[Tutor] python question

[Karl Pflästerer]

An unnamed person wrote:

> import string

> num_str=raw_input('Enter a number:')

> num_num=string.atoi(num_str)

> fac_list=range(1,num_num+1)
> print "BEFORE:", `fac_list`

> i=0

> while i < len(fac_list):
>       if num_num % fac_list[i]==0:
>               del fac_list[i]

>        i=i+1
> print "AFTER:", `fac_list`

> there is a bug in this program. i know what the bug is(doesn't work correctly
> when you enter an even number), but i'm not sure how to solve it. Can anybody
> give me a few hints? Thanks!

First to the bug.

********************************************************************
import string

num_str=raw_input('Enter a number:')

num_num=string.atoi(num_str)

fac_list=range(1,num_num+1)
print "BEFORE:", `fac_list`

i=0

while i < len(fac_list):
    if num_num % fac_list[i]==0:
        del fac_list[i]
        i =- 1
    i=i+1

print "AFTER:", `fac_list`
********************************************************************

That would be a correct version of your programm (but not a nice one).
If you delete a item you mustn't increment your index.

But now to the programm.

First: don't use string.atoi
Second: you filter a list so use either filter() or a list
comprehension.  Then you programm becomes:

********************************************************************
num_num=int(raw_input('Enter a number:'))

fac_list=range(1,num_num+1)
print "BEFORE:", `fac_list`

print "AFTER:", filter(lambda x: num_num % x != 0, fac_list)

#or

print "AFTER:", [x for x in fac_list if num_num % x != 0]
********************************************************************

IMO that's much more readable.


   Karl
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