Sorting an array of structures by field

Question

I have an array of initialized structures, I need to perform a simple sorting by field (name) in alphabetical order. I know that I need to use qsort() , but I can't do anything. Help please write a function that sorts the mass array.

 #include <stdio.h> #include <stdlib.h> #include <string.h> int my_compare(const void*,const void*); typedef struct test{ char name[100]; char surname[100]; char sname[100]; char posada[100]; char buff[100]; int staz; char buff2[100]; int oklad; }mass[8] ; int main() { /* struct test{ char name[100]; char surname[100]; char sname[100]; char posada[100]; char buff[100]; int staz; char buff2[100]; int oklad; }mass[8] ; */ FILE *fo; if((fo=fopen("C:\\Users\\andyb\\Desktop\\test2.txt","r"))==NULL) printf("error"); for(int i=1;i<8;i++) { fgets(mass[i].name,50,fo); fgets(mass[i].surname,50,fo); fgets(mass[i].sname,50,fo); fgets(mass[i].posada,50,fo); fgets(mass[i].buff,50,fo); mass[i].staz=atoi(mass[i].buff); fgets(mass[i].buff2,50,fo); mass[i].oklad=atoi(mass[i].buff2); } fclose(fo); for(int i=1;i<8;i++) { printf("%s%s%s%s%d\n%d\n",mass[i].name,mass[i].surname,mass[i].sname,mass[i].posada,mass[i].staz,mass[i].oklad); } qsort( mass, sizeof(mass)/sizeof(mass[0]), sizeof(mass[0]), my_compare ); for(int i=1;i<8;i++) { printf("%s%s%s%s%d\n%d\n",mass[i].name,mass[i].surname,mass[i].sname,mass[i].posada,mass[i].staz,mass[i].oklad); } return 0; } int my_compare(const void *a,const void *b) { const test *pa = (const test *)pa; const test *pb = (const test *)pb; /* Сравниваем имена и возвращаем 1, 0 или -1, в зависимости от результата (без учёта регистра): */ return strcmp( pa->name, pb->name ); }

Accepted Answer · 2016-10-07T02:33:55

This typedef ad

  typedef struct test{ char name[100]; char surname[100]; char sname[100]; char posada[100]; char buff[100]; int staz; char buff2[100]; int oklad; }mass[8] ;

introduces the mass type specifier as an alias for the struct test[8] . This declaration does not create any object of this type.

You should write

  typedef struct test{ char name[100]; char surname[100]; char sname[100]; char posada[100]; char buff[100]; int staz; char buff2[100]; int oklad; } test; ^^^^^^

Then inside main you can declare an array of the appropriate type as follows

 #define N 8 int main( void ) { test mass[N]; ^^^^^^^^^^^^^

Note that in C, the main function with no parameters is declared as

 int main( void ) ^^^^

In addition, it does not make much sense to declare buff and buff2 data members in a structure in order to use them as buffers for reading integer data members of the structure. You can simply enter a local variable into main for this purpose, such as

 char buff[100];

In this case, the structure declaration will look like

  typedef struct test{ char name[100]; char surname[100]; char sname[100]; char posada[100]; int staz; int oklad; } test;

Instead of the magic number 8, it is better to use a named constant declared as

 #define N 8

or if the compiler supports variable-length arrays, then how

 const int N = 8;

Array indices in C start at 0. Therefore, this loop statement

 for(int i=1;i<8;i++) {

wrong. Should write

 for ( int i = 0; i < N; i++ ) {

It is also not clear why in sentences like this

 fgets(mass[i].name,50,fo);

the magic number is 50, whereas the corresponding member of the structure, declared as an array, has 100 elements.

It would be better to write

 fgets( mass[i].name, sizeof( mass[i].name ), fo );

Keep in mind that the fgets function adds a newline character to the array in which it reads data. It is advisable to remove this newline character from the array. This can be done as follows.

 fgets( mass[i].name, sizeof( mass[i].name ), fo ); mass[i].name[strcspn( mass[i].name, "\n" )] = '\0';

In the function my_compare you have a typo:

 int my_compare(const void *a,const void *b) { const test *pa = (const test *)pa; ^^^ const test *pb = (const test *)pb; ^^^

Must be

 int my_compare(const void *a,const void *b) { const test *pa = (const test *)a; ^^^ const test *pb = (const test *)b; ^^^

Answer 2 · 2016-10-05T23:00:24

The qsort() prototype looks like this:

 void qsort(void *base, size_t nmemb, size_t size, int (*compar)(const void *, const void *));

Here base is a pointer to an array, nmemb is the number of items in it, size is the size of one item, and compar is a function to compare items. In your case, it might look like this:

 int my_compare( const void * a, const void * b ) { /* Теперь pa и pb указывают на сравниваемые элементы массива: */ const test *pa = (const test *)a; const test *pb = (const test *)b; /* Сравниваем имена и возвращаем 1, 0 или -1, в зависимости от результата (без учёта регистра): */ return strcmp( pa->name, pb->name ); }

So the call could be like this:

 qsort( mass, sizeof(mass)/sizeof(mass[0]), sizeof(mass[0]), my_compare );

I hope this information is enough to figure it out.

The correct typedef is :

 typedef struct { char name[100]; char surname[100]; char sname[100]; char posada[100]; char buff[100]; int staz; char buff2[100]; int oklad; } test; int main() { test mass[8]; }

The problem is that the function does not see the structure, since it is declared in the function main ()
She doesn’t need to “see” any structure, it’s enough that you pass the pointer to the array of structures you are sorting with the first argument.

Sergey Sergey 6.372 four nineteen · Answer 3 · 2016-10-06T02:16:50

So the program is not compiled. or not working?

Regarding the compilation, you have already been told how to fix the type definition.

And about the performance, I doubt this code:

 const test *pa = (const test *)pa; const test *pb = (const test *)pb;

I would venture to suggest that you wanted to write:

 const test *pa = (const test *)a; const test *pb = (const test *)b;

Sorting an array of structures by field

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