Using Racket ISL to define functions using list abstractions and local/lambda

Question

I am trying to design a function that, given a list of numbers, returns the squares of the even numbers in a supplied list. I do not want to compute squares that will not be used in the end result.

This is what I have using map and filter:

; even-squares-only : [List-of Number] -> [List-of Number]
; returns square of only even numbers in a supplied list

(define (even-squares-only lon)
  (map (λ (n) (sqr n)) (filter even? lon)))

How would I go about designing the same function using the foldr function so that I can iterate over the elements of the list once instead of twice (I am trying to use local or lambda in this function as well)?

Answer 1

In ISL with lambda:

(define (even-squares-only lon)
  (foldr (lambda (element result)
           (if (even? element)
               (cons (sqr element) result)
               result))
         '()
         lon))

This exercise can be even done without local and lambda , if you rewrite the anonymous function into a named one:

(define (reducing-fn element result)
  (if (even? element)
      (cons (sqr element) result)
      result))

(define (even-squares-only lon)
  (foldr reducing-fn
         '()
         lon))

Example:

> (even-squares-only '(1 2 3 4 5 6 7 8 9 10))
(list 4 16 36 64 100)