Improving the Quick sort

Question

If possible, how can I improve the following quick sort(performance wise). Any suggestions?

void main()
    {
      quick(a,0,n-1);
    }

    void quick(i         


        

Answer 1

Try another sort algorithm.

Depending on your data, you may be able to trade memory for speed.

According to Wikipedia

• Quick sort has a best case O(n log n) performance and O(1) space
• Merge sort has a fixed O(n log n) performance and O(n) space
• Radix sort has a fixed O(n . ) perfomance and O(n . ) space

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Edit

Apparently your data is integers. With 2.5M integers in the range [0, 0x0fffffff], my implementation of radix-sort is about 4 times as fast as your implementation of quick-sort.

$ ./a.out
qsort time: 0.39 secs
radix time: 0.09 secs
good: 2000; evil: 0

#include 
#include 
#include 

#define ARRAY_SIZE 2560000
#define RANDOM_NUMBER (((rand() << 16) + rand()) & 0x0fffffff)

int partition(int a[], int lower, int upper) {
  int pivot, i, j, temp;
  pivot = a[lower];
  i = lower + 1;
  j = upper;
  while (i < j) {
    while((i < upper) && (a[i] <= pivot)) i++;
    while (a[j] > pivot) j--;
    if (i < j) {
      temp = a[i];
      a[i] = a[j];
      a[j] = temp;
    }
  }
  if (pivot > a[j]) {
    temp = a[j];
    a[j] = a[lower];
    a[lower] = temp;
  }
  return j;
}

void quick(int a[], int lower, int upper) {
  int loc;
  if (lower < upper) {
    loc = partition(a, lower, upper);
    quick(a, lower, loc-1);
    quick(a, loc+1, upper);
  }
}

#define NBUCKETS 256
#define BUCKET_SIZE (48 * (1 + ARRAY_SIZE / NBUCKETS))

/* "waste" memory */
int bucket[NBUCKETS][BUCKET_SIZE];

void radix(int *a, size_t siz) {
  unsigned shift = 0;
  for (int dummy=0; dummy<4; dummy++) {
    int bcount[NBUCKETS] = {0};
    int *aa = a;
    size_t s = siz;
    while (s--) {
      unsigned v = ((unsigned)*aa >> shift) & 0xff;
      if (bcount[v] == BUCKET_SIZE) {
        fprintf(stderr, "not enough memory.\n");
        fprintf(stderr, "v == %u; bcount[v] = %d.\n", v, bcount[v]);
        exit(EXIT_FAILURE);
      }
      bucket[v][bcount[v]++] = *aa++;
    }
    aa = a;
    for (int k=0; k