Genuine copy of an element from an associative array

[Nick Treleaven]

On Thursday, 3 October 2024 at 12:23:27 UTC, Holzofen wrote:
> ```d
>     uint64 sod_decompose(const uint64 n, const uint64 mod, 
> const ref uint64[] primes, const ref uint64[uint64][uint64] 
> factorials)
>     {
>         auto result = factorials[n];
>
>         for (uint64 k = 2; k < n - 1; k++) {
>             auto backup = factorials[n];
>             tuple("1", backup).writeln;
>             foreach (p, e; factorials[k]) {
>                 backup[p] -= e;
>             }
>             tuple("2", backup).writeln;
>             foreach (p, e; factorials[n-k]) {
>                 backup[p] -= e;
>             }
>             tuple("3", backup).writeln;
>         }
>
>         return 0;
>     }
> ```
> uint64 is equivalent to ulong. Upon compilation I get this 
> error:
> Error: cannot modify `const` expression `backup[p]`

> Could anybody point me to a way how to make a straight copy 
> without meddling with the source array?

You can copy an AA using `dup`:
https://dlang.org/phobos/object.html#.dup

Unfortunately this doesn't work:

     uint64[uint64] backup = factorials[n].dup;

https://issues.dlang.org/show_bug.cgi?id=11725

Instead you can use:

     auto backup = cast(uint64[uint64]) factorials[n].dup;

I think casting away const is OK because the AA produced by `dup` 
is unique, and the element type doesn't have indirections.