6.3. Infinite loops

An endless source of amusement for programmers is the observation that the directions on shampoo, “Lather, rinse, repeat,” are an infinite loop because there is no iteration variable telling you how many times to execute the loop.

In some loops, like the countdown from last section, we can prove that the loop terminates because we know that the value of n is finite, and we can see that the value of n gets smaller each time through the loop, so eventually we’ll reach 0. Other times a loop is obviously infinite because it has no iteration variable at all.

Sometimes you don’t know it’s time to end a loop until you get half way through the body. In that case you can write an infinite loop on purpose and then use the break statement to jump out of the loop.

This loop is obviously an infinite loop because the logical expression on the while statement is simply the logical constant True:

Activity: CodeLens 6.3.1 (codelens531)

As you can see above, the Code Lens gives you a warning because it runs for over 1000 steps. If you make the mistake of running this code, you will learn quickly how to stop a runaway Python process on your system or find where the power-off button is on your computer. This program will run forever or until your battery runs out because the logical expression at the top of the loop is always true by virtue of the fact that the expression is the constant value True.

While this is a dysfunctional infinite loop, we can still use this pattern to build useful loops as long as we carefully add code to the body of the loop to explicitly exit the loop using break when we have reached the exit condition.

For example, suppose you want to take input from the user until they type done. You could write:

while True:
    line = input('Word: ')
    if line == 'done':
print ('Done!')

The loop condition is True, which is always true, so the loop runs repeatedly until it hits the break statement.

Each time through, it prompts the user with Word:. If the user types done, the break statement exits the loop. Otherwise, the program echoes whatever the user types and goes back to the top of the loop. Here’s a sample run:

Word: hello there
hello there
Word: finished
Word: done

This way of writing while loops is common because you can check the condition anywhere in the loop (not just at the top) and you can express the stop condition affirmatively (“stop when this happens”) rather than negatively (“keep going until that happens.”).

Construct a block of code that prints the numbers 1 through 5. Make sure you use correct indentation! Also, there will be three code blocks that aren’t used in the final solution.

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