accessing elements of list in scheme

Question

How do I change elements of list in Scheme. I want a procedure to change the minimum elements of a list to another number, so

If I have a procedure called proc and I give it two arguments ( a list and a number), my procedure would work like this: (proc (list 1 2 3 1) 9) returns '(9 2 3 9). so 9 take the place of the minimum value(s) of the list. I know I can apply min to get the minimum value, but I don't know how to modify a single element of the list.

Since Scheme doesn't have variables to hold values, I thought about using either let or letrec, but I don't know which would be the difference between using let and letrec.

Answer 1

This can be split into two distinct tasks - getting the lowest value in the list, then replacing that value with our new value. We can get the lowest value by running the sort function on our list and sorting by least to greatest, then using apply min to get the first element of the list.

After we've got that, we can use map to go through the list, replacing any instances of the lowest number with our new number. All in all, the complete function should look like this:

    (define (replace-least lst new)
      (let ((lowest (apply min lst)))
        (map (lambda (x) (if (= x lowest) new x)) lst)))

I tested this with DrRacket 5.3 and it performed perfectly as per the specifications provided in your question. If you have any trouble, let me know.

Answer 2

This is an improved, working solution using min:

(define (replace-min lst elt)
  (let ((m (apply min lst)))
    (map (lambda (x) (if (= x m) elt x))
         lst)))

Notice that min is the simplest way to find the minimum element in a list.