Info Catalog eintr: Recursive triangle function eintr: Recursion eintr: Recursive Patterns

eintr: Recursion with cond

 
 11.3.5 Recursion Example Using ‘cond’
 -------------------------------------
 
 The version of ‘triangle-recursively’ described earlier is written with
 the ‘if’ special form.  It can also be written using another special
 form called ‘cond’.  The name of the special form ‘cond’ is an
 abbreviation of the word ‘conditional’.
 
    Although the ‘cond’ special form is not used as often in the Emacs
 Lisp sources as ‘if’, it is used often enough to justify explaining it.
 
    The template for a ‘cond’ expression looks like this:
 
      (cond
       BODY...)
 
 where the BODY is a series of lists.
 
    Written out more fully, the template looks like this:
 
      (cond
       (FIRST-TRUE-OR-FALSE-TEST FIRST-CONSEQUENT)
       (SECOND-TRUE-OR-FALSE-TEST SECOND-CONSEQUENT)
       (THIRD-TRUE-OR-FALSE-TEST THIRD-CONSEQUENT)
        ...)
 
    When the Lisp interpreter evaluates the ‘cond’ expression, it
 evaluates the first element (the CAR or true-or-false-test) of the first
 expression in a series of expressions within the body of the ‘cond’.
 
    If the true-or-false-test returns ‘nil’ the rest of that expression,
 the consequent, is skipped and the true-or-false-test of the next
 expression is evaluated.  When an expression is found whose
 true-or-false-test returns a value that is not ‘nil’, the consequent of
 that expression is evaluated.  The consequent can be one or more
 expressions.  If the consequent consists of more than one expression,
 the expressions are evaluated in sequence and the value of the last one
 is returned.  If the expression does not have a consequent, the value of
 the true-or-false-test is returned.
 
    If none of the true-or-false-tests test true, the ‘cond’ expression
 returns ‘nil’.
 
    Written using ‘cond’, the ‘triangle’ function looks like this:
 
      (defun triangle-using-cond (number)
        (cond ((<= number 0) 0)
              ((= number 1) 1)
              ((> number 1)
               (+ number (triangle-using-cond (1- number))))))
 
 In this example, the ‘cond’ returns 0 if the number is less than or
 equal to 0, it returns 1 if the number is 1 and it evaluates ‘(+ number
 (triangle-using-cond (1- number)))’ if the number is greater than 1.
 

Info Catalog eintr: Recursive triangle function eintr: Recursion eintr: Recursive Patterns