getting an interval of a vector

Question

I want to take an interval of a vector in Scheme. I know there is a procedure named vector->values, but seems like it returns each element separately, while I want to get the result as a vector. How can I achieve this?

> (vector->values (vector 1 2 3 4 5) 0 3)
1
2
3

while I need:

#(1 2 3)

Answer 1

you want subvector:

(subvector (vector 1 2 3 4 5) 0 3)

Answer 2

The Scheme R6RS standard has make-vector, vector-ref, vector-set! and vector-length. With that you can write your own function subvector, which does not seem to be part of R6RS (!). Some Scheme implementation have something like subvector already.

You can also switch to Common Lisp, which provides the function SUBSEQ in the standard.

Answer 3

If you're using PLT, you have a few easy ways to get this:

(define (subvector v start end)
  (list->vector (for/list ([i (in-vector v start end)]) i)))

(define (subvector v start end)
  (build-vector (- end start) (lambda (i) (vector-ref v (+ i start)))))

(define (subvector v start end)
  (define new (make-vector (- end start)))
  (vector-copy! new 0 v start end)
  new)

The last one is probably going to be the fastest. The reason that there is no such operation that is built-in is that people usually don't do that. When you're dealing with vectors in Scheme, you're usually doing so because you want to optimize something so returning a vector and a range instead of allocating a new one is more common.

(And if you think that this is useful, please suggest it on the PLT mailing list.)

Answer 4

#lang scheme
(define (my-vector-value v l h c)
  (if (and (>= c l) (< c h))
      (cons (first v) (my-vector-value (rest v) l h (add1 c)))
      empty))

(list->vector (my-vector-value (vector->list (vector 1 2 3 4 5)) 0 3 0))

Ghetto? Yes, very. But it only took two minutes to write and gets the job done.

(I find it's generally easier to play with lists in Scheme)

Answer 5

Here is a portable R6RS version using SRFI 43:

#!r6rs

(import (rnrs base)
        (prefix (srfi :43) srfi/43:))

(srfi/43:vector-copy (vector 1 2 3 4 5) 0 3)