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Insertion Sort

The insertion sort, unlike the other sorts, passes through the array only once.  The insertion sort is commonly compared to organizing a handful of playing cards.  You pick up the random cards one at a time.  As you pick up each card, you insert it into its correct position in your hand of organized cards. 

The insertion sort splits an array into two sub-arrays. The first sub-array (like the cards in your hand) is always sorted and increases in size as the sort continues. The second sub-array (like the cards to be picked up) is unsorted, contains all the elements yet to be inserted into the first sub-array, and decreases in size as the sort continues.
 

Let's look at our same example using the insertion sort for descending order.
 

Array at beginning: 

84 69 76 86 94 91
 
= 1st sub-array
84 69 76 86 94 91
 
= 2nd sub-array
84 69 76 86 94 91
  84 76 69 86 94 91
  86 84 76 69 94 91
  94 86 84 76 69 91

2nd sub-array empty

94 91 86 84 76 69


The insertion sort maintains the two sub-arrays within the same array.  At the beginning of the sort, the first element of the first sub-array is considered the "sorted array".  With each pass through the loop, the next element in the unsorted second sub-array is placed into its proper position in the first sorted sub-array.

The insertion sort can be very fast and efficient when used with smaller arrays.  Unfortunately, it loses this efficiency when dealing with large amounts of data

// Insertion Sort Method for Descending Order
public static void InsertionSort( int [ ] num)
{
     int j;                     // the number of items sorted so far
     int key;                // the item to be inserted
     int i; 

     for (j = 1; j < num.length; j++)    // Start with 1 (not 0)
    {
           key = num[ j ];
           for(i = j - 1; (i >= 0) && (num[ i ] < key); i--)   // Smaller values are moving up
          {
                 num[ i+1 ] = num[ i ];
          }
         num[ i+1 ] = key;    // Put the key in its proper location
     }
}

    


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