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quick_select.c
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quick_select.c
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#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include "sorting_algorithms.h"
#define INPUT_SIZE 1000000
#define ITH_ORDER_STATISTICS 33
int find_group_median ( int[], int, int );
int find_median ( int[], int );
int special_partition(int[], int, int, int );
int select_ith( int[], int, int, int );
int main() {
int A[INPUT_SIZE];
int i;
for ( i = 0; i < INPUT_SIZE; i++ ) {
A[i] = random();
//printf("%d\n", A[i]);
}
clock_t start_time = clock();
printf( "%dth order statistics = %d\n", ITH_ORDER_STATISTICS, select_ith( A, 0, INPUT_SIZE - 1, ITH_ORDER_STATISTICS ) );
clock_t end_time = clock();
double execution_time = (double)(end_time - start_time) / CLOCKS_PER_SEC;
printf( "Execution time = %f\n ", execution_time );
return 0;
}
int find_group_median (int A[], int p, int r) {
insertion_sort_helper(A, p, r);
return A[(r - p) / 2 + p];
}
int find_median ( int A[], int n ) {
if( n <= 5 ) {
return find_group_median( A, 0, n - 1 );
}
// determine the number of groups
int medians_count;
if ( n % 5 == 0 ) {
medians_count = n / 5;
}
else {
medians_count = n / 5 + 1;
}
int medians[medians_count];
// determine the median for each group
int i;
for ( i = 0; i < (n - 4); i += 5 ) {
medians[i / 5] = find_group_median( A, i, i + 4 );
}
if ( n % 5 != 0 ) {
medians[medians_count - 1] = find_group_median( A, i, n - 1 );
}
// determine the median of medians
return find_median( medians, medians_count );
}
/*
** partition on a given pivot
*/
int special_partition(int A[], int p, int r, int median){
int i;
for ( i = p; i <= r; i++ ) {
if( A[i] == median ) {
swap( A, i, r );
break;
}
}
return partition( A, p, r );
}
int select_ith( int A[], int p, int r, int i ) {
int median = find_median( A, r - p + 1 );
int q = special_partition( A, p, r, median );
if ( (i - 1) == q ) {
return A[q];
}
else if ( (i - 1) < q ) {
return select_ith(A, p, q - 1, i);
}
else {
return select_ith(A, q + 1, r, i);
}
}