Friday, March 1, 2013

What is insertion sort? How to implement in C++?

Insertion sort similar to card ordering in your hand as you pick the cards one by one from the lot. The idea of the sort is to go through numbers from the beginning towards end one by one. At each time, check the number in question should be placed in the already visited numbers and move the numbers to make space for it. When all the numbers are visited, the list should be sorted. See below example output to see the operation.
  1. Average and Worst case performance are O(N^2), best case is O(N) (--> nearly sorted input).
  2. Need not have full list during start and can start as the numbers arrive.
  3. Stable and in-place.
  4. Below example, uses swapping of numbers, but one could directly move the number in question to its place and make efficient block move of the needed numbers.

Implementation of insertion sort

#include <iostream>

using namespace std;
const int INPUT_SIZE = 10;

void print(int *input)
{
    for ( int i = 0; i < INPUT_SIZE; i++ )
        cout << input[i] << " ";
    cout << endl;
}

void insertionsort(int* input)
{
    for (int i=1;i<INPUT_SIZE;i++)
    {
        for (int j=i;j>0;j--)
        {
            if (input[j]<input[j-1])
            {
                int tmp = input[j];
                input[j] = input[j-1];
                input[j-1] = tmp;
            } else
                break;
        }
        cout << "PASS: ";
        print(input);
    }
}

int main()
{
    int input[INPUT_SIZE] = {500, 700, 800, 100, 300, 200, 900, 400, 1000, 600};
    cout << "Input: ";
    print(input);
    insertionsort(input);
    cout << "Output: ";
    print(input);
    return 0;
}


Output:-
Input: 500 700 800 100 300 200 900 400 1000 600 
PASS: 500 700 800 100 300 200 900 400 1000 600 
PASS: 500 700 800 100 300 200 900 400 1000 600 
PASS: 100 500 700 800 300 200 900 400 1000 600 
PASS: 100 300 500 700 800 200 900 400 1000 600 
PASS: 100 200 300 500 700 800 900 400 1000 600 
PASS: 100 200 300 500 700 800 900 400 1000 600 
PASS: 100 200 300 400 500 700 800 900 1000 600 
PASS: 100 200 300 400 500 700 800 900 1000 600 
PASS: 100 200 300 400 500 600 700 800 900 1000 
Output: 100 200 300 400 500 600 700 800 900 1000 

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