the loop does not execute correctly

Question

class cga(object):
    ''''''
    def __int__(self,i,o):
        ''''''
        self.i = i
        self.o = o

    def get(self):
        ''''''
        self.i = []
        c = raw_input("How many courses you have enrolled in this semester?:")
        cout = 0
        while cout < c:
            n = raw_input("plz enter your course code:")
            w = raw_input("plz enter your course weight:")
            g = raw_input("plz enter your course grade:")
            cout += 1 
            self.i.append([n,w,g])
if __name__ == "__main__":
    test = cga()
    test.get()

my problem is the if i type 5 when program ask how many courses i enroll. The loop will not stop, program will keep asking input the course code weight grade. I debugged when it shows program has count cout = 6, but it doest compare with c and while loop does not stop.

First, fix your indentation. Not all of your code seems to be formatted properly. Second, add print statements (or functions) to show the values of `cout` and `c` so we can see what's happening. — S.Lott, May 05 '11 at 19:31
You know, if you don't want to use docstrings, you can leave them out instead of simply putting a blank one in there. — bgw, May 05 '11 at 19:36

score 6 · Answer 1 · answered May 05 '11 at 19:33

6

The problem is, that raw_input returns a string (not a number), and for some odd historical reasons, strings can be compared (for ordering) to any other kind of object by default, but the results are... odd.

Python 2.6.5 (r265:79063, Oct 28 2010, 20:56:23) 
[GCC 4.5.0 20100604 [gcc-4_5-branch revision 160292]] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 1 < "2"
True
>>> 1 < "0"
True
>>> 1 < ""
True
>>>

Convert the result to an integer before comparing it:

c = int(raw_input("How many courses you have enrolled in this semester?:"))

answered May 05 '11 at 19:33

Dirk

30,623
8
82
102

2

"for some odd historical reasons"? Not really. It's a perfectly sensible reason: "Explicit is better than Implicit". An implicit conversion is the wrong thing to do. – S.Lott May 05 '11 at 19:38
The error should be made *explicit* by raising an exception rather than *implicitly* seeming to work but producing counter-intuitive results. The fact that it does this is indeed a historical artifact. – kindall May 05 '11 at 20:02
1

@S.Lott: what's explicit about this behavior? Wouldn't it be more explicit to raise a `TypeError`? – Eli Bendersky May 05 '11 at 20:07
@Eli Bendersky: "Wouldn't it be more explicit to raise a `TypeError`"? Apparently not. It appears that it's more explicit to compare the objects and return `False`. – S.Lott May 05 '11 at 20:21
@S.Lott: Firstly, the comparison returns `True` (!) in Py2. Note, that the behaviour has been dropped (and rightly so in my opinion) in Py3 in favour of a `TypeError`: the above example yields `TypeError: unorderable types: int() < str()` – Dirk May 05 '11 at 20:39
@Dirk: Excellent point. I was only suggesting that the historical reason isn't "odd", there was some sense to it. It may not be the very *best* sense, but it wasn't random, either. – S.Lott May 05 '11 at 20:40

score 2 · Answer 2 · answered May 05 '11 at 19:32

raw_input returns a string, not an int. Your evaluation logic is flawed. You'll have to check whether the user input a valid value (positive integer, presumably less than some maximum value allowed). Once you validate this, then you'll have to cast c to an int:

c=int(c)

Only then will your comparison logic work how you expect.

the loop does not execute correctly

2 Answers2