implement quickselect function

scratchpad/quickselect/quickselect.c

@@ -2,7 +2,83 @@
 #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(){
-  printf("Hello world!\n");
+  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;
 }