[racket] how can I change only one element in the vector?

[김태윤]

hello

from the following code, I expected

'#(#(#("2" "2" "3") #("1" "2" "3")) #(#("1" "2" "3") #("1" "2" "3")))

but 

the interpreter pop the 

'#(#(#("2" "2" "3") #("2" "2" "3")) #(#("2" "2" "3") #("2" "2" "3")))

how can I change only one element in the vector?

===following code===

#lang racket

(define v (make-vector 2 

                       (make-vector 2 

                                    (vector "1" "2" "3"))))

(vector-set! (vector-ref (vector-ref v 0) 0) 0  "2")

v

[Neil Van Dyke]

"make-vector" is creating a new vector populated with *references* to a
single #("1" "2" "3") vector, *not* *copies* of the #("1" "2" "3") vector.

Since you've created one #("1" "2" "3") vector, changing it through one
reference changes all references to it.

(Aside to PLT people: This might be an example of having the default
writer show sharing could actually be helpful to beginners.)

--
http://www.neilvandyke.org/
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[김태윤]

thank you !

could you tell me how to make new vector that does not refer to a single vector?

[김태윤]

is this code right way to do it?

#lang racket

(define v (build-vector 2 (λ (n) 

                            (build-vector 2 (λ (n) 

                                              (vector "1" "2" "3"))))))

  

  (vector-set! (vector-ref (vector-ref v 0) 0) 0  "2")

  v

[Neil Van Dyke]

김태윤 wrote at 11/20/2010 03:31 AM:
> thank you !
> could you tell me how to make new vector that does not refer to a
> single vector?

"make-vector" is not the correct way, but there are many other ways.

This will make the nested vectors like you originally intended:

(vector (vector (vector 1 2 3)
(vector 1 2 3))
(vector (vector 1 2 3)
(vector 1 2 3)))

You can also use procedures:

(define (make-xyz) (vector (make-yz) (make-yz)))
(define (make-yz) (vector (make-z) (make-z)))
(define (make-z) (vector 1 2 3))
(make-xyz)

Or, if you want to represent multiple dimensions, and you know the size
of each dimension, you can use a single vector and then do arithmetic of
the X, Y, Z to find the right index:

(vector 1 2 3 1 2 3 1 2 3 1 2 3)