In Perl, not in MUMPS. MUMPS arrays are more complex, allowing nodes
to have both values and children. Hashes in Perl only associate a
value (which may be a reference) to each key (subscript).
In general, a hash is an algorithm that computes a number from an
object of some type in such a way that it is unlikely (not
impossible) for different objects to have the same has value. If you
think of an array with numeric subscripts, it is easy to locate the
ith element of the array in memory. Bear in mind that this is not
something programmers usually need to do, the compiler just takes
base address of the array and adds 4 time i (assuming a word size of
4). The programmer just writes a[i] (in C or Pascal) and doesn't
have to worry about the internals. But this scheme doesn't work for
languages like MUMPS that allow strings as subscripts. There are
various possible implementation strategies, one of which is to
compute a hash of the subscripts that the integer used to look up the
value. Another strategy would be to organize the array nodes into a
treelike structural (called a binary heap) where you basically make a
series of lookups based on the collating sequence of the subscript
(i.e., where it falls in "alphabetical order"). This can require a
lot of memory references, and memory references are VERY slow, but a
strategy called caching, where recently accessed values are stored
for fast lookup. Unlike the data structure I described before, this
kind of caching is typically done in hardware, but there are software
caches, too, and it's software caches that makes global references in
MUMPS fast.
===
Gregory Woodhouse
[EMAIL PROTECTED]
"The whole of science is nothing more than a refinement
of everyday thinking." -- Albert Einstein
On Aug 24, 2005, at 4:01 PM, James Gray wrote:
Very good. However, I still do not understand hash tables or
hashes. Does the % in front of the a make it a hash?
Jim Gray
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