Using global variables in python

Question

I am new to python. I am trying to convert one of my c programs to corresponding python program, however I am not able to use global variable in python. My code in both c and python are :

#include <stdio.h>
int globalcount;

void noofways(int firstnumchosen,int sum,int numofnum)
{
if(sum<0)
    return;

if(sum==0 && numofnum!=0)
    return;

if(sum==0 && numofnum==0){
    globalcount++;
    return;
}

if(numofnum<=0)
    return;

if(firstnumchosen>sum)
    return;

noofways(firstnumchosen+1,sum,numofnum);
noofways(firstnumchosen+1,sum-firstnumchosen,numofnum-1);
}

int main()
{
noofways(1,8,3);
printf("Required number: %d",globalcount);
return 0;
}

def noofways(firstnumchosen, sum, numofnum):
    global count
    count=0
    if sum<0:
        return 
    if sum==0 and not(numofnum==0):
        return
    if sum==0 and numofnum==0:
       count+=1
       return
    if numofnum<=0:
       return
    if firstnumchosen>sum:
       return
    noofways(firstnumchosen+1,sum,numofnum)
    noofways(firstnumchosen+1,sum-firstnumchosen,numofnum-1)

res=noofways(1,8,3);
print count

I think I know how to declare a global variable in python, but I am having problem in figuring out how to use that variable with recursion.

In what way is the code not working? It looks to me like it is printing 0, which is what I would expect it to do. Furthermore, why are you using a global variable with recursion? Why not just return the variable you're interested in? — David Robinson
– David Robinson, Commented Oct 19, 2012 at 5:02
I would note that Python and C are so different in terms of abstraction, trying to reimplement code from one language to another is a bad idea. The best thing is to ask yourself what the original was trying to do, then write it from scratch - that way you'll end up with Python that works in a Pythonic way, which will be far easier to read and maintain. — Gareth Latty
– Gareth Latty, Commented Oct 19, 2012 at 5:03
It should return 2. I need to increment the count once in a while (when i get the sum). — OneMoreError
– OneMoreError, Commented Oct 19, 2012 at 5:04
@J.F.Sebastian : Then the interpreter says "global name 'count' is not defined" — OneMoreError
– OneMoreError, Commented Oct 19, 2012 at 5:05

John La Rooy · Accepted Answer · 2012-10-19 05:02:34Z

4

Each recursive call will set the count back to 0

def noofways(firstnumchosen, sum, numofnum):
    global count
    # don't set count to 0 here
    if sum<0:
        return 
    if sum==0 and not(numofnum==0):
        return
    if sum==0 and numofnum==0:
       count+=1
       return
    if numofnum<=0:
       return
    if firstnumchosen>sum:
       return
    noofways(firstnumchosen+1,sum,numofnum)
    noofways(firstnumchosen+1,sum-firstnumchosen,numofnum-1)

# set count to 0 here
count = 0
res=noofways(1,8,3);
print count

answered Oct 19, 2012 at 5:02

John La Rooy

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