The bubble sort gets its name because as elements are sorted they gradually "bubble" (or rise) to their proper positions, like bubbles rising in a glass of soda. A bubble sort repeatedly compares adjacent elements of an array, starting with the first and second elements, and swapping them if they are out of order. After the first and second elements are compared, the second and third elements are compared, and swapped if they are out of order.

This process continues until the end of the list is reached. Remember that a bubble sort will continue until no swaps have occurred, meaning that the array is in the proper sorted order. When the end is reached, the bubble sort returns to elements one and two and starts the process all over again.

So, when does it stop? The bubble sort knows that it is finished when it examines the entire array and no "swaps" are needed (thus the list is in proper order). The bubble sort keeps track of the occurrence of swaps by the use of a flag. The table below follows an array of numbers before, during, and after a bubble sort for descending order.

A "pass" is defined as one full trip through the array comparing and if necessary, swapping, adjacent elements. Several passes have to be made through the array before it is finally sorted. Array at beginning: 84 69 76 86 94 91 After Pass #1: 84 76 86 94 91 69 After Pass #2: 84 86 94 91 76 69 After Pass #3: 86 94 91 84 76 69 After Pass #4: 94 91 86 84 76 69 After Pass #5 (done): 94 91 86 84 76 69



// Bubble Sort Method for Descending Order

public static void bubble_sort( int [ ] num )

{

     int j;

     boolean flag = true;   // set flag to true to begin initial pass

     int temp;   //holding variable
     while ( flag )

     {

            flag= false;    //set flag to false awaiting a possible swap

            for( j=0;  j < num.length -1;  j++ )

            {

                   if ( num[ j ] < num[j+1] )   // change to > for ascending sort

                   {

                           temp = num[ j ];                //swap elements

                           num[ j ] = num[ j+1 ];

                           num[ j+1 ] = temp;

                          flag = true;              //indicates that a swap occurred

                  }

            }

      }

}

The bubble sort is an easy algorithm to program, but it is slower than many other sorts. With a bubble sort, it is always necessary to make one final "pass" through the array to check to see that no swaps are made to ensure that the process is finished. In actuality, the process is finished before this last pass is made.

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