This is my (feeble) source code, for testing purposes only:

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]
which concerns these two lines: backup[p] -= e;

Without the const statement in the function declaration the source array will be modified which is definitely not desirable. Could anybody point me to a way how to make a straight copy without meddling with the source array?

Thank you!

On Thursday, 3 October 2024 at 12:23:27 UTC, Holzofen wrote:

    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;
    }

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.

On Sunday, 6 October 2024 at 10:53:28 UTC, Nick Treleaven wrote:

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

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

Thanks a lot!