10.6. Infinite Loops

In the last section, we saw what happens if a loop’s control variable is never updated. It will run forever.

Activity: CodeLens 10.6.1 (csprepeatnumbers_infinite1)

Getting a computer to repeat a set of statements is simple. Sometimes it can be tricky to get it to stop. Remember that a while loop will execute as long as the logical expression is true. What happens if the logical expression is always true?

Activity: CodeLens 10.6.2 (csprepeatnumbers_infinite2)

Since 1 will always be equal to 1, the two print statements will just be repeated over and over and over again and the logical expression will never be false. We call that an infinite loop, which means a loop that continues forever or until it is forced to stop.

In that program, it is obvious that the loop will be infinite. It is not always so clear. Maybe we don’t update the loop control variable. Or maybe we don’t write the logical expression in a way that it will ever become false. Look at this next program. Can you tell why it has an infinite loop?

Try running it. The problem will freeze up and eventually your browser tab will likely crash! To stop the program you will have to close the browser tab and reopen the page (copy the page address from the address bar of your browser before closing it).

counter = 1 while counter != 10: print(counter) counter = counter + 2

If you run in Codelens mode, there will be an error displayed that tells you Codelens stopped the run after 1000 lines of execution. You can watch the program run and see what it is doing up until that point. Try doing so. The loop says to keep going as long as the counter is not 10. The counter never hits 10 - it skips directly from 9 to 11!

The program was supposed to display the numbers 1,3,5,7,9. What operator should have been used in while counter ______ 10: instead of !=?

You have attempted of activities on this page