# [plt-scheme] Doubt in folding

 From: Matthias Felleisen (matthias at ccs.neu.edu) Date: Thu Oct 22 14:32:54 EDT 2009 Previous message: [plt-scheme] Doubt in folding Next message: [plt-scheme] Doubt in folding Messages sorted by: [date] [thread] [subject] [author]

```
Two changes:

#lang scheme

;; my-fold-left : (X X -> X) X (listof X) -> (listof X)
;; applies operation to each element of lox, and
;; combines them in a new list. Works from left to right
;; in lox.
(define (my-fold-left combinator initial lox)
(cond
[(empty? lox) initial]
[else
(combinator (first lox) (my-fold-left combinator initial (rest
lox)))]))

;; plt's built in foldl
(foldr cons empty (list 1 2 3 4))       ; evals to what I expected ->
(4 3 2 1)

;; mine
(my-fold-left cons empty (list 1 2 3 4))   ; evals to ((((() . 4) .
3) . 2) . 1), but why ?

;; ----

Hint: Foldl is an accumulator function.

On Oct 22, 2009, at 2:25 PM, Eduardo Bellani wrote:

> Small doubt in a fuction I've been playing with:
>
> ;; my-fold-left : (X X -> X) X (listof X) -> (listof X)
> ;; applies operation to each element of lox, and
> ;; combines them in a new list. Works from left to right
> ;; in lox.
> (define (my-fold-left combinator initial lox)
>  (cond
>    [(empty? lox) initial]
>    [else
>     (combinator (my-fold-left combinator
>                               initial
>                               (rest lox))
>                 (first lox))]))
>
> ;; plt's built in foldl
> (foldl cons empty (list 1 2 3 4))       ; evals to what I expected -
> > (4 3 2 1)
>
> ;; mine
> (my-fold-left cons empty (list 1 2 3 4))   ; evals to ((((() . 4) . 3)
> . 2) . 1), but why ?
>
>
> Thanks for the time.
> --
> Eduardo Bellani
>
> www.cnxs.com.br
>
> I object to doing things that computers can do.
> - Olin Shivers
> _________________________________________________