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digitalmars.D.bugs - [Issue 11415] New: Assign range to array

reply d-bugmail puremagic.com writes:
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



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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next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=11415


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

           What    |Removed                     |Added
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                 CC|                            |shammah.chancellor gmail.co
                   |                            |m



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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prev sibling next sibling parent d-bugmail puremagic.com writes:
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yazan.dabain gmail.com changed:

           What    |Removed                     |Added
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                 CC|                            |yazan.dabain gmail.com



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.



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prev sibling next sibling parent d-bugmail puremagic.com writes:
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monarchdodra gmail.com changed:

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





 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.
 



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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prev sibling next sibling parent d-bugmail puremagic.com writes:
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 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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prev sibling next sibling parent d-bugmail puremagic.com writes:
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bearophile_hugs eml.cc changed:

           What    |Removed                     |Added
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                 CC|                            |bearophile_hugs eml.cc



See also Issue 10176 for an array append of a lazy range.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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 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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prev sibling next sibling parent d-bugmail puremagic.com writes:
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  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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prev sibling next sibling parent d-bugmail puremagic.com writes:
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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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prev sibling next sibling parent d-bugmail puremagic.com writes:
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 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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2013-11-02 17:24:13 PDT ---
You mean what you get right now with that syntax for arrays?  None of those
array vector style operations turn into SIMD instructions.  That's what we have
std.simd; for.

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