On Monday, 23 July 2018 at 11:49:58 UTC, Alex wrote:
Hi all,
I'm looking for a d-ish way to solve a basic
"split-apply-combine" workflow. The idea is described (and
solved) here:
https://stackoverflow.com/questions/39922986/pandas-group-by-and-sum
So, given a structure with some fields, say
´´´
struct S
{
string s;
int i;
}
´´´
I create an array of them, like
´´´
void main()
{
import std.experimental.all;
S[] sarr;
sarr.length = 6;
sarr[0].s = "a";
sarr[1].s = "a";
sarr[2].s = "b";
sarr[3].s = "b";
sarr[4].s = "c";
sarr[5].s = "c";
sarr[0].i = 1;
sarr[1].i = 2;
sarr[2].i = 4;
sarr[3].i = 8;
sarr[4].i = 16;
sarr[5].i = 32;
auto res = sarr.group!((a, b) => a.s == b.s);
//writeln(res);
}
´´´
I'm also able to group them by a field, see last line.
But now the problems begin:
- The group operation tries to use the structure itself as a
key, despite I provide a custom binary predicate.
- I could ignore the fact above, but how, given the result of
the grouping operation I can merge by some function (like sum)
the group results?
At this moment, I assume, that I'm approaching the problem from
the wrong end, and simply don't see something trivial...
Anyway, does anybody has a hint for me?
You could use chunkBy:
auto res = sarr.chunkBy!((a, b) => a.s == b.s).map!(a =>
tuple(a.front.s, a.map!(b => b.i).sum));
https://run.dlang.io/is/TJOEmf
chunkBy:
---
[S("a", 1), S("a", 2)]
[S("b", 4), S("b", 8)]
[S("c", 16), S("c", 32)]
---
group:
---
Tuple!(S, uint)(const(S)("a", 1), 2)
Tuple!(S, uint)(const(S)("b", 4), 2)
Tuple!(S, uint)(const(S)("c", 16), 2)
---
