Computers are often used to automate repetitive tasks. Repeating identical or similar tasks without making errors is something that computers do well and people do poorly. In a computer program, repetition is also called iteration.

Because iteration is so common, Python provides language features to make it easier.

The while Statement

The while statement allows you to repeatedly execute a block of statements so long as a condition is true. It is considered to be a condition-controlled statement.

The simplest form of the while statement is this:

while 
[condition]:
    [do something]

Just like with the if statement, the Boolean expression after while is called the condition. If the condition is true, the body of the while statement executes. Once the body is completed, the condition is re-evaluated. If it is found to still be true, then the body executes again. This repeats until the condition is found to be false. At that point, the while statement stops and the rest of the program continues to run.

More formally, here is the flow of execution for a while statement:

Determine whether the condition is true or false.
If false, exit the while statement and continue execution at the next statement.
If the condition is true, run the body and then go back to step 1.

This type of flow is called a loop because the third step loops back around to the top.

Pretest Loop

The while statement is considered to be a pretest loop – this means it tests the condition before it decided to execute. So if you need to run the body of the while statement at least once, you may need to prime the while statement by setting the condition to be true ahead of time.

Example: Bad Joke

Here is a program which tells a knock knock joke using a while statement:

knock_knock = True

while 
knock_knock:
    print("\nKnock, 
knock!")
    print("  Who's 
there?")
    print("Banana.")
    print("  
Banana who?\n")
    
    again = 
input("Continue the joke? y/n 
")

        if 
again!="y":
            knock_knock 
= False

print("\nKnock, knock!")
print("  Who's 
there?")
print("Orange.")
print("  Orange who?")
print("Orange you 
glad I didn't say 'banana'?")

Output

Knock, knock!
  Who's 
there?
Banana.
  Banana who?

Continue the joke? y/n 
y

Knock, knock!
  Who's 
there?
Banana.
  Banana who?

Continue the joke? y/n 
n

Knock, knock!
  Who's 
there?
Orange.
  Orange who?
Orange you glad I didn't say 
'banana'?

How it Works

We've set a condition control knock_knock equal to True. The while statement checks that knock_knock is true, then performs the actions in the body of the statement. In this case, the user is prompted to enter a value, the value is evaluated using a conditional statement.

Code	Output	Explanation
`knock_knock = True`	None	First, we assign the Boolean value `True` to the variable `knock_knock.`
`while knock_knock:`	None	We begin the `while` statement with the condition of `knock_knock`. This is a shortcut way of writing `knock_knock == True`. We first check this condition. Since `knock_knock` IS True, the condition evaluates as true. We move on to execute the body.
`print("\nKnock, knock!") print(" Who's there?") print("Banana.") print(" Banana who?\n")`	`Knock, knock! Who's there? Banana. Banana who?`	We display the beginning of a bad knock knock joke.
`again = input("Continue the joke? y/n ")`	`Continue the joke? y/n`	We accept input from the user.
`if again!="y": knock_knock = False`	None	We evaluate the input. If the user enters something other than 'y' we set the value of `knock_knock` to `False`. If the user enters 'y' then nothing happens and the `while` loop continues. In our example, we entered 'y' so the `while` loop repeats.
`print("\nKnock, knock!") print(" Who's there?") print("Orange.") print(" Orange who?") print("Orange you glad I didn't say 'banana'?")`	`Knock, knock! Who's there? Orange. Orange who? Orange you glad I didn't say 'banana'?`	After we set the value of `knock_knock` to `False`, the program continues. In this case, we display the punchline.

Example: Countdown

Here is a program which counts down to 0 using a while statement:

n = 5
    
while n > 
0:
    print(n)
    n = n – 
1
    
print('Blastoff!')

Output

5
4
3
2
1
Blastoff!

How it Works

You can almost read the while statement as if it were English. It means, "While n is greater than 0, display the value of n and then decrement n. When you get to 0, display the word Blastoff!"

Code	Output	Explanation
`n = 5`	None	First, we assign the literal constant value `5` to the variable `n.`
`while n > 0:`	None	We begin the `while` statement with the condition of `n > 0`. We first check this condition. Since `n = 5` and `5 > 0`, the condition evaluates as true. We move on to execute the body.
`print(n) n = n – 1`	`5`	We display the value of `n`, then reassign `n` to be one less.
`while n > 0:`	None	We re-evaluate the condition again. `n = 4` and `4 > 0`, so the condition again evaluates as true.
`print(n) n = n – 1`	`4`	We display the value of `n`, then reassign `n` to be one less.
`while n > 0:`	None	We re-evaluate the condition again. `n = 3` and `3 > 0`, so the condition again evaluates as true.
`print(n) n = n – 1`	`3`	We display the value of `n`, then reassign `n` to be one less.
`while n > 0:`	None	We re-evaluate the condition again. `n = 2` and `2 > 0`, so the condition again evaluates as true.
`print(n) n = n – 1`	`2`	We display the value of `n`, then reassign `n` to be one less.
`while n > 0:`	None	We re-evaluate the condition again. `n = 1` and `1 > 0`, so the condition again evaluates as true.
`print(n) n = n – 1`	`1`	We display the value of `n`, then reassign `n` to be one less.
`while n > 0:`	None	We re-evaluate the condition again. `n = 0`, but `0` is not greater than 0, so the condition evaluates as false. The `while` statement is finished and the program moves on.
`print('Blastoff!')`	`Blastoff!`	We display the word Blastoff! The program is finished.

Infinite Loops

The body of the loop should change the value of one or more variables so that the condition becomes false eventually and the loop terminates. Otherwise the loop will repeat forever, which is called an infinite loop. An endless source of amusement for computer scientists is the observation that the directions on shampoo, "Lather, rinse, repeat", are an infinite loop.

In the case of the countdown, we can prove that the loop terminates: if n is zero or negative, the loop never runs. Otherwise, n gets smaller by one each time through the loop, so eventually we have to get to 0.

For some other loops, it is not so easy to tell. For example:

while 
n != 
1:
    
    print(n)
    
    if 
n % 2 == 0:
        # n is 
even
        n = n / 
2
    else:
        # n 
is odd
        n = n*3 + 1

The condition for this loop is n != 1, so the loop will continue until n is 1, which makes the condition false.

Each time through the loop, the program outputs the value of n and then checks whether it is even or odd. If it is even, n is divided by 2. If it is odd, the value of n is replaced with n*3 + 1. For example, if the argument passed to sequence is 3, the resulting values of n are 3, 10, 5, 16, 8, 4, 2, 1.

Since n sometimes increases and sometimes decreases, there is no obvious proof that n will ever reach 1, or that the program terminates. For some particular values of n, we can prove termination. For example, if the starting value is a power of two, n will be even every time through the loop until it reaches 1. The previous example ends with such a sequence, starting with 16.

The hard question is whether we can prove that this program terminates for all positive values of n. So far, no one has been able to prove it or disprove it! (See http://en.wikipedia.org/wiki/Collatz_conjecture .)

else: Alternative Execution

A while statement can have an optional else clause. This code will execute once the condition is no longer true. We can re-write the Countdown program using the else clause like this:

n = 5
      
while n > 
0:
    print(n)
    n = n – 
1
else:
    print('Blastoff!')

We will get the exact same output.

Example: Number Guessing, Part 2

In this program, we are still playing the guessing game, but the advantage is that the user is allowed to keep guessing until he guesses correctly - there is no need to repeatedly run the program for each guess, as we have done in the previous section. This aptly demonstrates the use of the while statement.

number = 23
running = True

while 
running:
    guess = int(input('Enter an integer: 
'))

    if guess == 
number:
        print('You guessed 
it!')
        running = 
False
    elif guess < 
number:
        print('No, you are 
too low!')
    else:
    
    print('No, you are too 
high!')
else:
    print('The while loop is 
over.')

print('Done')

Output:

Enter an integer: 50
No, you are too 
high!

Enter an integer: 22
No, you are too 
low!

Enter an integer: 23
You guessed it!

The 
while loop is over.
Done

How It Works

Code	Output	Explanation
`number = 23 running = True`	None	First, we assign the literal constant value `23` to the variable `number` using the assignment operator (`=`). We also assign the boolean value of `True` to the variable `running`.
`while running: guess = int(input('Enter an integer: ')) if guess == number: print('You guessed it!') running = False elif guess < number: print('No, you are too low!') else: print('No, you are too high!')`	Varies depending on the user’s input	We move the `input` and `if` statements to inside the `while` loop and set the variable `running` to `True` before the while loop. First, we check if the variable `running` is `True` and then proceed to execute the corresponding while-block. After this block is executed, the condition is again checked which in this case is the `running` variable. If it is true, we execute the while-block again, otherwise we continue to execute the optional else-block and then continue to the next statement. Notice that when the correct number is guessed, we change the value of `running` to `False`. This will stop the while loop and continue in our program.
`else: print('The while loop is over.')`	`The while loop is over.`	The `else` block is executed when the `while` loop condition becomes `False` - this may even be the first time that the condition is checked. If there is an `else` clause for a `while` loop, it is always executed unless you break out of the loop with a `break` statement.
`print('Done')`	`Done`	The `while` statement is finished. The program continues and the word `Done` is displayed.

Lesson 3: Iteration

The while Statement

Pretest Loop

Example: Bad Joke

Output

How it Works

Example: Countdown

Output

How it Works

Infinite Loops

else: Alternative Execution

Example: Number Guessing, Part 2

Output:

How It Works