programming-for-dummies

398
Using Shell Sort
32 9 74 21
Unsorted list
32
9
74 21
Each low value is yanked out of the list . . .
9 32 74 21
. . . and inserted in its proper, sorted location at the
beginning of the list.
Figure 1-4:
Insertion
sort only
examines a
list once to
sort it.
21
9 32 74
9 21 32 74
Final sorted list
One major advantage of the insertion sort algorithm is that it only needs to
examine an entire list once to sort it. In comparison, bubble sort must repetitively
examine the entire list multiple times, and selection sort must repetitively
examine progressively smaller lists multiple times. As a result, the
insertion sort algorithm is much faster while being easy to implement as
well.
Using Shell Sort
To speed up the performance of the insertion sort algorithm, Donald Shell, a
computer scientist, created a variation of the insertion sort algorithm
dubbed shell sort.
One problem with insertion sort is that it must examine one value at a time.
In a long list, this can take a long time. Shell sort works by dividing a long list
into several smaller ones and then performing an insertion sort on each
smaller list. After each smaller list gets sorted, the shell sort algorithm uses
an ordinary insertion sort to sort the entire list one last time. By this time,
the longer list is nearly sorted so this final insertion sort occurs quickly, as
shown in Figure 1-5.
Basically, shell sort works like this:
1. Divide a long list into multiple smaller lists.
2. Arrange each list in a grid or table consisting of rows and columns.