Structure Rule Three: Alternation

If-then-else is sometimes called alternation (because there are alternative choices). In structured programming, each choice is a code block. The decision box (the diamond) picks the correct branch so that the entry conditions for that block are correct.

If alternation is arranged as in the flowchart at right, then there is one entry point (at the top) and one exit point (at the bottom). The structure should be coded so that if the entry conditions are satisfied, then the exit conditions are fulfilled (just like a code block).

Rule 3 of Structured Programming: The alternation of two code blocks is structured.

An example of an entry condition for an alternation structure is: register $8 contains a signed integer. The exit condition might be: register $8 contains the absolute value of the signed integer. The branch structure is used to fulfill the exit condition.