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Appending to Dispatch[] tables

• To: mathgroup at smc.vnet.net
• Subject: [mg119393] Appending to Dispatch[] tables
• From: Szabolcs Horvát <szhorvat at gmail.com>
• Date: Wed, 1 Jun 2011 06:57:14 -0400 (EDT)

Is there an efficient way to add more rules to a Dispatch[] table?  I 
see that it contains a HashTable object, but the format of HashTable is 
not documented.

Adding new DownValues for a symbol is much more efficient than 
extracting the rule-list from a Dispatch, adding to it, and re-building 
the Dispatch table, however Dispatch[] is more convenient for certain 
applications than a symbol with DownValues (save for the limitation of 
adding new values).

Is there any inherent reason why it's not possible to efficiently add to 
a Dispatch, possibly by modifying the HashTable object contained within? 
  One reason I could imagine is that this type of modification would 
involve appending to lists (both the rule list and the HashTable within 
Dispatch), and appending is quadratically slow (because of the need to 
re-allocate the list).

Do Dispatch[] and symbols w DownValues use the same code in the background?

Any pointers on the topic are most welcome!

  -- Szabolcs