std.algorithm.remove strange behavior (removing items for the dynamic array)

[mezozoysky]

On Friday, 11 May 2012 at 03:18:41 UTC, Jonathan M Davis wrote:
> On Friday, May 11, 2012 04:45:18 mezozoysky wrote:
>> Hello!
>> I'm noticed that something non-obvious is happening with my 
>> code
>> today and i've found that the trouble somewhere close to the
>> removing items from the dynamic arrays e.i. close to
>> std.algorithm.remove function in this case.
>> 
>> I wrote a little test example using this function:
>> 
>> 
>> module test.app;
>> 
>> import std.stdio: writefln;
>> import std.algorithm;
>> 
>> int main(string[] args) {
>> 
>> 	int[] a = [2, 4, 8, 16, 32, 64, 128];
>> 	writefln("a before: %s", a);
>> 	a.remove(3);
>> 	writefln("a after : %s", a);
>> 	a.remove(1);
>> 	writefln("a after2: %s", a);
>> 
>> 	return 0;
>> }
>> 
>> 
>> ...and got the following output:
>> 
>> 
>> a before: [2, 4, 8, 16, 32, 64, 128]
>> a after : [2, 4, 8, 32, 64, 128, 128]
>> a after2: [2, 8, 32, 64, 128, 128, 128]
>> 
>> 
>> I'm confused.
>> Please tell me is it normal behavior of this function or is it 
>> a
>> bug?
>> Maybe i'm doing something wrong?
>> Maybe i need another "remove" or maybe it's normal to use 
>> slicing
>> to remove array's items (like a = a[0..i] ~ [i+1..$]) ?
>> 
>> Thanx for attention.
>> 
>> P.S. I'm sorry if my english confuses you.
>
> No. As the documentation for remove explains, this is 
> completely expected.
> remove removes elements from _the range_, not the container. It 
> can't remove
> elements from the container (regardless of the container or 
> range type),
> because it doesn't understand anything about the container. It 
> shifts the
> elements forward in the range and returns a range which is 
> reduced in length
> by the number of elements removed, but the original range is 
> not reduced in
> size, nor is the underlying container reduced in size (all of 
> which is
> slightly more confusing with dynamic arrays, because the range 
> _is_ the
> container, which is not the case in general). Also, some ranges 
> don't even
> _have_ an underlying container, so remove _definitely_ can't 
> remove anything
> from the container itself - only shift elements. C++'s erase 
> function has the
> exact same problem.
>
> If you would just assign the result back to a, then your 
> example would work as
> you expect.
>
> import std.stdio: writefln;
> import std.algorithm;
>
> int main(string[] args)
> {
>     int[] a = [2, 4, 8, 16, 32, 64, 128];
>     writefln("a before: %s", a);
>     a = a.remove(3);
>     writefln("a after : %s", a);
>     a = a.remove(1);
>     writefln("a after2: %s", a);
>
>     return 0;
> }
>
> a before: [2, 4, 8, 16, 32, 64, 128]
> a after : [2, 4, 8, 32, 64, 128]
> a after2: [2, 8, 32, 64, 128]
>
> - Jonathan M Davis


Thank you, Jonathan!

Thanks to you I saw that the documentation for std.algorithm 
contains useful information about "remove", not just the phrase 
"Remove current item from the target.", which opens when you 
click on the "remove" link at the top of the page :)

Really thank you. It works for me.