Comments on snippet: 'list-index (Scheme)'

random_number said on 12/13/09

Sun, 13 Dec 2009 00:21:40 GMT

switch the order of the first two conditional pairs so (empty? los) is checked before (first los)

random_number said on 12/12/09

Sat, 12 Dec 2009 23:03:36 GMT

Both versions could result in repeated recursion if los contains s as res has to be fully evaluated in order to determine if it equals -1. A helper function would come in handy. In Clojure:

`
    (defn list-index  
      ([s los] (list-index 0 s los))  
      ([i s los]  
        (cond  
          (= (first los) s) i  
          (empty? los) -1  
          :else (recur (inc i) s (rest los)))))  
`

the_coder said on 6/4/08

Wed, 04 Jun 2008 22:47:32 GMT

Slightly better:

(define (list-index s los)

  (cond ((null? los) -1)

        ((eq? s (car los)) 0)

        (else

         (let ((res (list-index s (cdr los))))

           (if (= res -1)

               -1

               (+ res 1))))))