Conditionals If/Else/Elif

Quick reference

• 00:04 In the last video, we looked at comparison operators.
• 00:06 In this video, we want to take those comparison operators, and
• 00:10 turn them into conditional statements that allow us to make decisions,
• 00:13 do different things based on different conditions.
• 00:16 The three main statements we're going to be looking are if, else, and elif.
• 00:21 So let's just go ahead and create one of these, let's create a variable, and
• 00:24 let's just call it num, and set it equal to 10.
• 00:26 So now, let's create our conditional statement.
• 00:28 Let's say, if num is greater than 5, and we have this colon, so
• 00:32 this is our conditional statement.
• 00:34 And you can see, we're using our comparison operator, greater than.
• 00:38 So now, if this is true, we want to do something.
• 00:42 So let's print, num is greater than 5, and that's it.
• 00:48 So you'll notice right here, I tabbed over, this tab is important.
• 00:53 So we're creating a block of code here, and
• 00:55 everything after this colon that is tabbed over will get executed, if this is true.
• 01:01 If it's not true, the program will drop down to the next line that's not indented,
• 01:06 and continue on from there.
• 01:08 So let's save this and run it, and we get num is greater than 5.
• 01:12 So let's change this to 4, so now, is 4 greater than 5?
• 01:16 No, that will evaluate to false, so what happens then?
• 01:20 Nothing, because it skips this line, because this is false.
• 01:24 So that is if, and you'll use this sometimes.
• 01:26 But for the most part, when you're dealing with these conditional statements,
• 01:30 you'll use an if-else statement in connection with each other.
• 01:33 So what that does is it says, if something, do something,
• 01:38 else do something else.
• 01:39 So if this is true, do this, otherwise do something else.
• 01:42 And to designate what that something else is, we just type in else and
• 01:47 a colon and again, indent.
• 01:49 And it has to be the same indenting, so you can't go like that,
• 01:52 these things all have to line up.
• 01:54 So we can print, num is not greater than 5,
• 01:59 and let's wrap these in quotes, of course.
• 02:04 So if we save this, our number is 4, 4 is not greater than 5.
• 02:08 So we would expect this to print out, and it does, num is not greater than 5.
• 02:13 We can change this back to 10, run it again, and
• 02:16 we're going to get num is greater than 5, because 10 is greater than 5.
• 02:20 And look at this flow, right, it starts up here.
• 02:23 It says, if this is true, do this and then stop, don't do this, right?
• 02:29 Now, otherwise, if we change this to 4, look at the flow again, it starts,
• 02:34 is 4 greater than 5?
• 02:35 No, it's not, so it skips this, and it goes right to this else, and
• 02:39 then it evaluates, and does whatever we've put here.
• 02:42 So that's if, that's if-else, the next thing is elif,
• 02:46 and elif stands for else if.
• 02:48 And this allows us to do other things, make other comparisons besides this.
• 02:54 In this one right here, this if-else, we're making one comparison, right,
• 02:58 we're saying, is 4 greater than 5?
• 03:00 If yes, do this, if no, do that, and then it ends.
• 03:03 We might want to test for something else, as well, so we would do that with an elif.
• 03:07 And here we pass in another condition, another comparison operator.
• 03:12 So if we said, if num == 4, and let's go print,
• 03:18 num is 4, woo-hoo, right?
• 03:22 We have a little error, we need a colon there.
• 03:24 So every time you create a conditional, comparison operator,
• 03:27 conditional statement, you have to put the colon after it.
• 03:30 So let's save this and run it, and we get, num is 4.
• 03:33 So let's look at the flow of this really quickly.
• 03:36 We have number 4, is 4 greater than 5?
• 03:38 No, it's not, so it drops down to the next elif statement.
• 03:41 It runs this conditional, is num == 4?
• 03:44 Yes it is, so it prints this out and then it ends, it does not do this.
• 03:49 So if we change this to 3, for instance, and saved it and
• 03:52 ran it, you see num is not greater than 5.
• 03:55 So let's look at this, is 3 greater than 5?
• 03:57 No, its not, so it skips this line.
• 03:59 It runs this new conditional, is 3 equal to 4?
• 04:03 No, it's not, so it skips this line, and then it ends with this else.
• 04:06 You notice, a lot of programming languages,
• 04:09 they end their if-else blocks with the word end, or something.
• 04:12 Python does not do that, and the reason why is because of this indentation.
• 04:16 It knows that when you stop indenting, if we were to say, print("Hello")?
• 04:23 Python knows this print("Hello") is not part of this whole block,
• 04:27 because it's not indented, right?
• 04:29 So the indentation is very important, so keep that in mind.
• 04:32 So those are conditional statements, very important.
• 04:35 In the next video, we're going to look at multiple conditionals.

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