Conditional Evaluation

• Suppose: if your income is less than $10,000 you pay no income tax. Otherwise, you pay 20%.

• Scheme's if expression:

      (if test-expr then-expr else-expr) 

  • test-expr must evaluate to true or false

  • then-expr is evaluated if test-expr is true

  • else-expr is evaluated if test-expr is false

• Procedures that return true or false are predicates. Examples: <, >, =, >=, <=, zero?, even?, equal?

• Example: (if (= 2 3) -10 37.5) evalutes to 37.5

• then-expr and else-expr can be any expressions