if arr is a single array the follow code works(removing [i]) but if it is a double array it fails. arr as type double[A][B]. arr[i] has type double[A].
v ~= arr[i].map!(a=>to!string(a)~",").join;

  with `Range = double[21]`
  must satisfy the following constraint:
`       isInputRange!(Unqual!Range)`

On Tuesday, 17 December 2019 at 07:40:28 UTC, AlphaPurned wrote:
>
> if arr is a single array the follow code works(removing [i]) but if it is a double array it fails. arr as type double[A][B]. arr[i] has type double[A].
> v ~= arr[i].map!(a=>to!string(a)~",").join;
>
>   with `Range = double[21]`
>   must satisfy the following constraint:
> `       isInputRange!(Unqual!Range)`

The problem isn't that it's a multi-array but that it's a
fixed-size array. As the error suggests: it complains about a
double[21], and not about a double[][]

Fixed size arrays you need to slice to make them valid input
ranges. (std.container.array is the same.)

At a terminal:

  $ rdmd --eval 'int[3] x; x.map!"a+1".writeln'
  ...
    with Range = int[3]
    whose parameters have the following constraints:
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    > isInputRange!(Unqual!Range)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Tip: not satisfied constraints are marked with >

  $ rdmd --eval 'int[3] x; x[].map!"a+1".writeln'
  [1, 1, 1]

Note the [] in x[].map