Re: vector-fill! seems giving out the wrong result

[Stephen Compall]

aladdin wrote:
> I had a problem when using vector-fill! to fill a dimension of 2D array. 
> Here is my code. Please note the second section of the code. First it 
> generate a 2x2 array with nested make-vector. Then, it called 
> vector-fill! to fill the second row of the array with value 10. But it 
> turns out that the whole 2x2 array is filled with 10.
>
> Running the code under mzscheme works out the same result. Is it a bug? 
> Or is there something wrong with my code?

Something is wrong with your code.  Consider this:

> (define v '#(#(1 3) #(2 4)))
> (display v)(newline)
> (vector-fill! (vector-ref v 1) 10)
> (display v)(newline)
>
> (set! v (make-vector 2 (make-vector 2 1)))

Make-vector is a function.  That means the above line is equivalent to:

(set! v (let ((b (make-vector 2 1)))
          (make-vector 2 b)))

Make-vector doesn't know how to copy every object, so they assume you 
want *the same object* in each position.  Try it out with pairs:

(let ((v (make-vector 2 (list 1))))
  (write v)
  (newline)
  (set-car! (vector-ref v 0) 42)
  (write v)
  (newline))

--
;;; Stephen Compall ** http://scompall.nocandysw.com/blog **
But you know how reluctant paranormal phenomena are to reveal
themselves when skeptics are present. --Robert Sheaffer, SkI 9/2003