Programs often need to take different actions depending on the data. The if statement allows this:
| (if | test | thenform | ) | |
| (if | test | thenform | elseform | ) |
The test is evaluated first. If it returns a value that is considered true ( anything other than #f), the thenform is evaluated and its value is the value of the if. If the value of the test is #f, the elseform is evaluated and its value is the value of the if.
(define (abs x)
(if (< x 0)
(- x)
x ) )
Note that the thenform and elseform are single forms; if multiple things need to be done, they must be enclosed in a begin.