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Nested Loops

In addition to the rules which apply to single loops, the following rules apply to nested loops:

Each nested FOR-NEXT loop must begin with its own FOR-TO statement and end with its own NEXT statement.

An outer loop and an inner (nested) loop cannot have the same running variable.

Each inner (nested) loop must be completely embedded within an outer loop, the loops cannot overlap.

Control can be transferred from an inner loop to a statement in an outer loop or to a statement outside of the entire nest. However, control cannot be transferred to a statement within a nest from a point outside the nest.

The following example shows the structure of a nested loop.


The inner loop (statements 90 through 115) is completely embedded within the outer loop (statements 65 through 140). Each loop begins and ends with its own FOR-TO and NEXT statements, and each loop has its own running variable. You will notice the running variable of the outer loop (X) is used as the initial value for the running variable of the inner loop (Y). This is allowed since the value of X is not changed within the inner loop.

By using nested loops, you are able to perform repeated sets of instructions within another set of instructions.


The outer loop (lines 30-1 10) will be executed 10 times, while the inner loop (lines 60-90) will be executed 3 times for each time the outer loop is executed. This means the inner loop is executed a total of 30 times.


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