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In previous lessons,

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we learned about using the if and else statements to check whether if somebody is

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over 120 centimeters or not and allow them to actually purchase a ticket if they

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are over a certain height. Now in addition to the height,

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there's another condition that we need to check for, namely their age.

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If somebody is over 18 years old and they should be paying the adult price,

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which is let's say $12. But if they are 18 or under,

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then they should only be paying $7. So how can we represent this extra condition

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that we need to check for in our code? Well,

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we could use something called a nested if/else statement.

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We've seen our,

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if/else statements look like this where it's only got two choices.

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If this condition is true, do this, otherwise do that.

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But in a nested if statement, once the first condition has passed,

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we can check for another condition. And then we can have another if/else statement

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inside this if condition. In order for this thing to happen,

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this has to be true and this also has to be true. In order for this to happen,

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this condition has to be true but this condition has to be false.

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So essentially the computer first looks at the larger picture,

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which is this first condition and decides on whether if it should go into the

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else block here or if it should go into the nested block inside the if

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statement.

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So now this is what our flow chart looks like. In the first if statement we check

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whether if their height is over 120 centimeters. If no,

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then the if statements all end. You can't ride, you can't buy a ticket.

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But if yes,

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we actually take them to yet another if statement where we check their age.

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If their age is 18 or under, then we give them a $7 ticket.

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If they're over 18 then they have to pay $12. So the place where we're going to

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nest our if statement is inside here. Notice how it's indented.

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So it's already inside this if block,

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and this already has to be true.

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Now here we're going to create another if and else statement and the condition

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checks for their age. So we better ask them for an age.

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Let's say age equals convert the inputs to an int and "what is your age?"

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So now that we've gotten hold of their age,

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we can see if their age is less than or equal to 18. Well in this case we're

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going to give them the $7 ticket.

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But else namely, if this is not true,

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if their age is over 18, well, in that case, we're going to give them the $12

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ticket.

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So now we have a nested if statement,

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because this if and else statement lives inside this if statement.

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So this condition will only be checked if this is already deemed to be true.

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Now let's say our situation got a little bit more complex. The boss comes over,

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checks our code and says, wait a wait, wait, wait,

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there's actually more price tiers than that. In fact,

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if you're less than 12 years old, you pay $5.

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If you're between 12 and 18 you pay $7 and if you're over 18 then you pay the

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full adult price,

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which is $12. Now there are three possibilities,

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so how do we represent this in our if statement? Well,

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we could use something called the elif. Instead of having a simple if/else statement

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where there's only one condition. If it's true, do this.

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Otherwise do that.

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You can add as many elif conditions as you want. So we can check for condition

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1. If that's true, then do a, but if that's not true,

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then we can continue and check for condition 2.

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If condition 2 is true, well then we can do B. And finally,

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if none of those conditions were true, we can do this final thing.

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Our flow chart now looks something like this and this is the logic that we're

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trying to program. Once we're inside this nested if statement,

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we're going to check if the age is under 12 in which case they should pay $5. If

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they're between 12 and 18 then they should pay $7 and finally if they're over 18

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then they should pay $12. Now the first thing I'm going to check is if the age is

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less than 12. Under this condition,

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they should pay $5. So let's change that to five.

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Now the next condition should be created using an elif,

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which stands for else if. So it means,

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if this is not true else if, can you check if this is true?

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Well in that case then we should do this. For example,

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if the age is not less than 12, so they're over 12, then are they under 18? Well then

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this condition basically catches everybody who's between 12 and 18. And finally,

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if they're not less than or equal to 18 and they're not less than 12 then that's

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everybody else who is over 18.

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Now remember that we can use as many elif conditions between the if and else as we

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like.

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So I could add another elif that checks whether if the age is less than,

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say, 22 well in this case, do something else.

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And then I can keep going with these elifs until I'm done with all my

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conditions. So have a play around with this code,

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write it yourself, see if it makes sense,

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and then mess around with the elifs so that it does what you expect it to do.

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And then once you're ready,

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go ahead and head over to the next lesson where I've got a coding challenge for

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you.