Trouble with populating student's marks - c

Question

This is in response to a similar threat I posted the other day with reading a file into the requisite data structure with the file data like so, I can't remember who said it but yes there's four subjects. (I wanted to post an overall reply to all responses but could only comment on each post made):
131782 Mathematics 59
075160 Mathematics 92
580313 Physics 63
073241 Mathematics 32
487476 Mathematics 73
075160 Physics 98
472832 English 44
...
I'm using fscanf() now to parse the data and this is a much better approach. I made another thread yesterday about removing duplicate strings. I've scrapped that idea now and just used qsort on the student IDs and created a for loop that skips every four elements and rings the unique student IDs into the structure. I did a printf() command earlier and they're successfully stored. Now I've got the IDs stored I'm now ready to search for that ID and populate their marks and I "think" it's almost there except for a slight problem inside the update_student() function.
If you look at my code, or even compile it, it's not liking the line that's supposed to populate the mark for the student, student_data[idx].marks[buffer_subjects]=marks. But buffer_subjects is a string but if you look at my defines those strings are constants which is the whole idea when it gets to this stage.
How can I fix this?
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define STUDENTS 20
#define COMPUTING 0
#define ENGLISH 1
#define MATHEMATICS 2
#define PHYSICS 3
#define SUBJECTS 4
#define ROWS 80
#define SIZE 100
int string_compare(void const *x, void const *y)
{
return strcmp(*(char**)x, *(char**)y);
}
struct student
{
char student_ID[SIZE];
int marks[SUBJECTS];
};struct student student_data[STUDENTS];
int find_student(char buffer_IDs[])
{
int j;
for(j=0;j<STUDENTS;j++)
if(strcmp(student_data[j].student_ID,buffer_IDs)==0)
return j;
}
void update_student(char buffer_IDs[], char buffer_subjects[], int marks[])
{
int idx = find_student(buffer_IDs);
student_data[idx].marks[buffer_subjects] = marks;
}
int main(void)
{
FILE *input;
int i,j, data_items;
char buffer_IDs[ROWS][SIZE];
char buffer_subject[ROWS][SIZE];
int marks[ROWS][SIZE];
char *string_ptrs[ROWS];
if((input=fopen("C:\\marks\\marks.txt","r"))==NULL)
perror("File open failed!");
else
{
for(i=0;i<ROWS;i++)
{
while((data_items=fscanf(input, "%s %s %d", buffer_IDs[i], buffer_subject[i], marks[i])!=3));
printf("%s %s %d\n", buffer_IDs[i], buffer_subject[i], *marks[i]);
string_ptrs[i]=buffer_IDs[i];
}
putchar('\n');
qsort(string_ptrs, sizeof(string_ptrs)/sizeof(char*), sizeof(char*), string_compare);
for(i=0;i<ROWS;i=i+4)
{
j=0;
strcpy(student_data[j].student_ID,string_ptrs[i]);
printf("%s\n",student_data[j].student_ID);
j++;
}
for(i=0;i<ROWS;i++)
update_student(buffer_IDs[i], buffer_subject[i], marks[i]);
}
return 0;
}
> Blockquote

Answer 1

There are numerous failures in this block of code. I would seriously recommend building it up block by block, and using a debugger to confirm that the data in each stage is as expected, and not proceeding till each of the lower level blocks work (and do not produce compile errors).
Running it through GDB with some sample data suggests problems even at the point of reading in and parsing data from the file on disk.
We can assist with individual issues as they arise from this approach.