7.6. while Loops

There is another Python keyword that can also be used for iteration—while. Unlike a for loop, which defines a loop variable and a specific starting and ending point, a while loop uses a single condition to determine whether or not to continue running.

This condition (a boolean expression) controls the iteration. The body of a while loop repeats as long as the expression evaluates to True.

7.6.1. while Loop Syntax

The general syntax of a while loop looks like this:

while boolean expression:
   loop body

A while loop continues to repeat as long as the boolean expression evaluates to True. This condition usually includes a value or variable that is updated within the loop. Eventually, the condition becomes False, and the loop stops.

Just like with for loops, the body of a while loop must be indented, and it may contain any number of statements.

Example

The following loop repeats until total < 1000 returns False:

total = 0
increase_by = 14

while total < 1000:
   total += increase_by
   print(total)

print("Not in the loop!")

7.6.2. Control Flow

We can visualize the program flow of a while loop as follows.

Flow of execution of a while loop

1. Python evaluates the condition, which returns a value of True or False.

2. If the condition is False, exit the while loop and continue the program at the next statement after the loop body.

3. If the condition is True, run the loop body and then go back to step 1.

7.6.3. for Loops Rewritten as while Loops

We can use while to create any type of loop we want, including anything we previously did with a for loop. For example, consider our first for loop that printed the numbers 0 - 20:

for num in range(21):
   print(num)

This can be rewritten as a while loop:

num = 0

while num < 21:
   print(num)
   num += 1

Instead of using the range function to produce the values for num, we need to produce them ourselves in a while loop. To do this, before starting the loop. In this example, we create the variable num and assign it a value of 0 in line 1. Every iteration, line 5 increases num by 1. Eventually, num increases enough to make the condition num < 21 false, and the loop ends.

num plays the same role as the loop variable in the for loop, but we need to manage its value ourselves. The for and while loops in the examples above do exactly the same thing, but they solve the task slightly differently.

If we imagine giving instructions to someone, the for and while approaches might sound something like:

1. For: Repeat this task for each number 0 - 20.

2. While: Repeat this task as long as the number is less than 21.

7.6.3.1. Try It!

Let’s convert this for loop into the equivalent while.

letters = 'abcdefghijklmnopqrstuvwxyz'
for_string = ''
num_letters = 8

for index in range(num_letters):
   for_string += letters[index]

print(for_string)  # Displays 'abcdefgh'

Do the following in the editor below:

1. On line 5, define a counter variable called index. Assign it a value of 0.

2. Next, code the while statement. Here are two possibilities:

   • while index < num_letters:

   • while len(while_string) < num_letters:

3. Inside the loop, update while_string. Look at line 6 in the for loop for a hint about how to do this.

4. Also inside the loop, increase the value of index by 1.

5. Run the program to verify that it prints abcdefgh when num_letters = 8.

# Follow the instructions in the text to rewrite a for loop a while loop.
letters = 'abcdefghijklmnopqrstuvwxyz'
while_string = ''
num_letters = 8

print(while_string)

repl.it

7.6.4. Input Validation

while and for loops serve the same purpose—to repeat a block of code multiple times. However, while loops are more flexible than for loops. If you don’t know how many times the loop needs to run, a while loop is the best fit. Let’s look at one example of this.

Try It!

This program is an example of input validation, which checks what the user enters to make sure it won’t cause errors down the line.

In this case, the computer prompts the user to enter a positive number. If they enter 0 or any negative number, then they see an error message and are prompted again within the body of the loop. If the user keeps entering invalid numbers, the loop continues to iterate. As soon as the user chooses a valid number, the loop ends.

num_choice = 0

while num_choice <= 0:
   num_choice = int(input('Choose a positive number: '))
   if num_choice <= 0:
      print('Invalid number')

repl.it

This example shows the additional flexibility provided by while loops. for loops iterate a specific number of times, but in this case we have no way of knowing how many times we need to prompt the user for a number. By setting a single condition (num_choice <= 0) we can keep the while loop going until the condition returns False.

7.6.5. Which Loop To Use?

Each time we write a for statement, we tell Python exactly how many times the loop body must repeat. for char in "Hello" repeats once for each letter in the string (5 times). Similarly, for value in range(10) repeats 10 times, with value assigned the numbers 0 - 9.

Even when we use variables in range(start, stop, step), these variables store specific values. Python knows exactly how many times to repeat the loop body, and this is called definite iteration. The starting and ending points of are set inside the for statement.

When we write a while loop, we give Python a condition to evaluate. When the condition returns False, the loop stops. We do NOT need to know how many times to repeat the loop. It will keep going as long as necessary.

As we saw in the input validation example, we cannot not know ahead of time how many tries the user will need. Since a for loop repeats a specific number of times, it will not work for this case. Instead, a while loop works better. Whether the user needs 1, 2, 10 (or more) tries, the loop operates only as long as it has to.

Indefinite iteration refers to the case where we do not know how many times a loop needs to repeat.

So which type of loop should we use in our code? for loops do better when iterating over a collection or a fixed number of times. while loops get the job done when we do not know how long it will take to meet a given condition.

Here are some points of comparison between the two types of loops.

7.6.5.1. for Pros and Cons

1. Easier to set up than while loops.

2. Must have a definite start and end point, and these must be declared in the for statement.

3. Can loop through strings and collections without using an index value (e.g. for char in 'hello':).

4. Automatically updates the loop variable.

5. It is hard to accidentally create an infinite Python for loop.

6. Can be used in place of some while loops, but not all.

7. Do not work for input validation.

7.6.5.2. while Pros and Cons

1. More flexible than for loops.

2. ANY for loop can be re-written as a while loop.

3. Do not need to know beforehand how many times the loop needs to run.

4. Can be used for input validation.

5. while loops require more work to build.

6. Making an infinite while loop is easy.

7.6.6. Check Your Understanding

Question

Which of the following will cause this while loop to end? Select ALL that apply.

username = ''

while len(username) <= 5:
   username = input("Enter a username: ")

1. “Bob3”

2. “Anaconda”

3. “Willmore Crane Hastings III”

4. “Sally”

5. “LaunchCode”

Question

If you know the maximum number of times you need a loop to repeat, either for or while will work, but which option is the better choice?

1. while loop

2. for loop

Question

You are asked to program a robot to move tennis balls from one box (Box #1) to another (Box #2), one-by-one. The robot should continue moving balls until Box #1 is empty. However, balls may be added to the box after the robot begins its work.

Which type of loop should you use to write the program?

1. while loop

2. for loop

Question

You are asked to write a program similar to the one above, but a user gives the robot a specific number of balls to move from Box #1 to Box #2. (You can assume there will always be more than enough balls in the first box).

Which type of loop should you use to write the program?

1. while loop

2. for loop