On Monday, 23 November 2015 at 15:19:08 UTC, Steven Schveighoffer
wrote:
On 11/21/15 10:19 PM, Jon D wrote:
On Sunday, 22 November 2015 at 00:31:53 UTC, Jonathan M Davis
wrote:
Honestly, arrays suck as output ranges. They don't get
appended to;
they get filled, and for better or worse, the documentation
for copy
is probably assuming that you know that. If you want your
array to be
appended to when using it as an output range, then you need
to use
std.array.Appender.
Hi Jonathan, thanks for the reply and the info about
std.array.Appender.
I was actually using copy to fill an array, not append.
However, I also
wanted to preallocate the space. And, since I'm mainly trying
to
understand the language, I was also trying to figure out the
difference
between these two forms of creating a dynamic array with an
initial size:
auto x = new int[](n);
int[] y; y.reserve(n);
If you want to change the size of the array, use length:
y.length = n;
This will extend y to the correct length, automatically
reserving a block of data that can hold it, and allow you to
write to the array.
All reserve does is to make sure there is enough space so you
can append that much data to it. It is not relevant to your use
case.
The obvious difference is that first initializes n values, the
second
form does not. I'm still unclear if there are other material
differences, or when one might be preferred over the other :)
It's was
in this context the behavior of copy surprised me, that it
wouldn't
operate on the second form without first filling in the
elements. If
this seems unclear, I can provide a slightly longer sample
showing what
I was doing.
extending length affects the given array, extending if
necessary. reserve is ONLY relevant if you are using appending
(arr ~= x). It doesn't actually affect the "slice" or the
variable you are using, at all (except to possibly point it at
newly allocated space).
copy uses an "output range" as it's destination. The output
range supports taking elements and putting them somewhere. In
the case of a simple array, putting them somewhere means
assigning to the first element, and then moving to the next one.
-Steve
Thanks for the reply. And for your article (which Jonathan
recommended). It clarified a number of things.
In the example I gave, what I was really wondering was if there
is a difference between allocating with 'new' or with 'reserve',
or with 'length', for that matter. That is, is there a material
difference between:
auto x = new int[](n);
int[] y; y.length = n;
I can imagine that the first might be faster, but otherwise there
appears no difference. As the article stresses, the question is
the ownership model. If I'm understanding, both cause an
allocation into the runtime managed heap.
--Jon
