Sorting structures from files - c

Question

I recently got an assignment to sort members in a struct by last name and if they are the same to sort by first name. What i have so far only reads their name and age from the file but I am not properly grapsing how I would be able to sort it. So far I gathered the data from the file but im at a loss from there. I followed a code I saw but i didnt get a proper grasping of the process so i reverted back to step one.
struct Members{
int id;
char fname[50];
char lname[50];
int age;
}bio;
int main(){
int i=0;
FILE *fptr;
file = fopen("Members Bio.txt", "r");
while ( fscanf(file, "%d%s%s%d", &bio[i].id,bio[i].fname,bio[i].lname,&bio[i].age) != EOF)
{
printf("%d %s %s %d %d\n", bio[i].id,bio[i].fname, bio[i].lname, bio[i].age);
i++;
}
fclose(fptr);
}
Can anyone help me out on this one?

Answer 1

Code goes something like this for your case.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct Members{
int id;
char fname[50];
char lname[50];
int age;
};
typedef int (*compare_func)(void*, void*);
int struct_cmp(void* s1, void* s2)
{
int l_result = strcmp(((struct Members*) s1)->lname, \
((struct Members*) s2)->lname);
if (l_result < 0)
return 1;
else if (l_result > 0)
return 0;
else
return (strcmp(((struct Members*) s1)->fname, \
((struct Members*) s2)->fname) < 0 ? 1 : 0);
}
void sort(void* arr,long ele_size,long start,long end,compare_func compare)
{
// Generic Recursive Quick Sort Algorithm
if (start < end)
{
/* Partitioning index */
void* x = arr+end*ele_size;
long i = (start - 1);
void* tmp=malloc(ele_size);
for (long j = start; j <= end - 1; j++)
{
if ((*compare)(arr+j*ele_size,x))
{
i++;
// Swap is done by copying memory areas
memcpy(tmp,arr+i*ele_size,ele_size);
memcpy(arr+i*ele_size,arr+j*ele_size,ele_size);
memcpy(arr+j*ele_size,tmp,ele_size);
}
}
memcpy(tmp,arr+(i+1)*ele_size,ele_size);
memcpy(arr+(i+1)*ele_size,arr+end*ele_size,ele_size);
memcpy(arr+end*ele_size,tmp,ele_size);
i= (i + 1);
sort(arr,ele_size,start, i - 1,compare);
sort(arr,ele_size,i + 1, end,compare);
}
}
int main()
{
FILE* fp;
int bio_max = 3;
struct Members bio[bio_max]; // Define bio to be large enough.
/* Open FILE and setup bio matrix */
/* For testing */
bio[0].id = 0;
strcpy(bio[0].fname, "");
strcpy(bio[0].lname, "Apple");
bio[0].age = 0;
bio[1].id = 1;
strcpy(bio[1].fname, "");
strcpy(bio[1].lname, "Cat");
bio[1].age = 1;
bio[2].id = 2;
strcpy(bio[2].fname, "");
strcpy(bio[2].lname, "Bat");
bio[2].age = 2;
/* Sort the structure */
sort(bio, sizeof(struct Members), 0, bio_max - 1, struct_cmp);
/* Print the sorted structure */
for (int i = 0; i < bio_max; i++) {
printf("%d %s %s %d\n", bio[i].id, bio[i].fname, \
bio[i].lname, bio[i].age);
}
}
Output
0 Apple 0
2 Bat 2
1 Cat 1
If the strings are not sorting in the way you want, you can redefine the struct_cmp function. Code is self explanatory, the base logic in the code is pass an array and swap elements using memcpy functions. You cant use simple assignment operator if you want to be generic, so that is why the element size is explicitly passed.
Edit
The code was not handling the condition, if lname are same. I missed it thanks for #4386427 for pointing this out.

Answer 2

I think you should define bio to be an array. And google sort algorithms please. Also recommend you google how to use libc function qsort.