6.9. 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:
Remove or comment out
num += 1.
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.
- By removing
num += 1from the loop body, the value of
- 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
True, so the
while loop will NEVER stop.
6.9.1. 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.
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.
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.
Here’s an example of an infinite
for loop in Python:
# NOPE! Nothing to see here!
Well. That’s nice.
Although it is possible to create an infinite
for loop in Python, you
need to be very deliberate and think very hard about how to make it happen.
You are unlikely to create one by accident.
6.9.2. Fun Fact¶
Infinite loops are such an established part of programming that Apple even made one the address of its California campus.
6.9.3. Check Your Understanding¶
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)
- num starts at 10 and increases by 1 each time through the loop, so it will always be positive.
- answer starts at 1 and increases by num each time, so it will always be positive.
- You cannot compare num to 0 in a while loop. You must compare it to another variable.