Sorting structures from a file alphabetically in C

问题:

It is necessary to make sorting of structures on a field (char last_name [256];) structures Pers and display the user in the console. How to do it? Thank you in advance.

There is such a structure (with nested):

struct Pers {
    int id;
    char first_name[256];
    char last_name[256];
    struct {
        int age;
        int status;
    } st;
} Pers;


struct Pers sw[2];
char i=0;

reading from a file and output looks like this: Everything is displayed in the order of reading from the file

 FILE *file;
 file = fopen("1.txt", "r");
 while ( fscanf(file, "%d%s%s%d%d", &sw[i].id,sw[i].first_name,sw[i].last_name,&sw[i].st.age,&sw[i].st.status) != EOF) 
 {

     printf("%d %s %s %d %d\n", sw[i].id, sw[i].first_name, sw[i].last_name, sw[i].st.age, sw[i].st.status);
        i++;
 }

 fclose(file);

回答1:

To use qsort from stdlib for sorting of your structures, you should implement function that compares two elements. And to compare strings strcmp from string is used.

Details are in the references.

Example for case when both first_name and last_name are used for sorting (last_name is the first for comparison):

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

int compare (const void * a, const void * b)
{
  const struct Pers * first = (const struct Pers *) a;
  const struct Pers * second = (const struct Pers *) b;
  // compare last names and check result. can be also:
  // if( !strcmp(first->last_name, second->last_name) )
  if( 0 == strcmp(first->last_name, second->last_name) )
      // compare first names if last names are equal
      return strcmp(first->first_name, second->first_name);
  else
      return strcmp(first->last_name, second->last_name);
}

usage:

    printf("Before sorting:\n");
    for(i = 0; i < 2; i++)
    {
        printf("%d %s %s %d %d\n",sw[i].id,sw[i].first_name,sw[i].last_name,sw[i].st.age,sw[i].st.status);
    }
    qsort (sw, 2, sizeof(struct Pers), compare);
    printf("After sorting:\n");
    for(i = 0; i < 2; i++)
    {
        printf("%d %s %s %d %d\n",sw[i].id,sw[i].first_name,sw[i].last_name,sw[i].st.age,sw[i].st.status);
    }

Results on my data:

Before sorting:
1 John Smith 33 1
2 Jack Smith 18 1
After sorting:
2 Jack Smith 18 1
1 John Smith 33 1