Original subtitles

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Now in the last lesson,

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we saw how we could use if, elif, else to check for multiple conditions.

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But in this case we're only checking one condition even though we have multiple,

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because if this first condition is true,

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then we would do whatever it is we need to do and then we would bypass

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everything else.

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Now what if you were in a situation where you need to check for multiple

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conditions even if the previous one was already true?

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Coming back to our rollercoaster ticketing problem,

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if you're going on a good rollercoaster ride,

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you'd probably want to keep a picture for the memories, right?

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And our rollercoaster is no exception.

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We want to be able to charge users an extra $3 if they want to purchase a ticket

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that includes a photo. Now,

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this is quite interesting because this is completely independent of their age or

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their height.

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Even if we've already gotten their age and height and determined their ticket

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price, this is an extra question. Do you want a photo or not?

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Yes or no. If you do,

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then we're going to add $3 to your existing ticket price.

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To do this, we would write multiple if conditions.

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If condition 1 is true, then do a,

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but then the code is going to go to the next case and check if condition 2 is

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also true,

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in which case it will do B and if the final condition is also true,

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it's going to do C. Whereas on the example on the left

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here, only one of these things, A, B, or C will be carried out.

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Comparing the example on the left where we're using

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if, elif, else, only one of these things A,

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B or C will be carried out. But on the right hand side,

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all three conditions are checked and if it's so happens that all three

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conditions are true, then A,

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B and C will all be executed.

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Currently, our code operates like this. We check for their height.

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If they're over 120 centimeters,

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we check their age and depending on their age,

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we give them a different price ticket.

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What we now want is even after we've checked for their ticket price,

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we want to ask them a question, do you want photos with your ticket?

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And if the answer is yes, then we're going to add $3 to that bill,

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no matter which type of ticket they've got.

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And finally we give them the total bill. If they say no,

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then we jump straight to the total bill and just tell them the price of their

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ticket. So how do we implement this in our code?

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Well, first let's change these print statements.

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Instead of giving them the bill at each of these steps,

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I'm going to tell them which type of ticket they're eligible for.

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So the next thing to do is to ask them whether if they want a photo or not.

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So I'm going to need to use an input, but the question is where do I put that?

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Now the correct answer is that it has to be at the same indentation level as

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this if block. So you can imagine this whole thing with the,

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if elif and else as sort of belonging together.

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Because they all relate to one thing:

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what is their age and which type of ticket are they eligible for. Now,

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outside of that if statement,

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I'm going to create an input and ask the user,

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do you want a photo taken? Type Y for yes or N for no.

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And then I'm going to save their input inside a variable called wants_photo.

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So now I'm going to use an if statement to check what their answer was.

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If it was true, then I'm going to add $3 and if it was false,

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I'm just going to skip to giving them their ticket price.

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So here is where I write my if statement.

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Notice how it's at the same indentation level as this previous set of if

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statements.

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But it's not at the same indentation level as this set of if statement.

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So essentially what's happening is once I've checked their age,

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then I'm going to check whether if they want a photo or not.

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And this is going to apply to everybody no matter their age.

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So if wants_photo is equal to Y,

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well in this case I'm going to go and add $3 to their bill.

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But how can I add $3 the bill when I don't have a variable to vary?

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So instead of using these print statements,

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I'm going to create a variable up here called bill and

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I'm going to set it to equal zero.

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Now in addition to printing to the user how much their ticket is,

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I'm going to set the bill to the price that they're supposed to pay.

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So if age is less than 12 bill is equal to $5, if age is less than 18 then bill

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is equal to $7, and finally for everybody else,

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the bill is equal to $12. So now depending on these conditions,

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the variable bill is going to be changed to a different number.

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But once we land in this if statement,

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I'm going to have to add $3 to their bill

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no matter which value it is at the moment.

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So effectively, what I want to do is bill equals the current value of bill plus

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$3. So if bill was $7,

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then this new value bill should be $10. If bill was $12 then it should now be $15.

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Now in Python as well as many other languages, there's actually a slightly shorter way

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of writing this.

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When you want to increase the current value that's held in a variable and you

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wanna save it back into the variable,

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you can simply write plus equals.

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So bill += 3 is the same as bill = bill + 3.

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Now, no matter what the value of bill is before it reached this if statement,

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I'm still going to add $3 to it. Now after this

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if statement is completed,

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I don't actually have to write a companion else statement because in this case,

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if the answer is no, then we're simply going to do nothing.

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We're not going to do anything to the bill. Instead,

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I'm just going to skip ahead and print to the user their final bill.

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And I'm going to use fstrings to insert the value that the bill variable has

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into this print statement. Now for your code to work,

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the indentation matters a huge deal because the computer will think you want

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it to do different things depending on the indentation.

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So notice how this indentation shows that this bill plus three is to be executed

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when this condition is true,

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but this print statement is not indented to the same level and effectively it's

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going to happen after this if statement has been executed.

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So you can play around with indenting this print statements, un-indenting it

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and see what the difference is.

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But now I'm going to go ahead and hit command

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+ enter or control + enter on Windows to run my code and I'm going to go ahead and

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try and get a ticket.

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So let's say that I am 21 years old,

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tells me the adult tickets at $12 and then it asked me,

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do you want a photo taken? So yeah, I would love to have a photo taken.

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And it tells me that my final bill is 15,

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so I forgot a dollar sign here.

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But effectively it's now added the $3 to the $12 because of this additional if

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statement. So if this concept is still a little bit confusing to you,

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then I want you to really study this flow chart and just look at the difference,

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um, and visualize it because the,

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if elif else statement is only ever going to be true in one of these arrows,

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right? You can't be less than 12 and 18 and over.

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And similarly,

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it doesn't make sense for you to have two different ticket prices.

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But with the want photos,

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this condition has to be asked no matter whichever branch they landed on.

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So it's a completely separate from this if statement.

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And once you've had a look at this flow chart,

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then compare it against the code and the way that the code works should be a lot

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more obvious. And once you're happy with the code and this concept,

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then I want you to head over to the next lesson where I've got a code challenge

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for you. So for all of that, and more, I'll see you there.