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Lacking from the simple API of System`Utilities`HashTable is a method to enumerate the keys/values.


HashTable   HashTableAdd    HashTableContainsQ  HashTableGet    HashTableRemove

After creating a hash table

h = System`Utilities`HashTable[];
System`Utilities`HashTableAdd[h, "a", 1];

Various methods to extract its content fail.


Any way out?

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I wonder what HashTableGet[] does... :) – J. M. Jun 8 '13 at 15:17
@0x4A4D I think OP wants to list all available key-value pairs in the table, which I believe is not possible. – R. M. Jun 8 '13 at 15:21
If you want to be able to enumerate the contents, you're probably better off just to use ordinary downvalues rather than the System`Utilities`HashTable. – Oleksandr R. Jun 8 '13 at 19:44
Just to follow up on 0x4A4D, you have tried SystemUtilitiesHashTableGet[h, "a"]? – Seth Chandler Jun 9 '13 at 5:35

1 Answer 1

up vote 11 down vote accepted

I'm sure there must be a better way, but this works:

h = System`Utilities`HashTable[];
System`Utilities`HashTableAdd[h, "a", 1];
System`Utilities`HashTableAdd[h, "b", 2];

ToExpression[ToString[h, InputForm], InputForm, Hold][[1, 2]]
{{"a", 1}, {"b", 2}}

Mr. Wizard provides a more terse version:

ToHeldExpression[ToString[h, InputForm]][[1, 2]]
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Unless there is something like SparseArray[. . .]["NonzeroPositions"] for HashTable there very well may not be a better way. – Mr.Wizard Jun 9 '13 at 13:48
More terse: ToHeldExpression[ToString[h, InputForm]][[1, 2]] – Mr.Wizard Jun 9 '13 at 14:55
As with many things Mathematica I learned it from Leonid. It's an old function but still entirely, well, functional. – Mr.Wizard Jun 10 '13 at 10:13
+1. I still hope there is some more robust way though, since ToString-ToExpression transformation is not always robust. – Leonid Shifrin Jun 10 '13 at 10:28
@Leonid for testing purposes can you give an example where Simon's method fails? – Mr.Wizard Jun 10 '13 at 12:01

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