Computer Programming Concepts and Visual Basic David I. Schneider

188 Computer Programming Concepts and Visual Basic
Private Sub SwapData(index As Integer)
‘Swap entries
Call SwapStr(city(index), city(index + 1))
Call SwapNum(pop(index), pop(index + 1))
Call SwapNum(income(index), income(index + 1))
Call SwapNum(natives(index), natives(index + 1))
Call SwapNum(advDeg(index), advDeg(index + 1))
End Sub
Private Sub SwapNum(a As Single, b As Single)
Dim temp As Single
‘Interchange values of a and b
temp = a
a = b
b = temp
End Sub
Private Sub SwapStr(a As String, b As String)
Dim temp As String
‘Interchange values of a and b
temp = a
a = b
b = temp
End Sub
[Run, and click the command button.]
■ SHELL SORT
The bubble sort is easy to understand and program. However, it is too slow for really long
lists. The Shell sort, named for its inventor, Donald L. Shell, is much more efficient in such
cases. It compares distant items first and works its way down to nearby items. The interval separating
the compared items is called the gap. The gap begins at one-half the length of the list
and is successively halved until eventually each item is compared with its neighbor as in the
bubble sort. The algorithm for a list of n items is as follows:
1. Begin with a gap of g = Int(n / 2).
2. Compare items 1 and 1 + g, 2 and 2 + g, . . . , n – g and n. Swap any pairs that
are out of order.
3. Repeat Step 2 until no swaps are made for gap g.
4. Halve the value of g.
5. Repeat Steps 2, 3, and 4 until the value of g is 0.