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digitalmars.D.learn - Genuine copy of an element from an associative array

reply Holzofen <z5iph7ukvx yopmail.com> writes:
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!
Oct 03 2024
parent reply Nick Treleaven <nick geany.org> writes:
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.
Oct 06 2024
parent Holzofen <z5iph7ukvx yopmail.com> writes:
On Sunday, 6 October 2024 at 10:53:28 UTC, Nick Treleaven wrote:

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

 [...]



 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.


Thanks a lot!
Oct 06 2024