Re: Convert array of tuples into array of arrays.

[Ali Çehreli via Digitalmars-d-learn]

On 8/31/22 07:34, musculus wrote:
> Hi. I have an array of tuples that I would like to convert to an array
> of arrays.
I misunderstood as well and wrote the following program which makes 
separate arrays. You can make an array of arrays from those with the 
following syntax:

  auto arrayOfArrays = [ keys, values ];

Then I wrote a more general program after this first one:

// Compiles slow but is convenient
import std;

auto makeTestTuple(int i) {
  return tuple!("key", "value")((i * 2).to!string,
                                (double(i) / 10).to!string);
}

void main() {
  auto tuples = iota(10)
                .map!makeTestTuple
                .array;

  string[] keys;
  string[] values;

  writeln("Imperative:");

  foreach (t; tuples) {
    keys ~= t.key;
    values ~= t.value;
  }

  writeln(keys);
  writeln(values);

  writeln("Alternative:");
  keys = tuples.map!(t => t.key).array;
  values = tuples.map!(t => t.value).array;
  writeln(keys);
  writeln(values);

  writeln("More generic:");
  keys = tuples.memberArray!"key";
  values = tuples.memberArray!"value";
  writeln(keys);
  writeln(values);
}

auto memberArray(string member, T)(T[] tuples) {
  return tuples.map!(t => mixin("t." ~ member)).array;
}

Here is the second program to have fun and "competitive advantage" with 
D. (That phrase came up recently, so I remembered it here. :) )

// Compiles slow but is convenient
import std;

auto makeTestTuple(int i) {
  return tuple!("key", "value", "somethingElse")(
    (i * 2).to!string,
    (double(i) / 10).to!string,
    i * i);
}

mixin template matchingMember(string name, T) {
  mixin (T.stringof ~ "[] " ~ name ~ ';');
}

template TupleMemberArrays(T) {
  alias types = T.Types;
  alias fieldNames = T.fieldNames;
  enum memberCount = types.length;

  struct TupleMemberArrays {
    static foreach (i; 0 .. memberCount) {
      mixin matchingMember!(fieldNames[i], types[i]);
    }

    this(T[] tuples) {
      foreach (t; tuples) {
        static foreach (i; 0 .. memberCount) {
          mixin (format!q{
              %s ~= t.%s = t[%s];
            }(fieldNames[i], fieldNames[i], i));
        }
      }
    }
  }
}

void main() {
  auto tuples = iota(10)
                .map!makeTestTuple
                .array;

  alias S = TupleMemberArrays!(ElementType!(typeof(tuples)));
  auto s = S(tuples);
  writeln(s.key);
  writeln(s.value);
  writeln(s.somethingElse);
}

That second program defines a type that matches the members of a tuple. 
Given a tuple with member names "key", "value", and "somethingElse"; and 
types string, string, int; it generates a struct similar to the following:

struct YourAliasHere {
  string[] key;
  string[] value;
  int[] somethingElse;
}

Limitation: That implementation uses field names, which not all tuples 
use. But it can be changed to generate names file field0, field1, etc. 
when needed.

Ali