In racket, built in function to tell what position a variable is in

Question

I have two lists, one of symbols and one of numbers. I need to find the maximum of the list of numbers and output the associated symbol. I have a way to find the maximum, but seem unable to also fit in the first list.

So is there a way to input a number and a lit, and it tells what position the number is in? So then I can use list-ref on the list of symbols to find the proper symbol?

Unless someone can help me with the above problem to put it all in one function? I have this outline:

(define (max-list lon)
  (cond [(empty? (rest lon)) (first lon)]
        [(> (first lon) (max-list (rest lon))) (first lon)]
        [else (max-list (rest lon))]))

to find the max-value for the list of numbers.

Answer 1

You can use the argmax function to do this conveniently. You can use map with cons to "zip" the two lists into a series of pairs, then pass the result into argmax to find the pair you want.

(define num-list '(4 8 2 7))
(define sym-list '(a b c d))

(cdr (argmax car (map cons num-list sym-list)))
; => 'b

The way argmax works is that it takes an "optimizer" procedure that returns a numeric value for each element of the list, and the result of the function is the first element that maximizes the value.