From [Tuple!(A,B), ...] to Tuple!(A[], B[])

[Simen Kjærås]

On Monday, 17 February 2020 at 11:51:52 UTC, FeepingCreature 
wrote:
> Here you go:
>
> import std;
>
> // extract the types that make up the tuple
> auto transposeTuple(T : Tuple!Types[], Types...)(T tuples)
> {
>     // templated function that extracts the ith field of an 
> array of tuples as an array
>     auto extractArray(int i)()
>     {
>         return tuples.map!(a => a[i]).array;
>     }
>     // return tuple of calls to extractArray, one for each 
> tuple index
>     return tuple(staticMap!(extractArray, 
> aliasSeqOf!(Types.length.iota)));
> }
>
> void main() {
>     Tuple!(int, double)[] array;
>     array ~= tuple(1, 2.0);
>     array ~= tuple(3, 4.0);
>     Tuple!(int[], double[]) tuple = array.transposeTuple;
>     assert(tuple[0] == [1, 3]);
>     assert(tuple[1] == [2.0, 4.0]);
> }

One tiny thing: the above fails for tuples with named fields, 
like Tuple!(int, "a", string "b"). This code handles that case, 
and preserves field names:

import std.meta : staticMap, aliasSeqOf;
import std.typecons : Tuple;
import std.range : array, iota;
import std.algorithm : map;

alias ToArray(T) = T[];
alias ToArray(T...) = T;
alias ToArrayTuple(T : Tuple!U, U...) = 
Tuple!(staticMap!(ToArray, U));

auto transpose(T : Tuple!U, U...)(T[] arr) {
     auto extract(int i)() { return arr.map!(a => a[i]).array; }
     return ToArrayTuple!T(staticMap!(extract, 
aliasSeqOf!(T.Types.length.iota)));
}

unittest {
     alias T = Tuple!(int, "a", string, "b");
     auto a = [T(1, "a"), T(2, "b")];
     assert(a.transpose.a == [1, 2]);
     assert(a.transpose.b == ["a", "b"]);
}

--
   Simen