The Bubble Sort Algorithm
The algorithm for bubble sort requires a pair of nested loops. The
outer loop must iterate once for each element in the data set (of size
n) while the inner loop iterates n times the first time it is entered, n-1
times the second, and so on. Consider the purpose of each loop. As
explained above, bubble sort is structured so that on each pass through
the list the next largest element of the data is moved to its proper
place. Therefore, to get all n elements in their correct places, the
outer loop must be executed n times.
The inner loop is executed on each iteration of the outer loop. Its
purpose is to put the next largest element is being put into place.
The inner loop therefore does the comparing and swapping of adjacent
elements. To determine the complexity of this loop, we calculate the number
of comparisons that have to be made. On the first iteration of the outer
loop, while trying to place the largest element, there have to be n - 1
comparisons: the first comparison is made between the first and second
elements, the second is made between the second and third elements, and so
on until the n-1th comparison is made between the n-1th and the nth element.
On the second iteration of the outer loop, there is no need to compare the
against the last element of the list, because it was put in the correct place
on the previous pass. Therefore, the second iteration requires only n-2
comparisons. This pattern continues until the second-to-last iteration of
the outer loop when only the first two elements of the list are unsorted;
clearly in this case, only one comparison is necessary. The total number
of comparisons, therefore, is (n - 1) + (n - 2)...(2) + (1) = n(n - 1)/2 or
O(n2).
The best case for bubble sort occurs when the list is already sorted or
nearly sorted. In the case where the list is already sorted, bubble
sort will terminate after the first iteration, since no swaps were made.
Any time that a pass is made through the list and no swaps were made,
it is certain that the list is sorted. Bubble sort is also efficient
when one random element needs to be sorted into a sorted list, provided
that new element is placed at the beginning and not at the end. When
placed at the beginning, it will simply bubble up to the correct place,
and the second iteration through the list will generate 0 swaps, ending
the sort. Recall that if the random element is placed at the end,
bubble sort loses its efficiency because each element greater than it
must bubble all the way up to the top.
The absolute worst case for bubble sort is when the smallest element of
the list is at the large end. Because in each iteration only the largest
unsorted element gets put in its proper location, when the
smallest element is at the end, it will have to be swapped each time
through the list, and it wont get to the front of the list until all n
iterations have occurred. In this worst case, it take n iterations of
n/2 swaps so the order is, again, n2.
Best Case: n
Average Case: n2
Worst Case: n2
