Lesson video

Hello, I'm Mr. Hogan.

We're going to be learning about Python today.

I'm really pleased you are able to join me.

Should we make a start? This is the fourth lesson from the Introduction to Python Programming unit, and it's called Selection with Multiple Paths.

Today the outcome is I can write selection statements to provide three or more paths to follow within a programme.

There are three key words to this lesson, condition, which means an expression that evaluates to true or false, multi-path selection, which is selection when there is more than one path for a programme to follow, and nested, which is a block of code where a loop is contained within another loop.

These may seem new to you, but throughout this lesson, you'll get to grips with 'em and understand them.

So the lesson outlined today, we're going to be using multiple-path selection to control programme flow, and then move on to create a nested if statement.

So let's have a look at the first point, use multiple-path selection to control programme flow.

This selection checks a condition.

If true, it runs the code within its block, representing one path of the program's flow.

If false, it's skipped, and the programme continues.

So you can see on the right-hand side of the slide we have a flow chart to represent this.

So if the condition is true, represented by the line coming from the right hand side of the decision block, it runs a block of code, and then continues the programme.

If it's false, it skips that block of code, and runs the next code in the programme.

What if our programme needs to follow different paths based on more than one condition? What if there are multiple possible paths to follow? Multiple-path selection allows your programme to handle situations where there are three or more different options to choose from.

So you can see on the right-hand side of the slide where we have a flow chart and two conditions.

If the conditions are true, it runs the block of code.

If it's false, that block of code is skipped and the programme continues.

So let's have a look at a real life example.

A theme park ticket may have three different prices.

Under 12 is the children ticket, over 12, it's an adult ticket, and under six is our free.

Multiple-path selection can be used within a programme to select the correct ticket type.

Multiple-path selection checks conditions one by one and picks the first one that's true, then follows the instructions for that path.

As Alex is saying on this slide, you'll need an if with an elif block when there is more than two paths for your programme to follow.

Let's have a look at this example.

We've got if, elif, and else.

You can use multiple elif blocks.

The else and its block are optional.

Let's have a quick check In a multi-path selection statement, what does the elif keyword do? Is it A, it starts a new block of code regardless of previous conditions, B, it ends a selection statement, C, it introduces a condition to check only if the previous if or elif conditions were false? Take your time to think about the answer.

Remember you can pause the video at any time.

Okay, should we have a look at the answer? It's C.

Well done if you've got that correct.

And if you didn't, don't worry.

I'm sure the rest of the lesson you'll learn loads more about Python programming.

Andeep's asking a question.

I wonder if you can think about the answer.

Do you remember what the relational operators are? So the relational operators are used within conditions to compare values and determine whether the condition is true or false.

And here they are.

So we've got the greater than symbol, the less than symbol, the symbols used for not equal to.

And remember, it is a double equals symbol to say if it's equal to.

And then we've got the less than or equals to symbol.

We're gonna be using these over the next few slides.

So let's have a quick check.

Consider this code.

If the user enters 15, what will the output be? Will it be A, it's hot, B, it's warm, C, it's cool, or D, it's cold? Remember to think about your answer and work through the code starting from the very first line and go all the way down to the end on line 10.

Take your time.

Remember you can pause the video at any point.

It's C, it's cool.

Well done.

So let's work through the code with the input of 15.

So the first if, the condition is seeing if the temperature is greater than 30.

Well, it's false, so it goes to the next elif, which is asking if the temperature is greater than 20.

Well that's false.

So we go down to the next elif, and it's asking if the temperature is greater than 10.

Well, in this case it's true, therefore it prints it's cool.

Let's have a little practise.

Give advice for when the weather is cloudy, rainy, or snowy.

Use this starter file to help you.

So on line one, we're just using the print function to output what is the weather like now.

And then we've got a variable on line two, weather, that stores whatever the user enters.

Remember, you can pause the video at any time and take your time to think about the code that you are going to enter under line two.

So shall we have a look at the answer? So this is the suggested solution for you.

We've got various paths that this programme can take.

We've got various conditions within this programme too.

So we start on line three and the condition is weather == "cloudy".

And we have the advice variable on line four which will store "No sunglasses are needed." if that condition is true.

If it is false, we go down to the elif.

If the condition is weather == "rainy", if it's true, the string stored in advice will change to "You need an umbrella." If it's false, the next elif weather == "snowy", the string in advice will change to "Gloves are needed." And then we've got else on line 10 which has the last advice on line 11.

So the string will change to "Enjoy the day!" On line 12, which will print the string stored in advice.

Well done for getting this far in the lesson.

We've done some really, really hard work about multiple path selection and using that to control flow of programmes.

Well done.

We're going to move to the next partner now which is to create a nested if statement.

When a condition and a block of code is placed within another condition and a block of code, it is a nested if statement.

Any condition and block of code can be nested inside another one.

So here's some examples of nested if statements.

So you can see we've just got one nested if statement on the left because it's within the first if block.

And then we can see the middle example here where we've got if and else, which is nested inside the first if statement.

And then we've got if, elif, and else on the right-hand side, which is all nested within the first if statement.

This programme has nested code from line seven to line 12.

When the condition elif less than 12 is true, this block of code will run.

So here we can see that when the condition elif age < 12 is true, this code is run.

And within this code, there are nested if statements.

If it's false, it goes down to line 13 and runs the next else, which is ticket_price = 35.

Shall we take a look at this code a bit more closely? So we've got our first if and condition on line four.

So the condition is age < three.

So if that evaluates to true, we go to the next indented line of code, which is print ("You get in for free!").

If it's false, we go to line six.

Line six is an elif with the condition age < 12.

So if that is true, we run the indented code below this.

So on line seven, and now we're gonna ask a question and user is going to input the answer.

So the print function on line seven prints, "Is it peak season? (yes/no)" So it's asking a user to identify for the programme if it is peak season, and then we're gonna store the answer as is_peak_season on line eight.

This next line of code is our first nested if statement.

So if is_peak_season == "yes", we're going to run the indented code on the line below, which is ticket_price equals = 25.

So we're gonna change the value in ticket_price to be 25.

If it evaluates to false, we're going to run the code under else, which will change the ticket_price to 20.

So if it is peak season, the ticket price is 25.

If it's not, so evaluates to false, it becomes 20 because it's cheaper if you don't go during peak season.

If all that was false, so if age < 12 on line six evaluates to false, we run the code under else on line 13, which is change the ticket price to 35.

So in effect, you'll be over 12 years old, and the ticket price will cost you 35.

And then on line 16, we print the ticket price, and then the value stored in the ticket_price variable.

That is quite hard to get your head around, but hopefully you're doing really well, and we can carry on with the lesson, or if you want to pause it and go back and replay this part, that's fine too.

I mentioned indentation a bit on the last slide, and that's really important, especially with nested if statements.

indentation shows which code belongs to which if, elif, or else block.

Each if, elif, and else statement is followed by colon, the block of code after the colon should always be indented.

So we can see we've got the indentation here on this example.

The code block inside the if and else statements is indented further to the right.

So we can see on this example where we've got the if statement, we can still see after the colon of the if and the else, the code below that line on line 10 and 12 is indented.

Let's have a quick check.

There is an error in the code that will prevent it from running correctly.

Which line contains the error? Is it A line five, B line seven, or C, line 10? Remember you can pause the video at any time to think about your answer and to look through the code.

Okay, shall we have a look at the answer? It's C, line 10.

Well done for getting that correct.

And it is an indentation error.

So on line nine, we've got if is_peak_season == "yes": so therefore the colon is there, so the line below it should be indented.

So ticket_price = 25 should be indented.

Alex is asking, "Can I add a peak season ticket price for people age 12 and over?" Take a look at that code.

What do you think? Well, let's have a look at what is on line 7 to 12.

Could we replicate this slightly and put it maybe after line 14 and before 16? Yes, we could put it under the else statement as, again, nested ifs.

Let's have a quick check.

What condition would be used on line three as part of this nested if statement to set the ticket price to 35 if it is peak season.

Remember you can pause the video at any time.

Okay, so check your answer.

Hopefully we've got all the options.

It's A, is_peak_season == "yes".

So yes, always a double equal symbol remember for a condition.

So well done if you got that correct.

We're gonna do the final practise of the lesson.

So first of all, we want you to open the starter code file.

We're then going to add code on line 14 to create a new path with a condition that will check if the user is greater than or equals to 12, followed by colon.

Three, we're going to add nested code inside the condition to ask the user if it is peak season.

Remember, this code is already used in the programme so you can copy and paste it.

You don't have to type in the code.

Remember you can pause the video at any time to complete your code, and you can then come back or replay any parts of the video at all.

So take your time and hopefully you'll complete your code.

So moving on to four, add a nested if else statement that checks the response to is_peak_season and displays "Ticket price: £35" if the reply is yes or "Ticket price is £30" if the answer is not yes.

And then five, explain what nested selection means.

Remember, you can pause the video at any time or replay any part to help you.

Let's have a look at the answer.

So for two, I asked you to add code on line 14 to create a new path with a condition that will check if the user is greater than or equals to 12.

So the code is elif age >= 12: The third thing I asked you to do was to add nested code inside the condition to ask the user if it is peak season and remembering we've already used this in the programme before, so you can copy and paste it.

So the code is print("Is it peak season? (yes/no):") and then the line below it will then take an input from the user and store it as a variable called is_peak_season.

I then asked you to add a nested if else statement that checks the response to is_peak_season and displays "Ticket price: £35" if the reply is yes or "Ticket price is £30" if the answer is no.

So we've got the code here.

So we have, if is_peak_season == "yes": ticket_price variable will become 35 or the value within that will become 35, else the ticket_price variable will then change to 30.

The variable will have the value 30 stored in it.

You can see the solution code file on this link.

Then I ask you to explain what nested selection means.

So an example answer is nested selection is when a condition and a block of code is placed within another condition and a block of code.

The nested block of code should be indented correctly.

Indentation is really important.

So hopefully you've included that in your answer.

Well done.

Excellent, we've come to the end of the lesson.

Well done for getting this far.

So in summary, multi-pass selection statements within conditions are used to control the path through a programme when there are more than two options to follow.

Nesting allows you to place one block of code inside another, creating more complex programmes.

When nesting blocks of code, indentation is important.

We done for getting to the end of the lesson.