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