Sort an array of structures. Si

Question

There is a structure

struct internet_prov { char name[20]; unsigned int speed; float tariff; };

You need to write a function that performs an action on an array (size 10) of pointers to structures of this type. The function should display the K (user-defined) structures with the most expensive fare. That is, first we need to sort this array by the tariff field. Tell me, please, how to do this?

use the standard qsort function and redefine the comparator in it.
@pavel I planned to do so, but I don’t know how to indicate what needs to be sorted by the field of tariff
@ 0xdb Yes, how to sort the array exactly by the required field of the structure

Vlad from Moscow Vlad from Moscow one · Accepted Answer · 2016-12-15T20:46:51

You can use the standard C function qsort , declared in the header of <stdlib.h> .

Below is a sample program that shows how you can sort an array of pointers to a structure.

 #include <stdlib.h> #include <stdio.h> #include <string.h> int cmp(const void *a, const void *b) { const struct internet_prov *left = *(const struct internet_prov **)a; const struct internet_prov *right = *(const struct internet_prov **)b; return (right->tariff < left->tariff) - (left->tariff < right->tariff); } #define N 3 int main( void ) { struct internet_prov * prov[N]; for (size_t i = 0; i < N; i++) { char name[] = "A"; name[0] += i; prov[i] = (internet_prov *)malloc(sizeof(internet_prov)); strcpy(prov[i]->name, name); prov[i]->speed = rand() % 100; prov[i]->tariff = rand() % 100 / 10.0f; } for (size_t i = 0; i < N; i++) { printf("%s %u %f\n", prov[i]->name, prov[i]->speed, prov[i]->tariff); } printf("\n"); qsort(prov, N, sizeof(struct internet_prov *), cmp); for (size_t i = 0; i < N; i++) { printf("%s %u %f\n", prov[i]->name, prov[i]->speed, prov[i]->tariff); } for (size_t i = 0; i < N; i++) { free(prov[i]); } }

The output of the program to the console may look as follows.

 A 41 6.700000 B 34 0.000000 C 69 2.400000 B 34 0.000000 C 69 2.400000 A 41 6.700000

@avp In programming, the same problem can be solved in different ways. :)
@VladfromMoscow and what will the cmp function look like if I work with an array of structures and sort them in descending order?
@ Pasha The function body will look like const struct internet_prov left = * (const struct internet_prov *) a;
const struct internet_prov right = * (const struct internet_prov *) b;
return (left-> tariff <right-> tariff) - (right-> tariff <left-> tariff);
Will be return (left.tariff <right.tariff) - (right.tariff <left.tariff);
That is, the appeal to the members of the object through a point, since it is no longer a pointer.

Answer 2 · 2016-12-15T20:32:56

Use qsort with an appropriate comparison function, roughly

 int comp(const void * p1, const void * p2) { struct internet_prov * i1 = (struct internet_prov*)p1; struct internet_prov * i2 = (struct internet_prov*)p2; // Сортируем по тарифу if (i1->tariff > i2->tariff) return 1; if (i1->tariff < i2->tariff) return -1; // Потом сортируем по скорости if (i1->speed > i2->speed) return 1; if (i1->speed < i2->speed) return -1; // и по названию return strcmp(i1->name,i2->name); }

Sort an array of structures. Si

2 answers 2

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