eharetea

Recursive string substitutions in lists in Scheme.

Recursive string substitutions in lists in Scheme, a dialect of Lisp.

Date Created:Tuesday September 16th, 2008 12:35 PM
Date Modified:Tuesday September 16th, 2008 12:54 PM

Here we simply pass in a list, new and old word for replacing.

(define (substitute l old new)
  (if (null? l)
      '()
      (append 
              (if (list? (car l))
               (list (substitute (car l) old new))
               (if (equal? (car l) old)
                (list new)
                (list (car l)) 
               )
              )   
              (substitute (cdr l) old new)
      )
  )
)

#|

Example call: 
(substitute '((ha blah) (ha (ha blah)) ((ha blah) (ha (blah ha)))) 'blah 'ha)

|#

Here we can replace a list of text recursively by passing in an old list and a new list of words to replace.

(define (substitute l old new)
  (if (null? l)
      '()
      (append 
              (if (list? (car l))
               (list (substitute (car l) old new))
               (if (member? (car l) old)
                (list (item (return-n (car l) old 1) new))
                (list (car l)) 
               )
              )   
              (substitute (cdr l) old new)
      )
  )
)

(define (return-n seed haystack n)
 (if (equal? (item n haystack) seed)
     n
     (return-n seed haystack (+ n 1))
 )
)

Substitute words in lists by Dan Lynch
is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License
Based on a work at www.3daet.com
Permissions beyond the scope of this license may be available at http://www.3daet.com