7.7. Infinite Loops

Here’s the code for a simple while loop. Run the program first to make sure it works. Then make one of these changes, and run the program again:

  1. Remove or comment out num += 1.

  2. Replace the += operator with -=.

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    # Be careful what you type!
    num = 0
    
    while num < 21:
       print(num)
       num += 1      # -= anyone?
    

Yikes! What happened? The program just keeps running!

This is an example of an infinite loop, which is a set of code that repeats forever.

  1. By removing num += 1 from the loop body, the value of num remains 0.
  2. By changing the operator to num -= 1, the value of the variable decreases every iteration.

By making either of these changes, the expression num < 21 will ALWAYS return True, so the while loop will NEVER stop.

7.7.1. Coding Infinity

Simple mistakes in the code create infinite loops, and every programmer accidentally creates one from time to time. Consider it a rite of passage—when you launch your first, welcome to the club.

Tip

When this happens to you, holding down control-c will usually force your program to stop.

Infinite loops are usually created from small typos or missing statements. These mistakes set up a situation where the ending condition cannot be reached.

Example

Here’s another infinite while loop. The program is supposed to print a decreasing total until that total reaches 0.

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start_value = 26
total = start_value

while start_value > 0:
   total -= 5
   print(total)

In this case, the update statement is correct (line 5), but the error occurs in line 4. Instead of using total in the boolean expression, we used start_value, which never gets updated in the loop.

7.7.2. Check Your Understanding

Question

The following code contains an infinite loop. Which is the BEST explanation for why the loop does not end?

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num = 10
answer = 1

while num > 0:
   answer = answer + num
   num += 1

print(answer)
  1. num starts at 10 and increases by 1 each time through the loop, so it will always be positive.
  2. answer starts at 1 and increases by num each time, so it will always be positive.
  3. You cannot compare num to 0 in a while loop. You must compare it to another variable.