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Extracting a sublist

Problem

Solution

(require (lib "1.ss" "srfi"))

; sublist : list integer integer -> list
;  (sublist (list x0 x1 ... xn) s e)
;  => (list xs ... x_e-1)
(define (sublist a-list start end)
  (take (drop a-list end)
        (- end start)))

; sublist! : list integer integer -> list
;  the linear update version of sublist,
;  sublist! may alter a-list to produce the result
(define (sublist! a-list start end)
  (take! (drop a-list end)
         (- end start)))

This uses the take and drop functions from SRFI 1. The function take takes the first n elements from a list, while drop takes everything after the first n elements. So you can see that we're dropping start elements from the front of the list and taking {- end start) elements from that result.

Example usage:

> (sublist '(1 2 3 4 5 6 7 8)  3 5)
(4 5)
> (sublist '(1 2 3 4 5 6 7 8)  1 5)
(2 3 4 5)
> (sublist '(1 2 3 4 5 6 7 8)  1 1)
()
> (sublist '(1 2 3 4 5 6 7 8)  1 2)
(2)
> (sublist '(1 2 3 4 5 6 7 8)  7 8)
(8)

Discussion

A call to sublist will a return a newly allocated list holding the elements of the given segment.

The function sublist! will alter the given list, and will therefore not allocate a new list.

Note: The sublist function corrresponds to using the slice operator in Python.

-- GordonWeakliem - 29 Apr 2004, JensAxelSoegaard - 12 May 2004

Comments

I rewrote the text to use the language of SRFI1?, i.e. using "sublist" instead of "slice".

-- JensAxelSoegaard - 13 May 2004