recursive-each-plus-others.rkt

#lang racket/base

;; others: generates a list of characters in `str` *other*
;; than the one at index `pos`
(define (others str pos [i 0] [res '()])
  (cond [(= i (string-length str))
         (reverse res)]
        [(= pos i)
         (others str pos (add1 i) res)]
        [else
         (others str pos (add1 i) (cons (string-ref str i) res))]))

    
;; each+others: generates a list of lists of characters. Each
;; list's first element is a unique character of `str` and the
;; rest of the elements are the rest of the string's characters.
(define (each+others str [i 0] [res '()])
  (cond [(= i (string-length str))
         (reverse res)]
        [else (each+others str
                           (add1 i)
                           (cons (cons (string-ref str i) (others str i))
                                 res))]))