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Hi guys, and welcome back.

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In this video we're going to learn about if statements.

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An if statement allows us to use a Boolean

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to make a decision in our programme

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and potentially do different things,

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depending on the value of that Boolean.

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Of course, if statements are most useful

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when you have user input,

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such that your programme can potentially get different data,

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depending on who runs it and who's using your programme.

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So here we've got an input that asks the user

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for the day of the week,

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so what day of the week is it today?

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And then, we're going to check

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whether the current day of the week is Monday.

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So, how would you check

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whether the current day of the week is Monday?

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You would say day_of_week == the string "Monday".

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Let's print this out to see whether our Boolean works.

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So, all I've done is printed the Boolean out.

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And again, remember that this here

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will evaluate first, so Python will say,

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"Day of week, is it equal to Monday?"

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And if it is, then it will replace

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this entire thing by "true."

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Otherwise, it will be replaced by "false."

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And, if we run this,

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you see that we get our prompt,

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and you can type in "Monday,"

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and then you get True.

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But, if you type "Tuesday,"

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then you get False.

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So this is what we expected.

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Now, instead of printing this out,

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we can actually put it in an if statement.

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So the if statement is like this,

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you type in your if keyword, which is special in Python.

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Then you put a space, and then,

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without putting brackets or anything,

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you type your Boolean comparison.

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And then at the end you put your colon.

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This will evaluate as soon as the if statement runs,

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so as soon as Python reaches line 3,

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it's gonna calculate the result of this Boolean,

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which is gonna be true if the user entered "Monday,"

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and false if they didn't.

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And if it is true,

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then we will run the code under the if statement.

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And here's one of the most important things

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in Python programming,

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which is that there are a few spaces

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in front of this line here.

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This is called indentation in Python,

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and after a colon that defines a block of code,

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such as an if statement or a function or a class,

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we're gonna learn more about those later on,

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you need to put indentation.

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That is how Python tells which code

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is inside the if statement,

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i.e. which code you want to run when this is true.

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So, for example, you can print

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"Have a great start to your week!"

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Now, this is the line of code

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that is inside the if statement.

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If this is true, this line of code will run,

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because it's got these four spaces in front.

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I'll say at this point that

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you don't have to use four space in Python,

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you can use two, you can use six, or eight,

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whatever you want, but be consistent.

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So, always use the same number of spaces

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that's gonna make your code work,

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otherwise it may not work,

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and also it's gonna make your code look much better.

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If you've got different indentations

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in different parts of your programme,

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people are gonna be really confused

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when they look at your code.

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I recommend using four spaces for Python code.

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Now, if you print something and you remove the indentation,

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the spaces in front,

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for example, like this, what you're gonna get

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is that this code is always going to run,

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if this is true, or if it is false.

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Because it's not in the if statement,

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it's completely unrelated to the if statement.

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So, Python will run this code if it's true,

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it will run this code.

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If it's false, it will not run this code,

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and then it will jump to the next line

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and it will continue running from here.

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So, I'm gonna run this down here again,

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and I'm gonna type "Monday,"

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and then you see that we get,

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"Have a great start to your week!"

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and, "This always runs."

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But, if we type in "Tuesday,"

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then you just see "This always runs."

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and you don't see the "great start to your week".

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If you want to print something

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if the day of the week is not Monday,

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then you can do something like this:

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If day_of_week is not Monday, then print

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"Full speed ahead!"

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Let's run that, and then I'll type "Monday,"

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and you get "Have a great start to your week!"

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But, if we type "Tuesday,"

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then you see "Full speed ahead!"

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Now this is actually not great Python code,

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because you have two checks

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that will always be evaluated by Python.

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Here you've got day_of_week == "Monday"

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and this will always be checked,

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whether it is true or false, by Python.

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And then you have day_of_week is not equal to "Monday"

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and this will always be checked by Python,

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regardless of whether this one was true.

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Evidently, if this one is true,

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this one cannot be, and vice versa.

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So, Python provides you a very nice piece of syntax

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called the else syntax.

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And what this does is,

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if this is true, then we run this code.

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Otherwise, run this code.

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So this is much nicer and also is much easier to read.

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Let me say at this stage,

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that only one of these two will run.

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You cannot possibly run both,

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because you can't have this be true

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and also false at the same time.

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In addition, if you wanted to print something else,

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if the day of the week is Tuesday,

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you can do elif day_of_week == "Tuesday"

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And what this does is it creates a chained if statement,

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an if statement with three different branches.

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First, Python will check

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whether the day of the week is Monday,

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and it will print this if it is true.

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But if it's not true,

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then Python will check

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whether the day of the week is Tuesday,

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and it will print this if it is true.

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But if neither of those are true,

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then it will go to the else.

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An important part of this syntax here is that

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if the day of the week is Monday,

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then Python will print this stuff,

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and it will ignore the rest.

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So it will never calculate this comparison.

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So this is actually good for performance,

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and also it lets you have a comparison here

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that when it evaluates, or when it's true,

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it ignores the rest.

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So there's actually a lot of stuff you can do

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with this knowledge,

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and you'll encounter that plenty

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while you're doing Python programming.

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A common problem when you are dealing with user input

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is that they may not enter exactly what you expect.

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For example, it is quite common

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for a user to enter all lowercase letters

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when you ask them for something,

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just because they can't be bothered

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typing the uppercase letters.

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So here, the user has typed "monday"

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all in lowercase, and then we have printed out

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"Full speed ahead!" because the day of the week

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is not exactly equal to the string "Monday"

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with a capital M.

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So, you have to do something about it

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if you want your programme to be able to detect

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whether the day of the week is Monday

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in uppercase and in lowercase.

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What I'd recommend is that you get the user's input

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and just after you type .lower

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And then, this is going to turn this entire string

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into it's lowercase variant.

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Now, if they enter Monday with the capital M,

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then it is also gonna be turned into monday

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with a lowercase m,

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which means that monday =="Monday"

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with an uppercase M will not match.

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So, instead what you should do

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is turn everything to lowercase.

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By turning the user's input into lowercase,

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and your checks into lowercase,

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you're always going to have the same case,

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because they're both lowercase.

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So if we save that and run it,

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you'll see that now I can type "monday,"

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but I can also type "MONDAY" all in uppercase,

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and I can also type "Monday" as usual.

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So this is a good way of insuring that your programme

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will work, no matter what type of case the users type.

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Let's leave it at this,

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thank you for joining me in this video,

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I hope it's been useful,

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and I'll see you in the next one.