Why do I appear to have an infinite loop?

Question

This is part of a tutorial. My goal was to take an input from user and then increment it while a condition is true.

inputNum = raw_input("What is max number?")

def incrementF(greatest):
    i = 0
    numbers = []

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

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

    print "The numbers:"

    for num in numbers:
        print num

incrementF(inputNum)

When I run the script and type anything in for inputNum, rather than a few lines of output, I appear to run into an infinite loop.

For example, is the raw input prompt gave 10, I'd have expected to see something like:

At the top of the list numbers is 0,
Numbers now: 0,
0
At the top of the list numbers is 1,
Numbers now: 1,
1

I've read over my script several times and cannot see. I suspect it could have something to do with indenting? But I'm not sure.

score 10 · Accepted Answer · edited May 23 '17 at 12:03

10

As quoted in the docs for raw_input:

The function then reads a line from input, converts it to a string (stripping a trailing newline), and returns that.

So, greatest will be a string because you used raw_input to get it.

Furthermore, in Python 2.x, strings are always evaluated as being greater than integers*:

>>> 1 < 'a'
True
>>> 100000000000 < 'a'
True
>>>

This means that the condition of your while-loop will always evaluate to True because greatest will always be evaluated as being greater than i. Thus, you get an infinite loop.

You can fix this problem simply by making greatest an integer like i:

inputNum = int(raw_input("What is max number?"))

*Note: If you would like some more information on this, see How does Python compare string and int?

edited May 23 '17 at 12:03

Community

1
1

answered Dec 17 '13 at 01:08

1

Does `string < int` always return non-`False`? – zneak Dec 17 '13 at 01:08
1

Damn, that's quick. +1. – aIKid Dec 17 '13 at 01:08
@zneak Yes. However, note that this behavior is fixed in Python 3, comparing int to string will raise a `TypeError`. – aIKid Dec 17 '13 at 01:10
Now it's +5 dammmmmmmmn – Andy_A̷n̷d̷y̷ Dec 17 '13 at 01:10
Thanks! Works now, accepting shortly. Will remember that raw_inputs are strings from now on. – Doug Fir Dec 17 '13 at 01:10
Actually, to add to my last comment... Why did it behave that way, even if it was a string? I'll research it – Doug Fir Dec 17 '13 at 01:12
@DougFirr - See [here](http://stackoverflow.com/questions/3270680/how-does-python-compare-string-and-int). Also, as aIKid said, this behavior is fixed in Python 3.x. – Dec 17 '13 at 01:13
Ah, I see my follow up question on why was answered by zneak above – Doug Fir Dec 17 '13 at 01:13

Why do I appear to have an infinite loop?

1 Answers1