Lesson video

Hello, my name is Mrs. Holbrook, and welcome to Computing.

I'm so pleased that you've decided to join me for the lesson today.

We are going to be exploring how we can iterate through a data structure to repeatedly add items to it or compare items held in the data structure.

Welcome to today's lesson from the unit Python Programming With Sequences of Data.

This lesson is called Iterating Through Data Structures.

And by the end of today's lesson, you'll be able to use iteration to repeatedly add items to a list and to check the contents of a string.

It would be useful if you had access to a device that you can create and amend code during today's lesson.

Shall we make a start? We will be exploring these keywords throughout today's lesson.

Iteration, iteration.

The process of repeating a sequence of instructions within a programme loop.

String, string.

A type of data that is a collection of characters.

Mutable, mutable.

Able to be changed.

Immutable, immutable.

Not able to be changed.

Today's lesson is broken into three sections.

We'll start by using iteration to repeatedly append items to a list.

We'll then go on to describe how strings are different from lists, and then we'll finish by iterating through a string to compare characters.

Let's make a start using iteration to repeatedly append items to a list.

This programme takes the name of an instrument and adds it to a list called band.

So you can see, on line 1, I've got an introduction that's printed out to the user.

On line 2, I'm initiating my list called band, but I'm not putting anything in it at this point.

So the square brackets are empty.

On line 3, I'm printing an instruction to the user to pick an instrument.

And on line 4, I'm taking in that input from the user and I'm storing it in the variable instrument.

On line 5, I'm then appending that instrument that the user has just typed in to my list called band.

And then on line 6, I'm just printing out the contents of the list.

This highlighted action, or lines of code, needs to be performed repeatedly in order to add all of the instruments required to form a band.

So you can see here, we've amended the code, and we've got some repeated lines of code.

Sam says quite rightly, "This doesn't seem like the most efficient way to do this!" What could you do to repeatedly perform actions in this programme? Maybe pause the video and have a think.

Ah, Jun's correct.

He says you can use iteration.

Sam says, "Yes, a while loop repeats an action until a condition is met." Time to check your understanding.

Which programming construct is used to repeatedly perform actions in a programme? Is it A, selection, B, sequence, or C, iteration? Pause the video whilst you have a think.

Did you select C? Well done.

Iteration is the programming construct which is used to repeatedly perform actions in a programme.

The programme has now been modified to use iteration.

So we can see here, on line 3 we've added a while loop, and we're using the len list method to check the length of the list.

The highlighted lines of code are repeated as long as the length of the list has not reached 3.

So let's have a look at some sample output.

The first instruction, Let's form a band, is printed, and then Pick an instrument is also printed.

The user responds by typing in Drum.

The loop continues, 'cause the list has not reached 3.

So it says Pick an instrument again.

So the user puts Guitar, it repeats it again, so Pick an instrument, and the user puts Piano, and then the loops stops because the length of the list has reached 3.

So the list now contains Drum, Guitar, Piano.

The band has specified that there must be a guitar player.

So you can see the condition in the while loop has been amended.

The not in operator is used to check that the item Guitar is not in the list.

Again, the lines of code highlighted are repeated as long as the item Guitar is not already in the list.

Let's have a look at some sample output.

So we print the instruction, Let's form a band, and then pick an instrument.

The user puts Drum, pick an instrument, Violin, pick an instrument.

None of these are Guitar yet, so it's still running.

Piano.

Pick an instrument, Guitar, and then the loop stops, because we now have Guitar in our list.

So at the end of this programme, the list contains the items Drum, Violin, Piano, Guitar.

Time to check your understanding.

A while loop will run a set number of times.

Is that true or false? Pause the video whilst you have a think.

That's right, it's false.

A while loop performs condition-controlled iteration, which means that it will run until a condition is met.

Okay, we are moving on to our first set of tasks for today's lesson, and you're doing a fantastic job.

So well done.

I'd like you to start by opening the starter programme at oak.

link/city-hopping.

I'd like you to add the lines of code below so that your programme randomly selects five cities from the cities lists and appends them to the trip list.

The lines of code you've been provided are not currently in the correct order.

I'd then like you to run the programme a few times to check that it randomly selects five cities.

Pause the video here whilst you complete the activity.

For part four, you may have noticed when you ran your programme a few times that the list of five cities sometimes contained a repeated city.

Amend your programme so that no duplicate cities are added to the trip list.

As a hint, you're going to need to use the list.

remove method.

For part five, you want to visit London.

Amend your programme so that rather than limiting the trip to five cities, the process of random city selection continues until London is selected and added to the trip list.

Pause your video here whilst you complete the activity.

How did you get on? Did you manage to create your code? Great work.

If you want to look at a full solution, you can go to oak.

link/city-hopping-solution.

So you can see, on line 8, I have a while loop, and it uses the len function to check that the length of the list trip is less than 5.

Whilst it is less than 5, it's going to assign city to an equal of choice from cities and then it's going to append that city to the trip list.

At the end, on line 11, the contents of the list trip are printed out.

For part four, you are asked to ensure that no cities were repeated twice in the list.

To do this, you needed to add this line on line 11, cities.

remove(city).

So once it had randomly picked the city from the list, it then removes it so that it can't be picked again.

For part five, you were asked to alter the condition so that London was always part of the trip list.

So the while loop, or the while condition, has been changed to say while "London" not in trip.

So this will keep adding cities to the trip until London is added.

Remember, if you need to go back and make any corrections to your code, you can pause the video now.

We're now moving on to the second part of today's lesson, where we are going to describe how strings are different from lists.

A string is a data type.

A string is a collection of characters that can include spaces, punctuation, numeric digits, and any other symbols, such as maybe the dollar or at symbol.

In Python, we represent a string by placing speech marks around the characters.

So here's an example.

name = "Laura", or password = "Password1%".

Note, if a string contains numeric digits, these will be handled as text characters, not numbers.

Time to check your understanding.

A string cannot hold numerical values.

Is that true or false? Pause the video whilst you have a think.

That's right, it's false.

A string can hold numerical values, but remember, these will be handled as text characters and not numbers.

Some Python functions can be used on both strings and lists.

Let's have a look at the len function.

So in a list, the len function returns the length, or number of items, of a list.

So here I've got the numbers 16, 76, and 11 stored in a list and then I've assigned the variable length to the len of numbers.

What do you think this might return? In a string, the len function returns the length, or number of characters, of a string.

So here, I've got the string Python, which is assigned to language, and I'm using length is equal to len(language).

Same syntax, same use.

What do you think these both might return? So the list us 3, and the length of the string is 6.

Now let's have a look at the in operator.

In a list, the in operator checks if a value is equal to any item in a list.

Expressions formed will evaluate to either true or false.

So again, I've got my list of numbers, 16, 76, and 11, and I've got found = 32 in numbers.

In a string, the in operator checks if a string is contained within another string.

Again, expressions formed will evaluate to either true or false.

So here I've got language = "Python" again, and then I've got contains = "ty" in language and contains = "on" in language.

What do you think these bits of code will return? Maybe pause the video and have a quick think.

So for the list, it'll return false, because the number 32 isn't in the list, For the contains ty, it'll return false.

Although the characters t and y are in the words Python, they're not next to each other, so they're not a string contained in the string Python.

on will return as true though, because those letters are a string within the string Python.

What about indexing? In a list, an item can be accessed through its index using the syntax list[index] in square brackets.

So here we've got the example of our numbers again, and then items = numbers[1] numbers[3] is equal to 32.

In a string, a character in a string can be accessed through its index, again using the same format, but this time, we put the name of the string and then the index in square brackets.

So again, I've got my string called language, which contains Python, and then I've got character = language[1].

What do you think these lines of code may return? Maybe pause the video again and have a think.

So item = numbers[1] will return 76, because that's the number held in position 1 in the list.

Remember, Python indexing starts at 0.

For the string, y will be printed, because y is held in position 1 of the string.

Again, the indexing still starts at 0.

Sam says, "So, I could update the characters in a string using the index value?" Oh, I'm not so sure about this, Sam.

It looks like this has thrown an error.

What do you think that error might be? You cannot assign a new value to an individual character in a string.

Lists are mutable, which means that their contents can be modified without having to create a new list.

Strings are immutable, so elements cannot be changed, added or removed.

If you want to update a string, you need to reassign the entire value held by the variable, which will overwrite the previous value.

Time to check your understanding.

Which code would you use to check how long the string "Oak National Academy" is? Is it A, len(string), B, in, or C, string[Index]? Pause the video whilst you have a think.

Did you select A? Well done.

len(string) will check how long the string "Oak National Academy" is and return a value.

Jun has a good question.

"Why am I getting the answer 20 and not 18?" Do you know why that might be? len(string) will return 20 because it counts the spaces too.

Okay, we're now moving on to our second set of tasks for today's lesson, and you're doing a fantastic job so far, so well done.

I'd like you to describe how strings are different from lists using the following terms, immutable and mutable.

Pause the video whilst you complete the activity.

How did you get on? Let's have a look at a sample answer together.

"Although lots of the same operations can be performed on strings and lists, they do behave differently.

A list is mutable, which means that values held in a list can be updated.

A string is immutable, which means that individual characters cannot be updated, replaced, or added." Okay, we are now moving on to the final part of today's lesson, where we're going to iterate through a string to compare characters.

Sam says, "How can I compare characters in a string?" Jun says, "Remember, you can use relational operators to compare values." Well remembered, Jun.

Some common comparison operators include the double equal sign, which means equal to, so to check that two values are the same, the exclamation and equals, which means not equal to, and not in, which checks if the value is held in the string.

Let's have a look at some examples.

Using the strings, name1 is equal to Laura and name2 is equal to Jun.

The first one, name1[0] == name2[0] will check that the first character of name1 is equal to the first character of name2.

Do you think this will return as true or false? The second one, len(name1) != len(name2) will check that the length of name1 is not equal to the length of name2.

Will this return true or false? Finally, the third one, name1[0] not in name2.

This will check that the first character of name1 does not appear anywhere in name2.

Will this return true or false? So the first one will return false.

That's because the first character in Laura is L, and the first character in Jun is J.

So they're not the same.

The second one will return true.

Laura contains five characters and Jun contains three.

So they're not the same.

And the last one will also return true, because the first character in name1, which is L, does not appear anywhere in the name Jun.

Time to check your understanding.

What code would you use if you wanted to check the value of the first letter held in the string language = "Python"? Is it A, B, or C? Pause the video whilst you have a think.

Did you select C? Well done.

Remember, in Python, indexing is zero-based, so position 0 will be the first character in the string.

What code would you use to check that the first letter of the string name1 is not the same as the first letter of the string name2? A, B, or C? Pause the video whilst you have a think.

That's right, it's C.

We're checking that it's not equal.

So we need the exclamation and the equal operator.

In programming, flags are used to tell a programme whether a condition has been met or not.

In this programme, while True is used as a flag and the programme will run the code forever as long as the flag is not changed.

So this is just gonna count up in increments of one.

Okay, we're moving on to the final set of tasks for today's lesson, and you've done a great job so far, so well done.

A game has been developed where a capital city is randomly selected from a list and then the user is asked to guess the capital city.

For part one, open the starter programme, oak.

link/guess-the-capital.

For part two, run the programme a few times to see how it works.

For part three, the programme currently only allows the user to guess the capital city once.

Add a while loop to line 10 that will run the code while the flag done is equal to False.

Hint, you're going to need to alter the indentation of the code within your while loop.

Pause the video whilst you have a go at the activity.

For part four, amend the code to give the user some help.

So for A, add an elif statement so that the programme reveals the first letter of the city to the user if the city's first letter is different from the first letter of the user's guess.

For B, add an elif statement so that the programme reveals the length, or number of characters, of city to the user if the city's length is different from the length of the user's guess.

And finally, for part C, add an elif statement so that the programme reveals the second letter of city to the user if the city's second letter is not contained anywhere in the user's guess.

So here's some sample output.

We ask the user to guess the capital and they guess Athens.

Now, the first letter of their guess isn't equal to the first letter of the answer, so we're going to say the first letter is S, and then ask 'em to guess the capital again.

So they're gonna guess Sofia.

Now, their guess isn't the same number of letters as the answer, so we're going to give them the clue, it has nine letters, and ask 'em to guess the capital again.

This time they guessed Stockholm, and we return that they've got it.

They didn't need to use the last elif statement here for part C, because they got it within two guesses.

Pause the video whilst you complete the activity.

How did you get on? Did you manage to complete your programme? Great work.

Well done.

Here's the solution code.

It's a little bit small on my screen, so if you want to open the full solution, you can do that at oak.

link/guess-the-capital-solution.

For part three, the programme currently only allows the user to guess the capital city once.

You were asked to add a while loop to line 10 that will run while the flag done is equal to False.

So on line 10, we've got the line of code while done == False.

For part 4a, you were asked to add an elif statement so that the programme reveals the first letter of the city to the user if the city's first letter is different from the first letter of the user's guess.

So we've got the line elif city[0] != guess[0].

And then we have a print statement which says, "The first letter is," and we return the first letter from the answer.

For 4b, you were asked to add an elif statement so that the programme reveals the length, or number of characters, of the city to the user if the city's length is different from the length of the user's guess.

So here we have an elif statement that says elif len(guess), the length of guess, is not equal to len(city).

And again, a print statement to give them the clue.

For 4c, you were asked to add an elif statement so that the programme reveals the second letter of city to the user if the city's second letter is not contained in the user's guess.

So we have the statement elif city[1] not in guess, and then we have a print statement to give them the clue.

Remember, if you need to make any changes to your code or you want to spend some time having a look at the solution, you can pause your video now.

Okay, we've come to the end of today's lesson, and you've done a fantastic job, so well done.

Let's summarise what we've learned today.

Condition-controlled iteration is implemented using while loops in Python.

Iteration can be used to repeatedly append items to a list.

A string is like a list in structure, but is immutable, so elements cannot be changed, added, or removed.

I hope you've enjoyed today's lesson, and I hope you'll join me again soon.

Bye.