squiggle.c/scratchpad/quickselect/quickselect.c

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

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;
    }
  }
}

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;

  // 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);
  return 0;
}