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Hash tables are a fast, powerful mechanism for storing large numbers of associations. MIT/GNU Scheme's hash tables feature automatic resizing, customizable growth parameters, and customizable hash procedures.
The average times for the insertion, deletion, and lookup operations on a hash table are bounded by a constant. The space required by the table is proportional to the number of associations in the table; the constant of proportionality is described below (see Resizing of Hash Tables).
(Previously, the hash-table implementation was a run-time-loadable
option, but as of release 7.7.0 it is loaded by default. It's no longer
necessary to call load-option prior to using hash tables.)