If

Conditional structure (I)

    (if logical_expression
        do_statement
    )

Using progn, the single do_statement can be replaced by a block of statements:

    (if logical_expression
        (progn
            do_statements
        )
    )

Examples

(if (< 0 1.5) "yes")	"yes"
(if (= 2.5 3.5) "yes")	nil

(defun test_if (input)
    (if (= input 0)
        (progn
            (princ "First, do this.\n")
            (princ "Then, do that.")
            (princ)
        )
    )
)

(test_if 0)
(test_if 1)