Some kind of memory mistake. Please review

The following code runs fine for denom <= 11, but it gives a segmentation fault with the gcc compiler and doesn't run at all with other compilers for denom > 11. Please help! Thanks.

Code:

#include <stdio.h>


void arrage(long *a, long *b);
void reduce(long *numer, long *denom, long gcd);
int getGCD(long a, long b);
int getLCD(long a, long b);

main()
{
  long denom, lcd, sumNumer = 1, sumDenom = 1;

  for(denom = 2; denom <= 11; denom++) {
    lcd = getLCD(sumDenom, denom);
    sumNumer = sumNumer * lcd / sumDenom + lcd / denom;
    sumDenom = lcd;
    reduce(&sumNumer, &sumDenom, getGCD(sumDenom, sumNumer));
  }
  printf("Sum is: %d/%d or %.5f in decimal form.\n", 
	 sumNumer, sumDenom, (double)sumNumer / sumDenom);
  getchar();
}

void arrage(long *a, long *b)
{
  int temp;
  if(*b > *a) {
    temp = *a;
    *a = *b;
    *b = temp;
  }    
}

void reduce(long *numer, long *denom, long gcd)
{
  *numer = *numer / gcd;
  *denom = *denom / gcd;
}

int getGCD(long a, long b)
{
  int r;
  arrage(&a, &b);
  r = a % b;
  if(r == 0)
    return b;
  else
    return getGCD(b, r);
}

int getLCD(long a, long b)
{
  return a * b / getGCD(a, b);
}

I think your recursive function is filling up the stack. It's not a question of < > 11, it's a question of hitting a number that causes your function to run so many times that if fills up the stack.

Originally Posted by Azuth

I think your recursive function is filling up the stack. It's not a question of < > 11, it's a question of hitting a number that causes your function to run so many times that if fills up the stack.

Try this in getGCD, with denom >= 12:

Code:

int getGCD(long a, long b)
{
  int r;
  arrage(&a, &b);
  printf("in getGCD: after arrag: a = %d, b = %d\n", a, b);
  r = a % b;
  printf("in getGCD: r = %d\n", r);
  if(r == 0)
    return b;
  else
    printf("Recursively Calling getGCD(%d, %d)\n", b, r);
    if ((b < 0) || (r < 0)) {
      getchar();
    }
    return getGCD(b, r);
}

(Ctrl-C gets you out of it.)

Regards,

Dave

When denom is 12, sumDenom is 27720 and sumNumer -68920. look at this stack frame:
getGCD(long 27720, long -68920) line 47
getGCD(long 27720, long -68920) line 51 + 13 bytes
getGCD(long 27720, long -68920) line 51 + 13 bytes
getGCD(long 27720, long -68920) line 51 + 13 bytes
getGCD(long 27720, long -68920) line 51 + 13 bytes
main() line 20 + 13 bytes
See? stack overflow!

Yup, and you don't wanna know what it looks like when denom hits 25 ! O_o

Thanks a lot everyone!

I understood what you all said, so I tried coding the recursive method iteratively. I still exceed LONG_MAX, however, and % doesn't apply to floating data types. Please help! This is the latest version of the code:

Code:

#include <stdio.h>

long getLCD(long num1, long num2);
long getGCD(long num1, long num2);

main()
{
      int i;
      long numerator = 0, denominator = 1, lcd;

      for(i = 1; i <= 50; i++)
      {
            lcd = getLCD(denominator, i);
            numerator = numerator * lcd / denominator + lcd / i;
            denominator = lcd;
            printf("On iteration %d: NUM = %d, DEN = %d\n", 
                     i, numerator, denominator);
            printf("Sum of 1/n from 1 to 50 is %d/%d in fractional
                     form, %.3f in decimal form.\n", numerator,
                     denominator, (double) numerator / denominator);
      }

      getchar();
}

long getLCD(long num1, long num2)
{
      return num1 * num2 / getGCD(num1, num2);
}

long getGCD(long num1, long num2)
{
      int temp, rlast, rcurrent;

      if(num1 > num2)
      {
            temp = num1;
            num1 = num2;
            num2 = temp;
      }

      rlast = num2;
      rcurrent = num1 % rlast;
      while(rcurrent != 0)
      {
            temp = rlast;
            rlast = rcurrent;
            rcurrent = temp % rlast;
      }

      return rlast;
}

You can use long long to get a 64-bit int.