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From: jack@cs.glasgow.ac.uk (Jack Campin)
Newsgroups: comp.lang.misc
Subject: testing whether a key is present in an ABC table
Message-ID: <5989@vanuata.cs.glasgow.ac.uk>
Date: 8 Aug 90 14:34:42 GMT
Organization: Computing Sci, Glasgow Univ, Scotland
Lines: 24

I've just been reading Geurts, Meertens and Pemberton's "ABC Programmer's
Handbook" (Prentice-Hall, ISBN 0-13-000027-2).  ABC has a useful type
constructor called the "table", which allows associative access to values
of any type based on keys of any type.  There seems to be one rather
important facility missing: a way to test if a key is present in a table.

There IS a direct way to test if a <key, value> pair is present, but the
only way the book describes to test for a key alone is by first
constructing a sorted list of the keys (using a standard function "keys")
and then testing the key's presence in the list.

They don't describe the way "keys" works, but nothing suggests that it's
lazy.  If it isn't, this is unacceptably inefficient for an application
that performs a long series of query-and-update transactions on a large
persistent table.

Workarounds?  Do I have to run "keys" at the start of the application and
then maintain an update log?  Not very elegant.

-- 
--  Jack Campin   Computing Science Department, Glasgow University, 17 Lilybank
Gardens, Glasgow G12 8QQ, Scotland   041 339 8855 x6044 work  041 556 1878 home
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