How do I take a slice of a list (A sublist) in scheme?

Question

Given a list, how would I select a new list, containing a slice of the original list (Given offset and number of elements) ?

EDIT:

Good suggestions so far. Isn't there something specified in one of the SRFI's? This appears to be a very fundamental thing, so I'm surprised that I need to implement it in user-land.

Answer 1

The following code will do what you want:

(define get-n-items
    (lambda (lst num)
        (if (> num 0)
            (cons (car lst) (get-n-items (cdr lst) (- num 1)))
            '()))) ;'

(define slice
    (lambda (lst start count)
        (if (> start 1)
            (slice (cdr lst) (- start 1) count)
            (get-n-items lst count))))

Example:

> (define l '(2 3 4 5 6 7 8 9)) ;'
()
> l
(2 3 4 5 6 7 8 9)
> (slice l 2 4)
(3 4 5 6)
> 

Answer 2

Strangely, slice is not provided with SRFI-1 but you can make it shorter by using SRFI-1's take and drop:

(define (slice l offset n)
  (take (drop l offset) n))

I thought that one of the extensions I've used with Scheme, like the PLT Scheme library or Swindle, would have this built-in, but it doesn't seem to be the case. It's not even defined in the new R6RS libraries.

Answer 3

You can try this function:

subseq sequence start &optional end

The start parameter is your offset. The end parameter can be easily turned into the number of elements to grab by simply adding start + number-of-elements.

A small bonus is that subseq works on all sequences, this includes not only lists but also string and vectors.

Edit: It seems that not all lisp implementations have subseq, though it will do the job just fine if you have it.

Answer 4

(define (sublist list start number)
  (cond ((> start 0) (sublist (cdr list) (- start 1) number))
        ((> number 0) (cons (car list)
                      (sublist (cdr list) 0 (- number 1))))
        (else '())))

Answer 5

Here's my implementation of slice that uses a proper tail call

(define (slice a b xs (ys null))
  (cond ((> a 0) (slice (- a 1) b (cdr xs) ys))
        ((> b 0) (slice a (- b 1) (cdr xs) (cons (car xs) ys)))
        (else (reverse ys))))

(slice 0 3 '(A B C D E F G)) ;=> '(A B C)
(slice 2 4 '(A B C D E F G)) ;=> '(C D E F)

Answer 6

Try something like this:

    (define (slice l offset length)
      (if (null? l)
        l
        (if (> offset 0)
            (slice (cdr l) (- offset 1) length)
            (if (> length 0)
                (cons (car l) (slice (cdr l) 0 (- length 1)))
                '()))))