Conditional Expressions
Abelson and Sussman introduce some dynamism into their procedures by using conditional statements. The procedures in the previous sections were simple and were unidimensional because they would only express what was initially given by the program. By incorporating conditional statements, the procedures can now respond to outcomes according to a set of rules. Conditions are set primarily with predicates, which are statements that return or evaluate to true or false. If the predicate returns true, the consequent expression is executed, while if the predicate returns false, an alternative is executed or the procedure is undefined.
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