Algorithms and Data Structures

N.Wirth. Algorithms and Data Structures. Oberon version 53
2.2.1 Sorting by Straight Insertion
This method is widely used by card players. The items (cards) are conceptually divided into a
destination sequence a 0 ... a i-1 and a source sequence a i ... a n-1 . In each step, starting with i = 1 and
incrementing i by unity, the ith element of the source sequence is picked and transferred into the destination
sequence by inserting it at the appropriate place. The process of sorting by insertion is shown in an
example of eight numbers chosen at random (see Table 2.1).
Initial keys 44 55 12 42 94 18 06 67
i=1 44 55 12 42 94 18 06 67
i=2 12 44 55 42 94 18 06 67
i=3 12 42 44 55 94 18 06 67
i=4 12 42 44 55 94 18 06 67
i=5 12 18 42 44 55 94 06 67
i=6 06 12 18 42 44 55 94 67
i=7 06 12 18 42 44 55 67 94
Table 2.1. A Sample Process of Straight Insertion Sorting.
The algorithm of straight insertion is
FOR i := 1 TO n-1 DO
x := a[i];
insert x at the appropriate place in a 0 ... a i-1
END
In the process of actually finding the appropriate place, it is convenient to alternate between
comparisons and moves, i.e., to let x sift down by comparing x with the next item a j , and either inserting x
or moving a j to the right and proceeding to the left. We note that there are two distinct conditions that may
cause the termination of the sifting down process:
1. An item a j is found with a key less than the key of x.
2. The left end of the destination sequence is reached.
PROCEDURE StraightInsertion; (* ADenS2_Sorts *)
VAR i, j: INTEGER; x: Item;
BEGIN
FOR i := 1 TO n-1 DO
x := a[i]; j := i;
WHILE (j > 0) & (x < a[j-1]) DO
a[j] := a[j-1]; DEC(j)
END;
a[j] := x
END
END StraightInsertion
Analysis of straight insertion. The number C i of key comparisons in the i-th sift is at most i-1, at least
1, and — assuming that all permutations of the n keys are equally probable — i/2 in the average. The
number M i of moves (assignments of items) is C i + 1. ). Therefore, the total numbers of comparisons and