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Introduction

PLT Scheme provides basic, but useful and fast ways of manipulating hashes. A hash table is used to store a collection of data values associated with keys.

To create a hash table use make-hash-table, which receives two optional arguments to create tables with weakly-held keys, and to use different comparison procedures. We will discuss this variations later, so for the time being something like:

(make-hash-table)

creates a new hash table. By default this table will compare keys using eq?. If you want to use keys that need a stronger equality test (i.e., strings), you can provide make-hash-table with the argument equal:

(make-hash-table 'equal)

In the following recipes we will discuss ways to manipulate our newly created hash table.

Recipes

• Adding an Element to a Hash (Recipe)
• Retrieving an Element from a Hash (Recipe)
• Testing for the Presence of a Key in a Hash (Recipe)
• Creating a Hash with Immutable Keys or Values (Recipe)
• Deleting from a Hash (Recipe)
• Traversing a Hash (Recipe)
• Printing a Hash (Recipe)
• Copying The Contents of a Hash Table (Recipe)
• Reading and Writing Hash Tables (Recipe)

Comments

Small comment: maybe we should avoid the use of the word "hash", since that's become a Perl term with lots of associations. Also, maybe we should have a breakdown of what Perl hashes are used for and how they map to different concepts and idioms in Scheme and PLT Scheme. (Someone who knows recent Perl versions better than I do would ideally volunteer...)

-- NeilVanDyke - 14 Jun 2004

I'd like to see some discussion of assoc lists vs. hashes. I always use assoc lists because it's convenient, what are the advantages of hashes over assq/assv/assoc?

-- GordonWeakliem - 17 Sep 2004

Contributors

-- KarlaRamirez - 19 May 2004

• Set ALLOWTOPICCHANGE = SchematicsEditorsGroup