C library function to perform sort

Question

Is there any library function available in C standard library to do sort?

Answer 1

qsort() is the function you're looking for. You call it with a pointer to your array of data, the number of elements in that array, the size of each element and a comparison function.

It does its magic and your array is sorted in-place. An example follows:

#include <stdio.h>
#include <stdlib.h>
int comp (const void * elem1, const void * elem2) 
{
    int f = *((int*)elem1);
    int s = *((int*)elem2);
    if (f > s) return  1;
    if (f < s) return -1;
    return 0;
}
int main(int argc, char* argv[]) 
{
    int x[] = {4,5,2,3,1,0,9,8,6,7};

    qsort (x, sizeof(x)/sizeof(*x), sizeof(*x), comp);

    for (int i = 0 ; i < 10 ; i++)
        printf ("%d ", x[i]);

    return 0;
}

Answer 2

C/C++ standard library <stdlib.h> contains qsort function.

This is not the best quick sort implementation in the world but it fast enough and VERY EASY to be used... the formal syntax of qsort is:

qsort(<arrayname>,<size>,sizeof(<elementsize>),compare_function);

The only thing that you need to implement is the compare_function, which takes in two arguments of type "const void", which can be cast to appropriate data structure, and then return one of these three values:

• negative, if a should be before b
• 0, if a equal to b
• positive, if a should be after b

1. Comparing a list of integers:

simply cast a and b to integers if x < y,x-y is negative, x == y, x-y = 0, x > y, x-y is positive x-y is a shortcut way to do it :) reverse *x - *y to *y - *x for sorting in decreasing/reverse order

int compare_function(const void *a,const void *b) {
int *x = (int *) a;
int *y = (int *) b;
return *x - *y;
}

2. Comparing a list of strings:

For comparing string, you need strcmp function inside <string.h> lib. strcmp will by default return -ve,0,ve appropriately... to sort in reverse order, just reverse the sign returned by strcmp

#include <string.h>
int compare_function(const void *a,const void *b) {
return (strcmp((char *)a,(char *)b));
}

3. Comparing floating point numbers:

int compare_function(const void *a,const void *b) {
double *x = (double *) a;
double *y = (double *) b;
// return *x - *y; // this is WRONG...
if (*x < *y) return -1;
else if (*x > *y) return 1; return 0;
}

4. Comparing records based on a key:

Sometimes you need to sort a more complex stuffs, such as record. Here is the simplest way to do it using qsort library.

typedef struct {
int key;
double value;
} the_record;

int compare_function(const void *a,const void *b) {
the_record *x = (the_record *) a;
the_record *y = (the_record *) b;
return x->key - y->key;
}

Answer 3

For sure: qsort() is an implementation of a sort (not necessarily quicksort as its name might suggest).

Try man 3 qsort or have a read at http://linux.die.net/man/3/qsort

Answer 4

While not in the standard library exactly, https://github.com/swenson/sort has just two header files you can include to get access to a wide range of incredibly fast sorting routings, like so:

#define SORT_NAME int64
#define SORT_TYPE int64_t
#define SORT_CMP(x, y) ((x) - (y))
#include "sort.h"
/* You now have access to int64_quick_sort, int64_tim_sort, etc., e.g., */
int64_quick_sort(arr, 128); /* Assumes you have some int *arr or int arr[128]; */

This should be at least twice as fast as the standard library qsort, since it doesn't use function pointers, and has many other sorting algorithm options to choose from.

It's in C89, so should work in basically every C compiler.

Answer 5

try qsort in stdlib.h.

Answer 6

I think you are looking for qsort.
qsort function is the implementation of quicksort algorithm found in stdlib.h in C/C++.

Here is the syntax to call qsort function:

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

List of arguments:

base: pointer to the first element or base address of the array
nmemb: number of elements in the array
size: size in bytes of each element
compar: a function that compares two elements

Here is a code example which uses qsort to sort an array:

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

int arr[] = { 33, 12, 6, 2, 76 };

// compare function, compares two elements
int compare (const void * num1, const void * num2) {
   if(*(int*)num1 > *(int*)num2)
    return 1;
   else
    return -1;
}

int main () {
   int i;

   printf("Before sorting the array: \n");
   for( i = 0 ; i < 5; i++ ) {
      printf("%d ", arr[i]);
   }
   // calling qsort
   qsort(arr, 5, sizeof(int), compare);

   printf("\nAfter sorting the array: \n");
   for( i = 0 ; i < 5; i++ ) {   
      printf("%d ", arr[i]);
   }
  
   return 0;
}

You can type man 3 qsort in Linux/Mac terminal to get a detailed info about qsort.
Link to qsort man page

Answer 7

Use qsort() in <stdlib.h>.

@paxdiablo The qsort() function conforms to ISO/IEC 9899:1990 (``ISO C90'').

Answer 8

There are several C sorting functions available in stdlib.h. You can do man 3 qsort on a unix machine to get a listing of them but they include:

• heapsort
• quicksort
• mergesort

Answer 9

GNU qsort source in stdlib shows that it is quicksort.