squiggle.c/scratchpad/quickselect/quickselect.c

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#include <math.h>
#include <stdio.h>
#include <stdlib.h>
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void swap_pointers(double *x, double* y){
double tmp = *x;
*x = *y;
*y = tmp;
}
void swap_in_array(int i, int j, double xs[]){
double tmp = xs[i];
xs[i] = xs[j];
xs[j] = tmp;
}
void array_print(double xs[], int n){
printf(" [");
for(int i=0; i<n;i++){
printf("%f, ", xs[i]);
}
printf("]\n");
}
int partition(int low, int high, double xs[], int length){
// srand(2);
array_print(xs, length);
int pivot = low + floor((high-low)/2); // low + floor(rand() % (high - low + 1));
double pivot_value = xs[pivot];
printf("pivot_value: #%f\n", pivot_value);
swap_in_array(pivot, high, xs);
int gt = low; /* This pointer will iterate until finding an element which is greater than the pivot. Then it will move elements that are smaller before it--more specifically, it will move elements to its position and then increment. As a result all elements between gt and i will be greater than the pivot. */
for(int i=low; i<high; i++){
printf("Step: #%d\n", i);
printf(" Before: \n");
printf(" gt: %d\n", gt);
array_print(xs, length);
if(xs[i] < pivot_value){
printf(" Swaping element #%d: %f and element #%d: %f\n", gt, xs[gt], i, xs[i]);
swap_in_array(gt, i, xs);
gt++;
}
printf(" After: \n");
array_print(xs, length);
printf(" gt: %d\n", gt);
}
printf("Step: Swap with last item\n");
swap_in_array(high, gt, xs);
printf(" After: \n");
array_print(xs, length);
printf(" gt: %d\n", gt);
return gt;
}
double quickselect(int k, double xs[], int length){
int low = 0;
int high = length - 1;
for (;;){
if(low == high){
return xs[low];
}
int pivot = partition(low, high, xs, length);
if(pivot == k){
return xs[pivot];
}else if(k < pivot){
high = pivot - 1;
} else {
low = pivot + 1;
}
}
}
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int main(){
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double xs[] = {2.1, 1.0, 6.0, 4.0, 7.0, -1.0, 2.0, 10.0};
int length = 8;
int k = 2;
// partition(0, length-1, xs, length);
double result = quickselect(k, xs, length);
array_print(xs, 8);
printf("The item in pos #%d is: %f\n", k, result);
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return 0;
}