squiggle.c/scratchpad/quickselect/quickselect.c

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

void swp(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)
{
    // To understand this function:
    // - see the note after gt variable definition
    // - go to commit 578bfa27 and the scratchpad/ folder in it, which has printfs sprinkled throughout
    int pivot = low + floor((high - low) / 2);
    double pivot_value = xs[pivot];
    swp(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++) {
        if (xs[i] < pivot_value) {
            swp(gt, i, xs);
            gt++;
        }
    }
    swp(high, gt, xs);
    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;
        }
    }
}

int main()
{
    double xs[] = { 2.1, 1.0, 6.0, 4.0, 7.0, -1.0, 2.0, 10.0 };
    int length = 8;
    int k = 2;
    array_print(xs, 8);
    double result = quickselect(k, xs, length);
    printf("The item in pos #%d is: %f\n", k, result);
    array_print(xs, 8);
    return 0;
}