Quicksort in Linux or Unix

Posted on Edited on Word count in article: 1.7k Reading time ≈ 2 mins.
qsort is quicksort function in linux or Unix. The function prototype is 
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void qsort(const void* array, const int number, const size_t sizeof_element,int (*cmp_fun)(const void*,const void*));

maybe I write too many const here:) I will write some examples to sort numbers below:

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#include<stdlib.h>
// in c++ use cstdlib
int cmp(const void * a, const void * b){
	const int *tmp_a = (const int*)a;
	const int *tmp_b = (const int *)b;
	return *tmp_a-*tmp_b; // this is ASCE
} // this is function for comparing two Int type

int main(){
	int array[100],i,flag = 0;
	for(i =0; i <100; i++){
		scanf("%d",&array[i]);	
	}
	qsort(array,100, sizeof(array[0]), cmp);
	// here you can use sizeof(int) instead
	for(i = 0; i < 100; i++){
		if(!flag){
			printf("%d", array[i]);
			flag = 1;
		}
		else{
			printf(" %d", array[i]);
		}
	}
	printf("\n");
	return 0;
}

qsort function will change the content of array. If you want use complex object in qsort. You should just convert const void* to object*. If you will use point in array, you should compare with objects use **tmp_a-**tmp_b. If you want to compare double or float type, you should have a look at my another article: compare double numbers in c/c++ language.

If you want a stable compare function, you can use qsort also. First you should construct a class or struct, then and index or other information into struct. And compare when two values are equal.

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int cmp(const void * a, const void * b){
	const Object *tmp_a = (const Object*)a;
	const Object *tmp_b = (const Object *)b;
	if(tmp_a->value == tmp_b->value){
		if(tmp_a->index < tmp_b->index){
			return -1;
		}
		else{
			return 1;
		}
	}
	return tmp_a->value-tmp_b->value; // this is ASCE
} // this is function for comparing two Int type