To supplement John's resource (which explains that the behavior you
indicate is indeed a property of the language):
Currently (and in the version you're using), range indexing such as `x[:]`
returns a copy of `x`. So does applying an operation like `y * 4`. That's
why writing `y = x[:]` or `y = y * 4` doesn't reassign `y` in such a way
that subsequent modifications to `y` impact `x`.
On Saturday, July 25, 2015 at 11:13:33 AM UTC-4, Christopher Fisher wrote:
>
> I am writing a program (in .3.1) that takes a one dimensional array of
> indices (x) and creates a new one dimensional array of indices (x = y) and
> switches a given pair of elements in the new array. For some reason, the
> original array (x) changes when I change the new array (see below).
>
> In [16]:
>
> x = [1:5]
>
> y = x
>
> y[[3;4]] = y[[4;3]]
>
> [x y]
>
> Out[16]:
>
> 5x2 Array{Int64,2}:
> 1 1
> 2 2
> 4 4
> 3 3
>
>
>
> However, if I change y = x to y = x[:], the problem does not occur, even
> though x and x[:] appear to be the same type.
>
>
> In [15]:
>
> x = [1:5]
>
> y = x[:]
>
> y[[3;4]] = y[[4;3]]
>
> [x y]
>
> Out[15]:
>
> 5x2 Array{Int64,2}:
> 1 1
> 2 2
> 3 4
> 4 3
> 5 5
>
>
> The code below shows that this does not seem to be a general property of the
> language. As in the previous cases, I define y as x and change y.
>
>
> In [13]:
>
> x = [1:5]
>
> y = x
>
> y = y*4
>
> [x y]
>
> Out[13]:
>
> 5x2 Array{Int64,2}:
> 1 4
> 2 8
> 3 12
> 4 16
> 5 20
>
>
>
> Certainly, I could use y = x[:] or simply change the original array.
> Nonetheless, the inconsistency seems undesirable from my point of view. Is
> this a bug? Any help would be appreciated.
>
>
>
