Do not understand why my loop doesnt go frther than 2 - c

Question

I'm trying to write a program (c) that the input is integer between 2 to 20.
and the output will be all the irreducible fractions between 0 to 1, that their denominator<=(input number).
for example:
input num=7
the output will be:
For denominator 2
1/2
For denominator 3
1/3
2/3
For denominator 4
1/4
3/4
For denominator 5
1/5
2/5
3/5
4/5
For denominator 6
1/6
5/6
For denominator 7
1/7
2/7
3/7
4/7
5/7
6/7
my problem is that the the counter i of the first loop doesnt go farther than 2, and the printf line in the j loop doesn't happen
Cant's understant what im doing wrong.
Thanks for helping.
int main()
{
int num, i, k, j, a, b;
printf("Please enter natural denominator < 20\n");
scanf("%d", &num);
for (i = 2; i < num ; i++) { /* loop for denomiator */
printf("For denominator %d\n", i);
for (k = 1; k < i; k++) { /* loop for numerator */
for (j = k; j > 0; j--) {
a = k % j;
b = i % j;
if (a == 0 && b == 0) {
printf("%d/%d", k, i); /* this line doesn't print for any input */
}
}
}
}
return 0;
}

Answer 1

In addition to the problemyou are describing (for which starrify has given a solution) there is also a flaw in your logic.
Instead of skipping entries that can be simplified, you are printing them more than once.
The problem can be solved by modifying your innermost loop like this:
int canBeSimplified = 0;
for (j = k; j > 1; j--) {
a = k % j;
b = i % j;
if (a == 0 && b == 0) {
canBeSimlified = 1;
}
}
if (!canBeSimplified) {
printf("%d/%d\n", k, i);
}

Answer 2

I guess you shall have your standard library function prototypes introduced by including the header:
#include <stdio.h>
The problem, as I guess, is that you're using a 64-bit system (that's important since on such platforms sizeof(int)!=sizeof(void*)), and as you didn't include the prototypes for function scanf and printf, all its parameters (and also return values) are treated as of type int.
On my laptop with the header included there's no problem with your code. If you have any further problems or simply including the header doesn't solve your problem, please let me know, better with some method for me to reproduce your problem.
Also, I guess you may want to have a new line at your output by changing this line:
printf("%d/%d", k, i);
into this:
printf("%d/%d\n", k, i);

Answer 3

First - fix (i = 2; i < num ; i++) to (i = 2; i <= num ; i++), or your program won't process last number.
Second - "%d/%d\n" instead of "%d/%d" to write numbers on separate lines.
Third, I fixed previous two and now have:
Please enter natural denominator < 20
3
For denominator 2
1/2
For denominator 3
1/3
2/3
Press any key to continue...
And yes, if you had some changes in your code after it was built and tested, you have to build it again before running. Check that.