Re: C Hashmap implementation

Bernhard Roessmann <>
26 Apr 2007 09:42:44 -0400

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From: Bernhard Roessmann <>
Newsgroups: comp.compilers
Date: 26 Apr 2007 09:42:44 -0400
Organization: Compilers Central
References: 07-04-089
Keywords: symbols
Posted-Date: 26 Apr 2007 09:42:44 EDT

> Almost everyone I've talked to has said that Chained Hashmaps are much
> easier to implement than Open Addressed maps.

I dont think so, open addressed maps are very easy to implement. But
beware of the fill level (see below).

> Wikipedia suggests that an approach to resizing hashmaps is to
> allocate space for a newer hashmap and copy elements from to the new
> table, and in some cases do it incrementally.

If you have enough RAM and resizing occurs infrequently, why not. In an
embedded environment with limited RAM resources and/or without MMU, this
is not really a good idea.

> Quick question about the last point: I'm curious about a starting
> point. How much space should a hashmap allocate initially, and when
> it's how much would I typically increase it? I realize
> there are lots of different answers here, so a good starting point
> would really help out.

There's a lot of academic stuff about this, but if you want a practical
approach for an open addressed map:
"Nearly full" open addressed hash table maps performs VERY bad, so
estimate the maximum number of elements and use 125% as the hash table

Bernhard Roessmann
Don't Fear The Penguins!

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