[Issue 12592] New: std.algorithm.keep?
via Digitalmars-d-bugs
digitalmars-d-bugs at puremagic.com
Fri Apr 18 04:03:54 PDT 2014
https://issues.dlang.org/show_bug.cgi?id=12592
Issue ID: 12592
Summary: std.algorithm.keep?
Product: D
Version: D2
Hardware: All
OS: All
Status: NEW
Severity: enhancement
Priority: P1
Component: Phobos
Assignee: nobody at puremagic.com
Reporter: bearophile_hugs at eml.cc
This code uses ranges in a UFCS chain to compute the unique items of immutable
data items, and to return an array (if you don't need r to be an array you can
omit the last 'array' call):
void main() {
import std.array: array;
import std.algorithm: sort, uniq, map;
immutable data = [10, 3, 2, 3, 4, 10];
const int[] r = data.dup.sort().uniq.array;
assert(r == [2, 3, 4, 10]);
}
But that allocates two arrays (beside the data), one in 'dup' and one in
'array'. To avoid that you can write:
void main() {
import std.algorithm: sort, uniq, copy;
immutable data = [10, 3, 2, 3, 4, 10];
auto int[] r = data.dup.sort().release;
r.length -= r.uniq.copy(r).length;
assert(r == [2, 3, 4, 10]);
}
But:
- This code is not very easy to write correctly the first time (I have had to
compile and run it to be sure);
- Its semantics and purpose are not as immediately clear for the reader as the
first version;
- Now r can't be const;
- We have lost the nice single UFCS chain, and the code is longer.
So a possible solution is to add a new function similar 'uniq' that performs
that common operation (I don't know if release is needed here):
void main() {
import std.array: array;
import std.algorithm: sort, uniq, map;
immutable data = [10, 3, 2, 3, 4, 10];
const int[] r = data.dup.sort().release.keepUniq;
assert(r == [2, 3, 4, 10]);
}
But that seems to contain a common pattern that perhaps it's worth
generalizing, a pattern that is equally usable here too:
const int[] r2 = data.dup.sort().filter!(x => x > 3).array;
That could become:
const int[] r2 = data.dup.sort().release.keepFilter!(x => x > 3);
Generalizing it could become:
const int[] r2 = data.dup.sort().release.keep!(filter!(x => x > 3));
And applied on the original code:
const int[] r = data.dup.sort().release.keep!uniq;
Another example:
auto data2 = [10, 3, 2, 3, 4, 10];
data2 = data2.remove!(x => x > 3);
assert(data2 == [3, 2, 3]);
Becomes:
data2.keep!(remove!(x => x > 3));
But this is not a good example because in my opinion it's much better to fix
remove, see issue 10959 .
If you have more usage cases please add them below.
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