to my code and if for whatever reason you just prefer to think in terms of equal or not equal as opposed

to not equal or equal it's really up to you we could change this to be equals equals but i'm going to have to change my print statements to be in the opposite order so let me go ahead now and reverse these two here and move the second one first and the first one second so now when i execute this code i'm asking still just one question so it's still just as good just as succinct but now the diagram instead of looking like this is going to change the not equal to equal equal and we just

need to make sure that we print out the right thing accordingly and again here too just as the code is getting a little more compact a little more compact with fewer and fewer characters so are these diagrams these flow charts capturing the relative simplification of each of those programs too all right let me go ahead and pause here to see if there's any questions now on any of these versions of code yeah i have a couple of questions uh uh what if indentation is not used uh if indentation is not used your program will not

work so python is a little different from a lot of languages in that it enforces the indentation requirement some of you who have been programming for years might not necessarily be in the best habit of indenting your code properly and one of the features arguably of python is that it makes you indent your code or it will not just work and i think did you have one other question uh yeah is the colon necessary is the colon necessary yes the colon two is necessary so with python what you see is what you get here and

indeed it needs to be indented and the colon is necessary python does not use in the same way by convention as c and c plus plus and java curly braces to connote blocks instead it relies indeed on this indentation well let me propose that we introduce one other keyword here in python to see exactly how we might combine additional thoughts and that's going to be literally the word and a conjunction of one or two or more questions that we might want to ask at once and let me propose here that we explore this kind of

logic by way of another program altogether in vs code whereby i'll go ahead now and create a new program say called grade.pi let's consider exactly what grade a student should get based on their score on an exam or a test or a quiz or some other assignment like that i'm going to go ahead and run code of grade.pie to give myself a new file and i'm going to go ahead and start by just getting the user's score again on some assignment or test or the like and i'm going to store it in a variable called

score equal the return value of the int function which is going to convert whatever the user's input is when prompted for this score so again the user should just oblige by giving me a number like 0 or 1 or 2 or hopefully much higher than that like 97 98 99 100 assuming the test or assessment is out of 100 percentage points now how can i go about assigning a grade to the student's score well in the u.s it's very commonly the case that if you get between a 90 and 100 that's an a and if

it's between an 80 and an 89 it's a b if it's 70 and 79 it's a c and so forth all the way down to f which should be e but we'll see that there's a bit of a jump so how might i express this well i can use conditionals and i can ask a few questions and then print out the student's grade accordingly so let me express it like this if the student's score is greater than or equal to 90 and the student's score is less than or equal to 100 so it's in that

range let's go ahead and print out that their grade shall be an a because they're in the 90s above grades range l if the score is greater than or equal to 80 and the score is less than or equal to say 89 but here i have some options logically i can actually express myself in any number of ways and maybe just to be a little cleaner i'm going to say a score is less than 90 so i'm using less than instead of less than or equal to so i'm making sure that their boundaries between these

grades are correct then i'm going to go ahead and give the student a b if it's in the 80s l if score is greater than or equal to 70 and the score is less than 80. i'm going to go ahead and give them a c l if the score is greater than or equal to 60 and the score is less than 70 i'm going to go ahead and give them a d and here's where it's a little anomalous at least in some schools here else i'm going to go ahead and give them an f so

we're skipping e all together and we're going to give an f instead for the grade so that's the catch-all and i think logically i've gotten this correct at least based on where i went to school growing up such that it's going to give an a or a b or c or a d else it's going to assume that you got an f well let's try just a few of these here let's run python of grade dot pi my score is let's let's start strong 100. all right i got an a didn't do as well the

next time maybe it's a 95 still an a starting to slip further so i got an 89 the next time that's now say a b and let's say i really had a bad week and it's now like a 71 that's now a c or i didn't even submit it at all that's a an f all together right so it seems to work that's not really an exhaustive test but at least based on some sampling there my code seems to work as i expect but let's see if we can't tighten this up it's not wrong it's

correct and indeed according to my own specifications i dare say this code is correct but can we tighten it up can we reduce the probability of bugs now or down the line can we increase the readability of it and can we increase the efficiency of it can we get the computer to have to answer fewer questions and still get the same result well let's see what we might do let me just kind of switch things up if only to demonstrate that we can use these symbols in different ways i could say as i've done if

score is greater than or equal to 90 but i can actually do this i can flip it around instead of saying greater than or equal to let's say 90 is less than or equal to score and here let's say if 80 is less than or equal to score and here 70 is less than or equal to score and then lastly 60 is less than or equal to score so it's the same thing logically i'm just kind of switching things around just like you could do on paper pencil if you really wanted but now notice this

trick and this is not possible for those of you who have programmed in c or c plus or java or other languages notice what i can do here is actually combine these ranges notice that i'm asking two questions two boolean expressions is 90 less than or equal to score and is score less than or equal to 100 well python allows you to nest these things like this and chain them together and just like you would on paper pencil in the real world you can encode in python do this which is just a little cleaner right

it's tightening up the code a little bit it's fewer keystrokes it's faster to type it's easier to read moving forward so that's arguably better as well so that's one improvement it's largely aesthetic in this case it's still asking the same number of questions but it's doing it a little more succinctly still well what what more could i do here next well you know what each time i'm deciding these grades i don't think i have to ask two questions i don't have to ask is it greater than 90 and less than 100 is it greater than

80 and less than 90 if i kind of rethink my logic i can maybe do this better still let me propose that we simplify this further and just do this if we know the input for the moment is going to be within 0 and 100 we can make some assumptions we could say something like if the score is greater than or equal to 90 well the student gets an a l if the score is greater than or equal to 80 the student gets a b l if scores greater than or equal to 70 they get

a c l if the score is greater than or equal to 60 they get a d else they get an f so what have i done here well instead of asking two questions every time checking the lower bound and the upper bound of that range i'm kind of being a little more clever here by asking if the score is greater than 90 well they've obviously gotten an a or better if your score is greater than 80 well you either deserve an a if it's really strong or a b if it's just above 80 but because

of the if l if logic we've already checked as the student score greater than 90 and if it's not then we're asking the question well is it greater than 80. so you implicitly know it's somewhere in the 80 to 89 range else you know it's in the 70 to 79 reach else it's in the next range down so it's a minor optimization that allows us to ask fewer questions but again it's making the code arguably a little more readable certainly more succinct and then hopefully more maintainable longer term any questions then on these types of

changes and this type of logic with our code uh what if we don't use alif at all what if we uh write the code in f yeah so that's a good question because it's actually going to have an unintended effect here let me get rid of the f temporarily and just focus on a through d if we revert to where we began today's story with conditionals saying if if if now our cleverness here of using broader strokes and not using an upper and lower bound ranges is going to come back to be a downside let

me go ahead and run python of grade dot pi and suppose my score is uh 95 i am so darn excited i want my a but nope i just got an a a b a c and a d so logically that's broken things because if you don't make these conditions mutually exclusive every one of those questions is going to get asked and therefore answered and even if your grade is above a 90 it's also logically above an 80 above a 70 above a 60 and if i kept it in there i would have failed as

well with an f really good question other questions here on this form of logic like it would there be any i guess better way to kind of clean up even just this simple statement like we had before the previous one that you had with the ltis oh i i i like your enthusiasm for simplifying things further i'm gonna go out on a limb here and say this is about as good as it gets at least using only conditional statements i can if my mind wanders think of a slightly more clever way to do this maybe

with something called a loop or another programming construct we don't have that yet in our vocabulary but yes there's absolutely other ways to do it but i think not yet if we want to restrict ourselves to just words like if and or and else uh and elif and and and the like well let me propose that we pivot now to use another approach here that uses one other symbol that up until now we've not really had occasion to use let me propose that we implement a program uh that we'll call parity and mathematics parity can

refer to whether a number is even or odd and that's kind of an interesting question and turns out it can be useful in other applications too to i just asked the question is a given number even or odd maybe that the user typed in and let me go ahead and write a new program called parity dot pi via code parity.pi in my terminal and let me propose that we use this as an opportunity to introduce the last of those arithmetic symbols at least most of which we're familiar with addition subtraction multiplication division but there's been

on this list before this last one here a percent sign and it doesn't mean percentage in this case when used as an operator in programming in python rather it represents the so-called modulo operator for modular arithmetic or at least in our case we're going to use it to calculate the remainder when dividing one number by another well what do i mean by that well if you take a number like one divided by three three does not go into one eat cleanly so you have a remainder of one two divided by three has a remainder of

two three divided by three has a remainder of zero because it divides cleanly four divided by three has a remainder of one because you can divide it in once but then that leaves one so it has a remainder of one and then lastly something like five divided by three has a remainder of course of two so that's all we mean by remainder how much is left over after dividing one number by another well if i go back now to my code and i consider how i might implement the question is this number even or odd

let's consider how we might implement that since it's perhaps not necessarily obvious how we can use this additional building block but it turns out it's going to be very useful longer term well let's first just get a number from the user in a variable called x and i'm going to set that equal to the conversion to int of whatever the user inputs after asking them what's x question mark and we've done that before many times how do i now determine if x is even or odd well it turns out if i have access to a

programmatic operator that tells me the remainder i think i can do this in fact let me just ask the group and this is just from grade school math perhaps what does it mean for a number to be even to be clear a number like 0 2 4 6 8 10 12 14 16. those are all even numbers but what does that really mean elena if i'm saying that right uh even numbers it can divide it exactly by two for example two four six eight and ten perfect and we could go on all day long literally

since there's an infinite number of those even numbers but it's nice that you formulated it in terms of a question that we can ask very clearly is this number cleanly divided by two that is can we divide it by two with no remainder a remainder of zero well that's perfect because if we have this operator this percent sign that allows us to answer just that what is the remainder we can presumably check is the remainder is 0 or is it 1. do we have nothing left over or do we have 1 left over well let's

ask that if x divided by two has a remainder of zero as elena proposes let's go ahead and print out something like quote unquote even and just say as much to the user else i think we can assume that if a number is not even it's going to be odd if it's indeed an integer so i'm going to go ahead and print out quote unquote odd instead and let's go ahead and now run python of parity.pi in my prompt what's x let's start with 2. 2 is in fact even let's start with four is in

fact even let's get thing let's get interesting with three three is now odd and i think we could do that all day long and hopefully get back indeed exactly that answer but what more could we do here how could we improve upon this well recall that we have the ability to invent our own functions and let me just propose for the sake of discussion that we're going to eventually find that it's useful to be able to determine if a number is even or odd and so we'd like to have that functionality built in and i

don't think python has a function for telling me just that but i can invent it using code like just this so let me go into my earlier version here and let me propose that we do this let me go ahead and write a main function i'm going to get back into that habit of defining a main function to represent the main part of my program and i'm going to do what i did before i'm going to get an integer from the user's input asking them what's x question mark and then i'm going to ask this

question for the moment i'm going to naively assume that the function already exists but that's a useful problem-solving technique even if i have no idea yet where i'm going with this how i'm going to invent and function that determines if a number is even i'm just going to assume that there's a function called is even and i'm going to call it blindly like this if is even passing in x then go ahead and print quote unquote even so if this magical function called is even returns true as its return value i'm going to print out

that it's even else otherwise i'm going to assume that it's of course odd now the one problem with this program even if i call main over here is that is even does not exist and this program would break if i ran it right now but that's okay i have the ability recall to invent my own function so let me define with def a function called is even i want this function to take an argument and i'm going to call it n just a number generically i could call it x but again i don't want to

confuse myself as to which x is which so i'm going to give it a different name and that's fine i'm just going to call it more generically n for number and then i'm going to do this i'm going to say if n percent 2 equals equal 0 just like before then and here's the magic you the programmer can actually return what are called boolean values we've seen in python that python has stirs or strings ins or integers floats or floating point values all of which are different types of data in python python also has a

fourth data type called bool for a boolean value and even though this is just adding to our list the nice thing about bulls is that they can only be true or false an int can be any number of infinite possible values a bull can only be true or false and it must be capital t and capital f if you're writing itself so if i go back now to my code and i consider exactly what i want to return here well if x if n percent two equals equals zero that is if n divided by two

has a remainder of zero well i think it's even to elena your definition so let's return true capital t else if it doesn't have a remainder of zero i'm pretty sure mathematically it's got to have a remainder of one but it doesn't matter i know it's not even so i'm going to return false and we've returned false instead capital f and now that we've defined both main and is even and i'm calling main at the bottom i think i've got this right python of parodied up high enter what's x let's try something simple like 2

and it's even let's do it again what's x how about 4 even once more what's x how about 3 and it's odd now what have i done here i've just made the point that if i want to create my own function called is even that answers this question for me that i can now use in this program and heck maybe future programs that i write i now have a function that no one gave me i gave myself that i can use and reuse and i can even perhaps share it with others i'm using that function

now on line three just to make a decision i'm using a conditional up there and my boolean expression something that's true or false is going to be not something explicit like x less than y or y greater than x or the like it's going to be a function call i'm using a function as my boolean expression but that's okay because i know because i wrote it that that function is even returns true or it returns false and that's all i need in a conditional to make a decision to print even or print odd so let

me pause here to see if there's any questions now on how i've implemented is even using this bool hello hi david first of all thank you for this wonderful class uh day before yesterday and today uh i have just one query like bases on my background of java there when we used to pass the argument we can also pass the address of the variables so is there any sort of this concept in python short answer no those who are unfamiliar with java or other languages or c or c plus plus there's generally ways to pass

values in different mechanisms that allow you or disallow you to change them in python no everything we're going to see is actually in fact an object but more on that down the line how about time for one more question here on these bulls and these is evens so i actually had a question about um defining a function okay if that's okay so if you define one are you like within your code like you made it up are you allowed to use the dot operator like we did name dot strip and use it like that good

question if you've created your own function can you use other functions like dot strip or dot title or dot capitalize that we've seen in the past you can use those on strings those functions come with strings you can't necessarily use them on your own functions unless your function returns a string for the examples you gave i'm returning a bull bulls have no notion of white space to the left or the right you can't call strip you can't call capitalize but if you were writing a different function that returns a string absolutely you could use those

functions as well well let me turn our attention if i may back to this example here and consider as we now frequently do can we improve on the design of this code can i make this particular program better and i can there's a couple of ways here and i'll show you something that's now generally known as something pythonic there's actually this term of art in the python world where something is pythonic if it's just the way you do things in python which is to say we've seen already there's so many different ways to solve certain

problems and in the python community of programmers there tend to be some ways that are smiled upon more than others and they tend to relate to features that maybe only python has but not other languages and here's some syntax that you might not have seen in languages like java or c or c plus if you've programmed before and if you've never programmed before this too is going to be new instead of asking a question like this if else using four lines in python you can actually collapse this into just one more elegant line if you

will instead of asking if n divided by two has a remainder of zero return true else returned false let me delete all of that and just say this return true if n divided by 2 has a remainder of 0 else return false now those of you who do have prior programming experience might actually think this is kind of cool you can condense from four lines into one line that very same thought and one of the reasons why python is popular is that it does tend to read rather like english it's not quite as user friendly

as most english or most human languages but notice now the line does rather say what you mean return true if n divided by two has a remainder of zero else false i mean that's pretty darn close to something you might say logically in english be it about even an odd or really anything else so that program is going to work exactly the same python of peridot pi let me type in two it's still even let me type in three it's still odd but i can refine this even further and again consistent with this idea of

not just writing correct code but writing better and better code but still keeping it readable i can do one even better than this notice this value here is my boolean expression and it is going to evaluate to true or false is n divided by 2 having a remainder of 0 or not like that is by definition a boolean expression it has a yes no answer a true false answer well if your boolean expression itself has a true or false answer why are you asking a question in the first place why ask if why say else

just return the value of your own boolean expression and perhaps the tightest version the most succinct and still readable version of this code would be to delete this whole line pythonic though it is and just return n modulo 2 equals equals 0. if it helps let me add parentheses temporarily because what's going to happen in parentheses will happen first n divided by 2 either does or does not have a remainder of zero if it does the answer is true if it doesn't the answer is false so just return the question if you will you don't

need to wrap it explicitly with an if and an else and in fact because of order of operations you don't even need the parentheses so now this is perhaps the most elegant way to implement this same idea now which is better this is pretty darn good and it's hard to take fault with this because it's so very succinct but it's perfectly okay and just as correct to have an if and then an else even though it might be four total lines if that helps you think about your code more clearly and it helps other people

reason about it as well so it turns out there's another syntax that you can use to implement the same idea of a conditional whereby you do something optionally based on the answer to some boolean expression and the keyword that you can now use in recent versions of python is called this match match is a mechanism that if you've programmed before is similar in spirit to something called switch in other languages for instance let me go ahead here and close out parity dot pi and let me go ahead and create a new file called house dot

pi and in house stop pi i think what we're going to do is try to implement a program that prompts the user for their name and then just outputs what house they're known to be in in the world of harry potter so for instance let me go ahead and do this let me give myself a variable called name set it equal to the return value of the input function and i'll say something like what's your name question mark and then after that i'm just going to use a traditional if el if else construct to decide

what house this person is in so let me say if name equals equals say harry as in harry potter well let's go ahead and print out harry's house which is gryffindor in the world of harry potter el if the name is instead hermione then go ahead and print out also quote unquote gryffindor she's in the same house too l if name equals equals ron let's go ahead and similarly print out gryffindor quote unquote and let's make this a little more interesting now l if name equals quote unquote how about draco draco malfoy in the books

let's go ahead and print out quote unquote slytherin and just in case someone else's name gets inputted for now let's just pre-suppose that we don't recognize them and say by default else print out quote-unquote who question mark just to convey that we don't actually have a hard-coded response to that particular name let me go ahead now and run this as python of house dot pi enter and i'll go ahead and type in something like harry and voila we see that harry is indeed in gryffindor let's run it one more time python of house dot pi

let's type in draco this time slytherin and now let's type in an unrecognized name let's go ahead and rerun python of house dot pi and let's go ahead and type in padma enter and who because we haven't actually hard coded with an l if condition in this case uh what house padma is meant to be in all right well it turns out there's other ways to implement this indeed there's some redundancy here in that we're checking if harry or hermione or ron are all in gryffindor i feel like we can at least tighten this code

up a little bit using techniques we've seen already so let me go ahead and do this let me go up here and instead do something like this let's get rid of these two blocks of l-if's leaving just harry's for a moment and let's use that or keyword again and say or name equals equals quote unquote hermione or name equals quote unquote ron thereby consolidating all three cases if you will into just one if statement then we still have a separate lift for draco because he's not in fact in gryffindor and then the final else to

catch anyone else all right let me go ahead now and run this version of the program python of house dot pi i'll type in hermione this time she too is still in gryffindor let me try it with ron and that too still seems to be correct well it turns out there's another approach altogether that can perhaps make your code a little less verbose you could imagine how complicated this code might get if we had not just harry and hermione and ron but a whole bunch of other names as well for gryffindor for slytherin and for

all of the other hogwarts houses so you can imagine that code just getting pretty unwieldy pretty fast well it turns out another technique you can use is indeed this keyword called match which is very similar in spirit but the syntax is different and allows you to express the same ideas a little more compactly so let me go back to house dot pi and let me propose that i get rid of my current if elif else approach and instead do this literally use the keyword match and type the name of the variable or value that we

want to match on and then i'm going to go ahead and include a colon and then underneath that i'm going to include literally a keyword called case and the first case i want to consider is going to be harry and i'm going to put harry in quotes because it's a string or a stir and i'm going to have another colon at the end of this line and indented under that one i'm going to go ahead and for now print out gryffindor which of course is harry's house otherwise i'm going to have another case for quote

unquote hermione and similarly i'm going to have under that indented print quote-unquote gryffindor close quote now i'm going to have another case for ron also in quotes with a colon now print quote unquote gryffindor and now i'm going to have a other case for let's say draco this one gets a little more interesting because draco of course now is in slytherin and then i'm going to go ahead and leave it as that for now so let me go ahead and save this file and go back down to my terminal window running python of house stop

high enter and let's go ahead and try harry and he seems still to be in gryffindor let's run it again for hermione enter gryffindor let's skip ahead to draco and type in draco's name he's indeed in slytherin now let's try another name that we haven't handled a case for like padma again enter and we're just ignored there's no output whatsoever because there wasn't a case for padma now we could of course go back in and explicitly add one for padma but what if we similarly to the else construct just want kind of a catch-all that

handles anyone whose name is not explicitly specified well turns out the syntax for that using this new match statement is to still have another case but then to use this single underscore character which is used in other contexts in python but for here it's meant to say whatever case has not yet been handled go ahead and print out as we did before for instance quote unquote who with a question mark at the end now let's go ahead and rerun this python of house dot pi i'll type padma's name again and this time i think we're

at least going to get an explicit response indicating who whereas previously we did not have the equivalent of that now i think we've regressed a little bit we went from tightening things up by putting harry and hermione and ron all in the same line in the same if statement but here we have now three case statements again for all three of those well we can tighten these this code up as well but the syntax is going to be a little bit different i'm going to go ahead and delete these two middle cases for hermione and

ron and then up here next to harry's name before the colon i'm going to go ahead and use a single vertical bar and then a quote unquote hermione then another single bar and do quote-unquote ron and this is how using this relatively new match statement you can say the equivalent of harry or hermione or ron but more concisely than you could using an if statement alone as we implemented it previously so now one final run of the program with python of house stop high let's make sure that harry is still in gryffindor let's make sure

that hermione's still in gryffindor let's make sure that ron is still in gryffindor and indeed all three of them are now as always with python and programming more generally there's going to be different ways you can solve these problems this is just another tool in your toolkit arguably it has tightened things up arguably it's perhaps a little more readable because there's a little less syntax going on a little less duplication of equal signs and elif and elephant elif all over the place but ultimately this would be an equally correct approach to that same problem but

it turns out with the match statement you can do even more powerful forms of matching as well here we've used it simply to implement the same idea as that if l if else construct and it's worth noting if you've programmed in some other language the syntax here is indeed correct you do not need for instance a break statement as has been peppered throughout and you don't need something like default or something explicit you indeed just use this underscore as your catch-all at the end of the match so just by adding in some of these new

keywords here like if and l if and else we have now the ability to ask questions about values we have the ability to analyze input from users and ultimately make decisions about it and these then were our conditionals lying ahead is going to be the ability for us to not only use functions and variables and also these conditionals but also next loops the ability to do something now again and again