 |
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.
The 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.
|
|
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 |
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.
// Bubble Sort Function for Descending Order
void bubble_sort(apvector <int> &array)
{
int i, j, flag = 1;
// set flag to 1 to begin initial pass
int temp;
// holding variable
int arrayLength = array.length( );
for(i = 1; (i <= arrayLength) && flag; i++)
{
flag = 0;
for (j=0; j < (arrayLength -1); j++)
{
if (array[j+1] > array[j])
// ascending order simply changes to <
{
temp = array[j];
// swap elements
array[j] = array[j+1];
array[j+1] = temp;
flag = 1;
// indicates that a swap occurred.
}
}
}
return;
//arrays are passed to functions by address;
nothing is returned
}
|
