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Nesting is when one programming construct is included within another. Nesting enables powerful yet simple programming. It reduces the amount of code needed, while making it simple for a programmer to debug and edit.

Nested selection

When using selection, the number of possible paths at a decision point can be increased by including one selection within another.

This pseudo-code program uses nested selection:

SEND ‘How old are you?‘ TO DISPLAY
                    RECEIVE age FROM (INTEGER) KEYBOARD
                    IF age > 16 THEN
                         SEND ‘You are old enough to drive a car
                         and ride a moped’ TO DISPLAY
                    ELSE
                         IF age = 16 THEN
                              SEND ‘You are old enough to ride
                              a moped!’ TO DISPLAY
                         ELSE
                              SEND ‘Come back when you are older!’
                              TO DISPLAY
                         END IF
                    END IF

Here, there are two conditions that may be tested, resulting in three possible paths to follow. The second condition is only tested if the result of the first test is FALSE.

Nested selection can be built up over and over to include many possibilities and paths for a program to follow. It enables complex decisions to be made.

Nested iteration

Iteration can also be nested. This pseudo-code program uses two count-controlled loops - one within another - to print out the times table for all numbers from one to ten:

FOR x FROM 1 TO 10 DO
                         FOR y FROM 1 TO 10 DO
                              SET result TO y * x
                              SEND y & " * " & x & " = " & result
                              TO DISPLAY
                         END FOR
                    END FOR

For every iteration of x, y is iterated ten times.

This program uses condition-controlled WHILE loops, one within another, to print out the times table for all numbers from one to ten:

SET x TO 1
                    SET y TO 1
                    WHILE x < 11 DO
                         WHILE y < 11 DO
                              SET result TO y * x
                              SEND y & " * " & x & " = " & result
                              TO DISPLAY
                              y = y + 1
                         END WHILE
                         SET y TO 1
                         SET x TO x + 1
                    END WHILE

For every iteration of x, y is iterated ten times.