Thread: Value of structure can't be changed

Hi, I have encountered with a bizarre problem. Though i solved it in a different way, i can't understand why the problem occurs.

Here is the problematic code:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
typedef struct frac {
	int ust;
	int alt;

} frac;

frac fracs[13100];
frac fracs2[13100];

int cmp(const void *a,const void *b) {

	return ((frac*)a)->ust * ((frac*)b)->alt -  ((frac*)a)->alt * ((frac*)b)->ust;
}

FILE *in,*out;

void qs(int alt,int ust);


int main() {
	int num,i,j,x=1,y=0,m,n,f,g;
	clock_t a,b;
	frac z;

	a=clock();

	in=fopen("frac1.in","r");
	out = fopen("frac1.out","w");
	fscanf(in,"%d",&num);
	/*num=4;*/

	fracs[0].alt = 1;
	fracs[0].ust=0;

	for (i=1;i<=num;i++) {
				fracs[x].alt = i;
				fracs[x].ust=1;
				x++;
	}

	for (i=3;i<=num;i++) {
		for (j=2; j<i; j++) {
				n=j;
				for (m=2; m<=n;m++) { 
					if (!(i%m) && !(j%m)) {
						j++;
						break;
					}
				}
				if (i%j == 0) {
					fracs[x].alt = i/j;
					fracs[x].ust=1;
					x++;
				} else {
					fracs[x].alt = i;
					fracs[x].ust=j;
					x++;
				}
		}
	}

	for (i=0;i<=x;i++) {
		m=fracs[i].ust;
		for (j=2;j<=m;j++) {
			if (!(fracs[i].alt%j) && !(fracs[i].ust%j)) {
				fracs[i].ust =fracs[i].ust/j;
				fracs[i].alt =fracs[i].alt/j;
			}
		}
	}
				
	qsort(fracs,x,sizeof(fracs[0]),cmp);

	


	/*for (i=0;i<x;i++) printf("%d / %d \n", fracs[i].ust,fracs[i].alt);*/

	fracs2[0] = fracs[0];
	for (i=1;i<x;i++) {
		if ( (fracs2[y].alt) * (fracs[i].ust) == (fracs2[y].ust) * (fracs[i].alt)) fracs2[y] = fracs[i];
		else ++y,fracs2[y] = fracs[i],fprintf(out,"%d/%d\n", fracs2[y-1].ust,fracs2[y-1].alt);
	}	
	fprintf(out,"%d/%d\n", fracs2[y].ust,fracs2[y].alt);
	b=clock();
	printf("\n\n---%f---\n",((float)(a-b)/1000));

	return 0;
}

This way, the values in structure cannot be changed! when i changed the bold part with this code, the problem is solved!:

Code:

	for (i=0;i<=x;i++) {
		m=fracs[i].ust;
		g = fracs[i].alt;
		for (j=2;j<=m;j++) {
			if (!(g%j) && !(m%j)) {
				fracs[i].ust =m/j;
				fracs[i].alt =g/j;
			}
		}
	}

I couldn't figure out where i am mistaken.

Thanks in prior

Regards

fracs[i].alt is evaluated on every j loop iteration in your original program but not in the second version.