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':
break
print(line)
print ('Done!')
The loop condition is True, which is always true, so the
loop runs repeatedly until it hits the break statement.
Q-2: At what word will the while loop pictured above terminate?
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
finished
Word: done
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.”).
- 1
- Incorrect! All the statements that are indented 4 spaces to the right of the while are part of the body of the loop. Try again.
- 2
- Correct! There are two statements that are indented 4 spaces to the right of the while statement, so there are two statements in the body of this loop.
- 3
- Incorrect! There are three lines here total, but not all of them are in the body of the loop. Try again.
Q-3: How many lines are in the body of the while loop in shown above?
while 1==1:
print ("Looping")
print ("Forever")
Looping
Forever
Looping
Forever
Looping
Forever
Looping
Forever
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.