http://d.puremagic.com/issues/show_bug.cgi?id=11415

           Summary: Assign range to array
           Product: D
           Version: unspecified
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: druntime
        AssignedTo: nobody@puremagic.com
        ReportedBy: daniel350@bigpond.com


--- Comment #0 from daniel350@bigpond.com 2013-11-01 18:28:20 PDT ---
I would have expected the following to work:

int[] d = [1,2,3,4,5,6,7];
d[] = d.filter!(x => x > 3)[];

Where the rhs could have been assigned to the lhs. Unfortunately this gives the following:

Error: cannot implicitly convert expression (f.opSlice()) of type
FilterResult!(__lambda2, int[]) to int[]



For now, the only idiomatic solution I could find is this roundabout way:

auto t = d.filter!(x => x > 3).copy(d);
d = d[0 .. $ - t.length];

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Shammah Chancellor <shammah.chancellor@gmail.com> changed:

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                 CC|                            |shammah.chancellor@gmail.co
                   |                            |m


--- Comment #1 from Shammah Chancellor <shammah.chancellor@gmail.com> 2013-11-01 18:32:58 PDT ---
Seconded.   If an operator implements opSlice it should behave as expected for a r-value per the Array assignment documentation.  This can be converted to a similar foreach() loop that arrays are.

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yazan.dabain@gmail.com changed:

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                 CC|                            |yazan.dabain@gmail.com


--- Comment #2 from yazan.dabain@gmail.com 2013-11-02 02:09:07 PDT ---
Filter produces a lazy range. In other words, it is not an int array. To store
the result of filter in d[] you need to eagerly evaluate it. To do that you can
use std.array.array on the result of filter.
The code becomes:

int[] d = [1,2,3,4,5,6,7];
d = d.filter!(x => x > 3).array();

If this answers your question, please close the bug.

http://dlang.org/phobos/std_array.html#.array

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monarchdodra@gmail.com changed:

           What    |Removed                     |Added
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             Status|NEW                         |RESOLVED
                 CC|                            |monarchdodra@gmail.com
         Resolution|                            |INVALID


--- Comment #3 from monarchdodra@gmail.com 2013-11-02 02:44:19 PDT ---
(In reply to comment #2)
> Filter produces a lazy range. In other words, it is not an int array. To store
> the result of filter in d[] you need to eagerly evaluate it. To do that you can
> use std.array.array on the result of filter.
> The code becomes:
> 
> int[] d = [1,2,3,4,5,6,7];
> d = d.filter!(x => x > 3).array();
> 
> If this answers your question, please close the bug.
> 
> http://dlang.org/phobos/std_array.html#.array

Yes. Do note though that this will allocate a new array, and not *copy* filter "into" the "d" array. Not that it's wrong, I just want to highlight it, as I don't think its quite what the op wanted.

If you want to assign the *contents*, then std.algorithm.copy will do what you
want:
http://dlang.org/phobos/std_algorithm.html#copy
int[] d = [1,2,3,4,5,6,7];
d.filter!(x => x > 3)().copy(d);

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--- Comment #4 from monarchdodra@gmail.com 2013-11-02 02:46:09 PDT ---
(In reply to comment #0)
> I would have expected the following to work:
> 
> int[] d = [1,2,3,4,5,6,7];
> d[] = d.filter!(x => x > 3)[];

I'm surprised filter has opSlice() at all: It's not a range primitive, and is
useless. If anything, it leads to error (as you just tried to use it)

> Error: cannot implicitly convert expression (f.opSlice()) of type
> FilterResult!(__lambda2, int[]) to int[]

opSlice should be removed from filter, or any other range. It only makes sense for containers (including static arrays), or plain dynamic arrays.

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bearophile_hugs@eml.cc changed:

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                 CC|                            |bearophile_hugs@eml.cc


--- Comment #5 from bearophile_hugs@eml.cc 2013-11-02 03:02:45 PDT ---
See also Issue 10176 for an array append of a lazy range.

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--- Comment #6 from yazan.dabain@gmail.com 2013-11-02 03:18:54 PDT ---
@monarchdodra, that's true, however even this usage of copy is wrong as d should change length.

Printing d after the copy produces: [4, 5, 6, 7, 5, 6, 7] where it is meant to be [4, 5, 6, 7]

So you'll either have to go with what daniel350 had in his first post, or with the allocating array call.

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--- Comment #7 from monarchdodra@gmail.com 2013-11-02 04:36:18 PDT ---
(In reply to comment #6)
> @monarchdodra, that's true, however even this usage of copy is wrong as d should change length.
> 
> Printing d after the copy produces: [4, 5, 6, 7, 5, 6, 7] where it is meant to be [4, 5, 6, 7]
> 
> So you'll either have to go with what daniel350 had in his first post, or with the allocating array call.

Huh. I didn't even notice OP had gotten it right in his original post. Serves me for not fully reading.

I guess it really boils down to if you want a new array, or if you want to overwrite existing data. For example, copy can be used to write inside a slice taken from a stack allocated static array.

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--- Comment #8 from Shammah Chancellor <shammah.chancellor@gmail.com> 2013-11-02 16:47:07 PDT ---
The point is that the syntax is listed in http://dlang.org/arrays.html.  This syntax should be extended to forward ranges since that is how one would *EXPECT* it to work.  Either that or the array vector copy syntax should be abandoned.

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--- Comment #9 from monarchdodra@gmail.com 2013-11-02 17:21:22 PDT ---
(In reply to comment #8)
> The point is that the syntax is listed in http://dlang.org/arrays.html.  This syntax should be extended to forward ranges since that is how one would *EXPECT* it to work.  Either that or the array vector copy syntax should be abandoned.

The array vector copy is more than just convenient syntax: It's a request for a vectorized operation, which is built into naked arrays.

If this syntax did work, you'd *EXPECT* a vector operation too, but you'd be getting a plain foreach copy, which would be just as wrong, but less explicitly so.

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