Visual Basic.NET How to Program (PDF)

Chapter 7 Arrays 271
Fig. Fig. 7.11 7.11 Sorting an array with bubble sort (part 3 of 3).
The program contains methods cmdCreate_Click and cmdSort_Click.
Method cmdCreate_Click (lines 26–43) assigns 10 random values to the elements of
array and displays the contents of the array in txtOriginal. Method
cmdSort_Click (lines 46–62) sorts array by calling procedure BubbleSort from
modBubbleSort.
Procedure BubbleSort receives the array as parameter sortArray. The nested
For/Next structures in lines 10–20 of Fig. 7.10 performs the sort. The outer loop controls
the number of passes of the array. The inner loop (lines 12–18) controls the comparisons
and swapping (if necessary) of the elements during each pass.
Procedure BubbleSort first compares sortArray(0) to sortArray(1), then
sortArray(1) to sortArray(2), and so on until it completes the pass by comparing
sortArray(8) to sortArray(9). Although there are 10 elements, the comparison
loop performs only nine comparisons (because the comparisons each involve a pair of numbers).
The comparisons performed in a bubble sort could cause a large value to move down
the array (sink) many positions on a single pass. However, a small value cannot move up
(bubble) more than one position per pass. On the first pass, the largest value is guaranteed
to sink to the bottom element of the array, sortArray(9). On the second pass, the
second-largest value is guaranteed to sink to sortArray(8). On the ninth pass, the ninth
largest value sinks to sortArray(1), leaving the smallest value in sortArray(0).
Thus, only nine passes are required to sort a 10-element array (and, in general, only n-1
passes are needed to sort an n-element array).
If a comparison reveals that the two elements are in descending order, BubbleSort
calls procedure Swap to exchange the two elements, placing them in ascending order in the
array. Procedure Swap receives the array (which it calls swapArray) and the index of the
first element of the array to transpose (with the subsequent element). The exchange is performed
by three assignments
hold = swapArray(first)
swapArray(first) = swapArray(first + 1)
swapArray(first + 1) = hold