Iteration: WHILE loops in Python

Computer Science · Python

Be the program: find Sam

The user types Andeep, then Laura, then Sam. Step through and watch the condition get checked before anything inside the loop runs.

Before you press Next: how many times do you think "Try again, I'm looking for Sam" will be printed?

 1  name = input("Enter a name: ")
 2  while name != "Sam":
 3      print("Try again, I'm looking for Sam")
 4      name = input("Enter a name: ")
 5  print("Hello Sam")

Predict, then check

In the guess-the-lucky-number game, the flag guessed starts as False and the loop keeps asking while guessed is False.

The user guesses wrongly twice, then guesses the lucky number correctly. How many times does the loop ask for a guess?

Computer Science · Python

While loop or for loop?

For each task, decide which loop fits, then read why.

6 of 6 still to sort.

What you need to know

• Repeating instructions in a program is called iteration, and a loop statement does the repeating.
• A condition is an expression that works out to True or False.
• In Python, a while loop repeats its instructions for as long as the condition stays True.
• The condition is checked at the start of every pass, before the loop body runs.
• Have a goSay the condition comes out False the very first time it is checked. How many times does the loop body run?

  Zero times.

  The check comes before the body. A False result ends the loop straight away and the program carries on after it.

• If it is True, the body runs and the condition gets checked again.
• If it is False, the loop ends and the program carries on with the statement after the loop.
• Something inside the loop has to be able to change the thing the condition checks, or the loop never stops.
• A Boolean flag is a variable with just two values, True or False. Changing it can stop a while loop.
• Have a goA loop is controlled by the flag keep_counting. The user says they do not want to keep counting. What should the program do to the flag?

  Switch keep_counting to False.

  The loop's condition is the flag, so changing the flag to False makes the condition False and the loop ends.

• A loop that starts with while True: runs forever on its own, because True is a fixed value, not a variable.
• Use a while loop when the number of repeats can change. Use a for loop when you know the exact number.
• Four things to check in any while loop: capital T and F, == to test equal, a colon, and indentation.
• Have a goSpot the slip: while keep_counting == true: — which word is wrong?

  true. It needs a capital T: True.

  Python's Boolean values are written True and False, with a capital first letter, so lowercase true is not the value.

The big picture

A while loop repeats its instructions for as long as its condition is True. The condition is checked at the start of every pass, and the loop ends when it is False. So something inside the loop must be able to make the condition False, or the loop never stops. A Boolean flag variable such as keep_counting is one way to switch a loop off, but while True: is a fixed value that runs forever on its own.

⚠ Watch out

Thinking while True: is a flag the program can switch off. True is a fixed value, so on its own it runs forever. It is a flag VARIABLE such as keep_counting that can be set to False to stop a loop.

Memory hook

Question first, body second. A while loop asks "still True?" before every pass, and only a False lets you out.

Check yourself

A friend's while loop never stops. Without looking back, say what you would check first, and what they would need to add inside the loop.