Racket what does the procedure

Question

I have a procedure but actually I don't know what it does. Will anybody explain it?

(define (stj fun listt)
  (if (null? listt)
      `()
      (cons (fun (car listt)) (stj fun (cdr listt)))))

Answer 1

It's the map procedure, check the documentation. It takes a procedure and a list as parameters, and applies the procedure to each of the elements in the input list, producing an output list with the results:

(stj sqr '(1 2 3 4 5))
=> '(1 4 9 16 25)