Why is this a infinite loop

Question

a = raw_input ("enter a number")
i = 0
numbers = []

while i < a:
    print "At the top i is %d" % i
    numbers.append(i)

    i = i + 1
    print "Numbers now:", numbers
    print "At the bottom i is %d" % i

print "The numbers: "

for num in numbers:
    print num

so I'm following lpthw and was just messing with the code, why is it when I use raw_input and enter a number like 6 this loop turns into a infinite loop? shouldn't i = i + 1 be there to stop this from happening?

Because an integer is always less than a string in Python 2. — Ashwini Chaudhary, Feb 02 '14 at 19:15
I was wrong about this being the answer, but you *are* mixing tabs and spaces. This is **bad**. I don't know why it runs; maybe this is only a problem in your copy/paste to SO, but be sure you're using only spaces. — mhlester, Feb 02 '14 at 19:19
fix indents btw... like that the code will just not even compile. — Loïc Faure-Lacroix, Feb 02 '14 at 19:19
Is `i = i + 1` inside the while loop? Please fix your indentation. And it's better to write it as `i += 1`. — , Feb 02 '14 at 19:43

score 9 · Accepted Answer · edited May 23 '17 at 12:03

9

In Python 2.7, raw_input returns a str(string). So when you compare i < a, you are comparing an integer with a string and it will always return True.

To fix this, cast your input to an integer:

a = int(raw_input(...))

Note: As @AshwiniChaudhary commented, comparing (with <, <=, > or >=) an integer with a string in Python 3.x will raise an Exception:

TypeError: unorderable types: int() <operator> str()

edited May 23 '17 at 12:03

Community

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answered Feb 02 '14 at 19:16

Christian Tapia

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Also mention the fact that such comparison will raise error in Python 3. – Ashwini Chaudhary Feb 02 '14 at 19:18

score 1 · Answer 2 · answered Feb 02 '14 at 19:16

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raw_input return a string. Your variable a is a string and strings are greater than ints in python.
You need to convert a to an int:

a = int(raw_input("enter a number"))

answered Feb 02 '14 at 19:16

Ishaan

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score 1 · Answer 3 · answered Feb 02 '14 at 19:17

If you use raw_input, then a will be a string. You need an int, for the i < a comparision to work as expected. Use input instead of raw_input.

For future reference, input was removed in Python 3 and raw_input renamed to input, so you will need int(input (...)) in Py3.

score 1 · Answer 4 · answered Feb 02 '14 at 19:28

You are doing two things that can lead to the infinite loop.

The tabs and spaces are mixed up, python cares about indentation, so make sure that i = i + 1 is aligned with numbers.append(i)
The actual reason why it is keeping on going is that when python collects raw_input, it transforms it into a string, not an integer, so the comparison does not work as you'd expect. Change to this and it will work as expected:

a = int(raw_input ("enter a number"))

Why is this a infinite loop

4 Answers4