Shell Sort

Sorting Algorithm

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In this tutorial we will be learning about Shell sort algorithm.

About Shell Sort

It is a generalisation of the insertion sort. In this sorting algorithm we compare elements that are distant apart rather than adjacent.

We start by comparing elements that are at a certain distance apart. So, if there are N elements then we start with a value gap < N.

In each pass we keep reducing the value of gap till we reach the last pass when gap is 1.

In the last pass shell sort is like insertion sort.

C Code

#include <stdio.h>

//function declaration
void display(int arr[], int size);
void shellSort(int arr[], int size);

int main(void) {
  //unsorted elements
  int arr[] = {14,18,19,37,23,40,29,30,11};

  //size of the array
  int n = sizeof(arr)/sizeof(arr[0]);

  //output unsorted elements
  display(arr, n);

  //sort the elements
  shellSort(arr, n);

  //display sorted elements
  display(arr, n);

  return 0;
}

void display(int arr[], int size) {
  int i;
  for(i = 0; i < size; i++) {
    printf("%d ", arr[i]);
  }
  printf("\n");
}

void shellSort(int arr[], int size) {
  int i, j, gap, temp;

  //we start with a bigger gap
  gap = size/2;

  while(gap > 0) {
    //now we will do the insertion sort of the gapped elements
    i = gap;

    while(i < size) {
      temp = arr[i];

      //shifting gap sorted element to correct location
      for(j = i; (j >= gap) && (arr[j - gap] > temp); j -=gap) {
        arr[j] = arr[j - gap];
      }
      arr[j] = temp;

      //increase i
      i++;
    }

    //reduce the gap by half
    gap = gap / 2;
  }
}
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