Original subtitles

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Here's a question. Have you ever sat in your bath and wondered,

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why is it that no matter how forgetful I am,

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my bath never overflows like this?

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This is something I was thinking about the other day and I realized that it's

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not because I'm particularly diligent. I forget stuff all the time. I mean,

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pizza, anyone? This is what my pizzas usually look like. Um,

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but the reason why a bathtub or the sink doesn't overflow is because of this

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fantastic piece of engineering, the overflow.

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So this means that whenever the water reaches beyond a certain level about here,

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the water starts overflowing

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so the bathtub doesn't overflow and annoy your neighbors downstairs.

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In fact, we could represent this mechanism with a conditional statement.

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When the water level is say greater than 80 centimeters,

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then it should drain the water.

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But if the water level is not greater than 80 centimeters,

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it should continue filling up the tub.

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This type of conditional statement is known as an if/else statement.

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Depending on a particular condition,

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we would do either A or B.

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And when we want to write Python code to represent this,

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it looks something like this.

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There's the keyword if, and then the condition that we're testing for and then a

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colon and after the colon we've got an indented block of code which should be

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executed if this condition is met, if it's true.

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But if it's not true,

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then we will skip to the else block and it's just the else keyword with a colon

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and then this code block would execute if the condition is false.

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So we could represent that previous bathtub situation with code that looks a bit

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like this.

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Let's say our water level is at 50 centimeters and then we would test if the

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water level is greater than 80 centimeters. Well, if that is the case,

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then we should drain the water.

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But if it's not greater than 80 centimeters,

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in other words else, well in this case,

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we should just continue filling up the bathtub.

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Let's put this into practice with a real life problem.

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Now let's say that you've gotten a job at a theme park and your first job of the

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day is to write some code that replaces the ticket box.

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Now there's a couple of things that you'll need to think about. Firstly,

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in order for somebody to actually purchase a ticket to go on the rollercoaster

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ride, they will need to be over 120 centimeters.

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So we have to check what their height is because if they're too short,

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then we won't be able to sell them a ticket anyways. In the course resources,

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I've included a link to this flow chart that I've created on draw.io.

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This is a really,

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really useful tool for creating any sort of flow charts or diagrams and it's

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really easy to use. Now, if we take a look at this flow chart,

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this is basically the logic that we have to program using our,

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if and else statements.

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If the person who's trying to purchase a ticket is not over 120 centimeters,

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then they can't ride on the rollercoaster.

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But if their height is greater than 120 centimeters, then they can ride.

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So let's try it out. If you head over to the starting Repl.it,

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you'll find some starting code in there and you can go ahead and fork this

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Repl.it.

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All that I've got here is a print statement that says "Welcome to the

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rollercoaster" as well as an input asking the user for their height in

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centimeters and then converting the string into a whole number, an integer, and

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then I'm storing it inside this variable called height.

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Now we're going to check whether if the height that the user has typed in is

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greater than 120. So we use the keyword if and then we check if the height is

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greater than 120 and then we add the colon and now when I hit enter,

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you'll notice that the code editor has automatically indented me slightly over.

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I'm not over here because in Python the spacing and indentation is really,

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really important.

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It tells the computer that the code that I'm about to write is what should be

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executed when this condition is met.

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So what should happen? If your height is over 120 then we will just print

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You can ride the rollercoaster!.

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But what should we print if height is not greater than 120? Well,

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in this case, we would use the else statement to catch when that happens.

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And it's really important that you don't write the else here because this is

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indented over. Instead,

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you want it to be at the same indentation level as the if statement.

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These two are essentially a pair, if and else. Now,

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after the else keyword,

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we again add a colon and then we hit enter and we're now indented again.

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And here we can write the code that should happen if this condition is false.

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"Sorry, you have to grow taller before you can ride."

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So the really important things here are the condition which we're testing for, is

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the value of height greater than 120, the syntax of this code

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so the keywords

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if and else as well as the colons that come after each of these lines.

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And finally also the indentation.

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Everything that is indented after the

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if is a block of code.

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So this is indented and it's effectively inside this

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if. So

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this line of code lives inside the else statement.

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This block of code lives inside the if statement. And if you mess up on the

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indentation then you're probably going to get an indentation error telling you

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that this line 5 probably should be indented.

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But when we do actually correct it and we hit run,

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let's say that our height is 130 centimeters,

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then we get back, you can ride the rollercoaster.

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But if our height was say 90 centimeters,

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then we get a different outcome. We get, sorry,

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you have to grow taller before you can ride. So by using,

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if and else statements we're able to get our code to do different things,

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either printing this line or printing this line depending on a condition that

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we're testing for. Now, when we use this greater than sign,

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effectively what we're saying is that is the height greater than 120 which means

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that it does not include 120. In fact,

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if I run this code and if I type in my height as 120 then actually goes into the

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else block and prints this.

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So if we wanted to include 120 centimeters so that all the people who are

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exactly 120 centimeters can ride the rollercoaster,

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then instead of just using a greater than symbol,

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we have to write greater than or equals.

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So these two symbols have to be next to each other.

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And now when I run my code and I write 120, you'll see that it's now falling into

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this block of code and it's telling me that I can ride the roller coaster. Now,

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these are called comparison operators and we've already seen greater than, so

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lesser than is pretty self explanatory.

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But we've also seen greater than or equal to and lesser than or equal to.

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So if you want to include a particular number, a particular value,

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when you do these comparisons, you would be using these instead.

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Now in this table, there's also the equal to and not equal to.

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For example,

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if you wanted to check if somebody's height is equal to precisely 120 then you

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would use two equal signs. And very often it gets a little bit confusing,

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especially if you're new to programming when you're typing equal signs,

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because sometimes we're typing one and other times we're typing two.

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It's important to remember that when you have one equal sign,

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it means that you're assigning this value to this variable.

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But when you have two equal signs,

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you are checking to see if the value on the left is equal to the value on the

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right and they're completely different. Now,

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the good thing is that when you get it wrong,

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usually you'll get enough clues in the error to actually hint to you,

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Hey, maybe there's something that's wrong here, right?

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Cause the syntax doesn't look right. For this to be a condition,

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it has to be something that evaluates to true or to false.

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And height =120 does not evaluate to either.

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So by changing it to this,

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we're saying if height is equal to 120 then we'll execute this line of code

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or any other lines which are in the same block of code.

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But if the height is not equal to 120 then we're going to execute this block of

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code. So if I write 120 then that works.

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But if I write 121 then it does not work.

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Similarly, you can also check for not equals to, which just flips it around.

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But in our case, it makes sense to say if the height is greater than or equals

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120 then you can ride the rollercoaster.

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But otherwise you cannot ride the rollercoaster. By using,

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if and else statements,

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we can get our computer to do different things and respond differently depending

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on different conditions. So in the next lesson,

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I've got a coding exercise for you. I'll see you there.