Lesson video

Hello, I'm Mr. Hogan.

We are going to be learning about how to programme using Python today.

I'm really pleased you were able to join me for this lesson.

Shall we make a start? This is the fifth lesson from the Introduction to Python programming unit, and it's called, "Iteration using while loops." So the outcome for today's lesson.

I can use iteration to repeat sequences of code in a programme and use a flag to stop a loop from running.

I hope you enjoy it.

It may be really hard along the way and a bit challenging, however, keep with it and I'm sure we'll get through it together.

So we have three key words in today's lesson.

We have iteration.

This is the process of repeating a sequence of instructions within a programme loop.

We have condition.

This is an expression that evaluates to True or False.

And we have flag.

So this is a signal that is used to let a programme know whether a condition has been met or not.

We have two parts to today's lesson.

The first one is, use a while loop to repeat instructions.

And the second part is all about flags, so use flags as indicators for an event.

Let's have a look at our first part of the lesson.

So we're gonna use while loops to repeat instructions.

This programme, which you've seen previously, greets the user by name.

So line one prints, "What is your name?" Line two has the variable name, and the input from the user will be then stored in the variable name.

And then line three, we have print, "Hello," and the variable name.

So if I entered Mr. Hogan, it will say on line three, "Hello Mr. Hogan." But Alex is asking "What if I want the code to repeat until a specific name, like Alex, is entered?" Iteration is the process of repeating a sequence of instructions a number of times.

In a programme, a loop statement can be used to repeat a sequence of instructions.

Condition-controlled iteration is when a set of instructions is repeated based on whether a condition evaluates to True or False.

So let's have a look at condition-controlled iteration.

It's implemented using a while loop.

A while loop will repeat actions while a condition is met.

So you can see on the flow chart on the right-hand side, this loop, in effect, that's taking place after the condition.

So if the condition is True, it will loop around and continually check if the condition is met.

Let's do a quick check.

Which of the following is used to repeat a section of code until a specific condition is no longer True? Is it A, if, B, while, or C, print? Remember, you can pause the video at any time to have a think about it or replay any of the video to help you.

Let's have a look at the answer.

It's B, while.

Can you predict what the outcome of running this programme will be? Take a good look at the code.

So the first two lines we've seen before, and then line three is the first condition, with the while at the start of the code on line three.

And so therefore if it's True, it will print, "Try again, I'm looking for Sam." And if it's False, "Hello Sam." Let's have a closer look.

So it's these statements on lines four and five which will be repeated.

This condition on line three is checked at the beginning of each loop.

So this programme will keep asking the user for their name until the name Sam is entered, at which point, it will display, "Hello Sam" on line six.

Try the code to test it out and see how it works.

Izzy is saying something.

"Imagine trying to unlock your phone with a passcode.

You keep entering the code until you get it right." Alex is adding, "The phone's software uses a while loop.

It repeats the 'enter passcode' action while the code is incorrect." Let's have a quick check.

Take a look at this code.

What difference will it make if line five is removed when the first name entered is not Sam? Is it A, it will make no difference? B, the value for name is never modified, so an error will occur? Or is it C, the value for name is not modified, so the programme will never stop? Take a look at the code, have a think, and remember you can pause the video at any time.

The answer is C.

The value for name is not modified so the programme will never stop.

Well done if you got it right, and if you didn't, don't worry.

We can continue the lesson and you can always repeat parts of the video.

Let's have a quick practise.

Look at the Python programme.

Predict the program's output when the user enters the following names in sequence.

Andeep, Laura, Sam.

So take a look at the code.

Very similar to what we've seen before.

Number two, explain how the while loop controls the flow of the programme.

In your answer, explain, what condition is used at the start of the loop? What happens when the condition is True? What happens when the condition is False? Three, write a programme to simulate unlocking a phone with a passcode.

The correct passcode is 1234.

Write code that repeatedly asks the user to enter a passcode until they get the passcode correct.

You can use a while loop to keep asking for the passcode until the user enters the correct passcode.

If the passcode is correct, print, "Phone unlocked!" and end the loop.

If the passcode is incorrect, print, "Incorrect passcode.

Try again" and make the loop run again.

Remember, you can pause the video at any point to do these practise tasks, and you can repeat the video as well.

So let's have a look at the answer for one.

We asked you to look at the programme and predict the program's output when the user enters the following names in sequence, Andeep, Laura, and then Sam.

So on the right-hand side we've got a table with output, input, output.

So the programme on line one asks, "What's your name?" The input we considered first was Andeep.

So therefore, the condition evaluates to True, and the output is run under that condition, which is, "Try again, I'm looking for Sam." So again, when we're considering Laura, "What is your name?" The input is Laura.

This still evaluates to True, so therefore the output would be the same as the previous one, which is, "Try again, I'm looking for Sam." So the final one to consider is Sam.

So the input will be Sam.

Now, when we look at this condition, it evaluates to False because name is Sam.

So therefore, on line six, we will run the print function, "Hello Sam." Number two, explain how while loops control the flow of programme.

And we asked you to consider in your answer, what condition is used at the start of the loop? What happens when the condition is True? And what happens when the condition is False? The condition checks that the name entered is not equal to Sam.

If the condition is True, the name entered is not Sam, the code inside the loop runs.

So this prints, "Try again, I'm looking for Sam" and prompts the user to input the name again.

If the condition is False, the loop ends and the programme prints, "Hello Sam." I hope you got this right.

If not, don't worry.

You can repeat parts of the video to help you.

Write a programme to simulate unlocking a phone with a passcode.

The correct passcode is 1234.

So here's the code, and we can see on line one, we've got print function with, "What is the passcode?" And then the passcode variable.

This will store whatever the input is.

And we've got the while loop on line three.

So while passcode is not equals to 1234, so if it evaluates to True, print, "Try again." And then passcode equals input.

If it's False, on line six, print, "Phone unlocked!" And you can see the solution for this on the following link.

Excellent.

We are halfway through the lesson.

We're gonna move on to the next part, which is all about using flags to indicate for an event.

Flags are used to communicate different conditions.

For example, in car races, flags are used by marshals to indicate messages to the driver.

A green flag, start or resume the race.

A yellow flag indicates to slow down, there's a hazard on the track.

A red flag is, stop the race immediately due to a dangerous situation.

And a chequered flag means the race is finished.

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

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

This code will print the numbers 1, 2, 3, 4, and continue.

It will not stop.

Izzy's asking, "But what if I do want the programme to stop?" A flag is a special variable that acts like a signal or a switch.

Boolean flags only have two values, True or False.

Aisha's making a comment.

"You can change the status of a flag to make a block of code stop." Take a look at this programme.

It contains a flag called keep_counting that is switched to False when the user says they do not want to keep counting.

This stops the loop from running forever.

Let's have a quick check 'cause it can be really hard to understand this concept of a flag.

What are the two possible states a flag can have? Is it A, On and Off? B, True and False? Or C, Yes and No? Have a little think about your answer and remember you can pause the video anytime.

The answer is True and False.

So only two states that a Boolean flag can have, True or False.

This programme asks a user to guess a lucky number.

It then displays a message depending on whether the guess is correct or not.

So we can see that lines one and two, three that sets a variable lucky to 13 and then asks the user to "Guess my number." So the whole point is trying to guess that the lucky number's 13.

On line five, we've got an if with a condition, guess is equal to lucky.

So if it's True, print, "Amazing, you guessed it" else, so if it's False, print, "Sorry, it's not" what's stored in the guess variable.

Print, "Nice playing with you" if that is False.

This only allows the user to guess the lucky number once.

To improve the programme, you could do the following.

Create a flag variable called guessed.

Initialise guessed to False.

A value of False for guessed will mean that the user has not yet guessed the lucky number and the programme will keep asking the user to guess the lucky number.

A value of True for guessed will mean that the user has guessed the lucky number correctly and the programme will stop.

Let's have a look at this code.

The guessed flag remains set to False if the user guesses the lucky number incorrectly.

We can see this on line three.

The guessed flag is changed to True if the user guesses the lucky number correctly.

So on line eight, we've got, guessed = True.

"Why are flags so important?" Aisha's asking.

Well, flags help control the flow of a programme.

In the guess the lucky number game, the flag controls when the guessing stops.

Let's have a quick check.

In a game, a player collects coins to earn extra lives.

Which of the following situations would most likely cause a flag called Extra_Life to be set to True? Is it A, the player pauses the game? B, the player collects a certain number of coins? Or C, the player loses all their current lives? So have a think about it.

Try to think about the situation of a game.

A player collects coins to earn extra lives.

You may have played a game similar to that.

Remember, you can pause the video at any time.

The answer is B, the player collects a certain number of coins.

So yeah, a player collects coins to earn extra lives.

Izzy's making a comment, "My code doesn't work.

I have checked the F in False is capitalised and the condition uses a double equals symbol to check if the value of guessed is False." Aisha's also saying, "Have you remembered the colon after the condition in while? Have you used indentation to indicate which statements belong to the while block?" So these are the sort of things that you can start to check when you are doing your code when we move on to the practise.

The practise, then.

So firstly, open the starter code.

Then explain the purpose of the distance_travelled variable.

How is it updated within the while loop? So take a look at the code.

Remember, you can pause the video at any time.

Now describe what the while loop does in this code.

When does the loop start? When does it end? Remember, you can pause the video at any time or repeat parts of the video if you want to go back and to look at things again.

Four, modify the code to check if the distance_travelled has reached or exceeded 250 steps.

And lastly, number five, modify the code so that if the treasure is found, the treasure_found flag is set to True to stop the loop.

Remember, you can pause the video at any time or repeat any parts.

So let's have a look at some answers.

So two, we asked you to explain the distance_travelled variable.

So it keeps track of how far the adventurers have travelled in their search.

It starts at zero and increases by the amount of steps the user enters during each iteration of the loop.

Three was all about the while loop.

So the while loop simulates the ongoing search.

It starts when treasure_found is False, meaning the treasure hasn't been found yet, and it ends when treasure_found becomes True, meaning the treasure has been found.

Number four, we asked you to modify the code to check if the distance travelled has reached or exceeded 250 steps.

And then five, we modified the code so that if the treasure is found, the treasure_found flag is set to True to stop the loop.

So if we take a look at this code, we can see that on line nine we have, if distance_travelled is greater than or equal to 250.

So that's the answer to four.

And then on line 10, we've changed treasure_found to True.

So if line nine evaluates to True, we change the treasure_found to True and then it will stop the loop.

Well done.

You've reached the end of this lesson all about iteration while using while loops.

Well done.

So in summary, iteration in Python is implemented through while loops, which allow you to repeat actions as long as a specific condition remains true.

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

A flag can be used to stop a while loop from running.

Well done.

You've reached the end of the lesson.

I hope you've really, really gained some important knowledge about using iteration and using while loops to do that.

Well done.