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;
}