Using Recursion to Reverse String - C

Question

I don't know if I just don't understand arrays or what, but I've been looking for the problem in this code for the last couple of hours. When I run it, I get an infinite loop of returns.

Here's the code:

#include <stdio.h>
#include <string.h>

void reverse(char string2Reverse[80], int start, int end);

int main(int argc, char *argv[])
{
    char string[80];

    int cntr;
    for(cntr = 0; cntr < 80; cntr++)
        string[cntr] = '\0';

    int start = 0, end;

    while((scanf("s", string)) != EOF)
    {
    end = 0;
    printf("%s ", string);

    while(string[end] != '\0')
        end++;

    end--;
    reverse(string, start, end);
    end++;
    printf("%s\n\n", string);

    for(cntr = 0; cntr < end; cntr++)
        string[cntr] = '\0';
    }
}

void reverse(char string2Reverse[80], int start, int end)
{
    if(string2Reverse[start] != string2Reverse[end] && start != end)
    {
    char temp = string2Reverse[start];
    string2Reverse[start] = string2Reverse[end];
    string2Reverse[end] = temp;
    reverse(string2Reverse, start + 1, end - 1);
    }
}

Any help would be awesome! I can't seem to find my problem.

It is often helpful to only provide as much code as needed to exhibit the problem. Here replacing input with constants would be a good idea as it makes automatic verification harder. — pmr
– pmr, Commented Oct 17, 2012 at 19:15

hexist · Accepted Answer · 2012-10-17 19:20:11Z

If end-start is not even, you'll infinite loop because start will != end ever .. also i dont' know if you actually want to be comparing the string2Reverse[start] != string2Reverse[end] like that, that means if there is a place in the string where there is an 'a' and in the balanced position on the other side of a string there is also an 'a', it'll terminate. Try this:

void reverse(char string2Reverse[80], int start, int end)
{
    if(start < end)
    {
    char temp = string2Reverse[start];
    string2Reverse[start] = string2Reverse[end];
    string2Reverse[end] = temp;
    reverse(string2Reverse, start + 1, end - 1);
    }
}

And as everyone else pointed out here, you need "%s" instead of "s" in your scanf, the reason why you're looping right now is because end starts off as -1 (since it was a zero len string), so obviously start is increasing and end is decreasing, so your termination condition never met.

lccarrasco · Accepted Answer · 2012-10-17 19:12:38Z

1

scanf's format to read strings is %s, it seems you're missing the percent sign.

answered Oct 17, 2012 at 19:12

lccarrasco

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Comments

Chris · Accepted Answer · 2012-10-17 19:16:04Z

1

while((scanf("s", string)) != EOF) should be while((scanf("%s", &string)) != EOF) dont forget % and &.

answered Oct 17, 2012 at 19:16

Chris

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