Booleans

Scheme uses the syntax #t and #f for Boolean true and false value. For example the less-than function is named <. Its result is true if the first argument is less than the second (or if there are more than two arguments: that they are in increasing order):

(< 3 4) ⇒ #t
(< -3 -4) ⇒ #f
(< 2 3 5 7 11)) ⇒ #t

The if special form takes two or three sub-expressions: It evaluates the first expression. If that is true it evaluates the second expression; otherwise it evaluates the third expression, if provided:

(if (< 3 4) (+ 5 5) (+ 5 6)) ⇒ 10

We call if a special form rather than a function, because for a function all the arguments are evaluated before the function is called, but in a special form that is not neceassarily the case.

In addition to #t any value except #f counts as true when evaluating the first expression of an if:

(if 0 (+ 5 5) (+ 5 6)) ⇒ 11

You can use and, or, and not and to create complex boolean expressions. Of these and and or are special forms that only evaluate as many of the sub-expressions as needed.

(if (not (and (>= i 0) (<= i 9)))
    (display "error"))

You can use cond form as an alternative to if:

(cond ((> 3 3) ’greater)
      ((< 3 3) ’less)
      (else ’equal))       ⇒ equal