Question

Sorting a list of Colors.

If two colors’ red values are the same, then the one with smaller green value appears first in the ordering. If two colors have the same red and green value, then the one with smaller blue value appears first in the ordering.

Example: let the color list be

((72 75 0) (0 0 10) (255 255 255) (0 10 10) (0 10 8)(50 100 255))

Then the procedure would return

((0 0 10) (0 10 8) (0 10 10) (50 100 255) (72 75 0) (255 255 255))

Answer 1

You should write a custom comparator for sort:

(define color-comparator
  (lambda (c1 c2)
    (cond
      ((not (= (car c1) (car c2))) (< (car c1) (car c2)))     ; red
      ((not (= (cadr c1) (cadr c2))) (< (cadr c1) (cadr c2))) ; green
      (else (< (caddr c1) (caddr c2))))))                     ; blue

Then you can just pass this function to the sort procedure.

Guile:

(sort
  '((72 75 0) (0 0 10) (255 255 255) (0 10 10) (0 10 8)(50 100 255))
  color-comparator)

Chez Scheme:

(sort
  color-comparator
  '((72 75 0) (0 0 10) (255 255 255) (0 10 10) (0 10 8)(50 100 255)))