I've managed to write this following program which accepts values for 5 students and works on it. But, for the life of me, I can't figure out how to modify this for 'n' number of students! I've tried almost everything I know but I can't get it to work without errors! Does anyone have any ideas?? Thanks!

#include <stdio.h>
#include <conio.h>
#include <stdlib.h>

#define NUM_STUDENTS 5

#define IND_REGNO 0
#define IND_FAIL 1
#define IND_RANK 2
#define IND_TOTAL 3
#define IND_MARKS 4

int st[NUM_STUDENTS][9];
int sortarr[NUM_STUDENTS];

int sortfunc(const void* p1, const void* p2) {
int id1=*(int*)p1, id2=*(int*)p2;

return st[id2][IND_TOTAL]-st[id1][IND_TOTAL];
}

int main() {
printf("\n Enter the register number and marks in each subject");
int i, j, passers=0, passrate=0, topper=0;

for(i=0; i<NUM_STUDENTS; i++) {
scanf("%i %i %i %i %i %i", &st[i][IND_REGNO], &st[i][IND_MARKS+0], &st[i][IND_MARKS+1],
&st[i][IND_MARKS+2], &st[i][IND_MARKS+3], &st[i][IND_MARKS+4]);

for(j=0; j<5; j++) {
st[i][IND_TOTAL]+=st[i][IND_MARKS+j];
if(st[i][IND_MARKS+j] < 50) {
st[i][IND_FAIL]=1;
}
}

if(st[i][IND_FAIL] == 0) {
passers++;
}
}

for(i=0; i<NUM_STUDENTS; i++) {
sortarr[i]=i;
}

qsort(sortarr, NUM_STUDENTS, sizeof(int), sortfunc);

for(i=0; i<NUM_STUDENTS; i++) {
st[sortarr[i]][IND_RANK]=i+1;
}
passrate=(passers*100) / NUM_STUDENTS;
topper=st[sortarr[0]][IND_REGNO];
printf("\nNo \tM1 \tM2 \tM3 \tM4 \tM5 \tTOTAL \tP/F \tRANK \n \n \n");
for(i=0; i<NUM_STUDENTS; i++) {
printf("\n%i \t%i \t%i \t%i \t%i \t%i \t%i \t%s \t%i\n", st[i][IND_REGNO], st[i][IND_MARKS+0], st[i][IND_MARKS+1],
st[i][IND_MARKS+2], st[i][IND_MARKS+3], st[i][IND_MARKS+4], st[i][IND_TOTAL],
(st[i][IND_FAIL]==0) ? "PASS" : "FAIL", st[i][IND_RANK]);
}

printf("\nPass Percentage - %i%%\nTopper - %i\n", passrate, topper);
getch();
return 0;
}

You can't use a standard array structure - you have to use a dynamic malloc procedure to do this. It is a lot more complicated but it will help you understand C much better.