## Booleans

Scheme uses the syntax `#t` and `#f` for Boolean true and false value, respectively. 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` 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 the `cond` form as an alternative to `if`:

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

The null value (written as `#!null` in Kawa or `null` in Java) is also considered as false.