►The Racket Guide
▼3 Built-In Datatypes
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3.10 Hash Tables

A hash table implements a mapping from keys to values, where both keys and values can be arbitrary Racket values, and access and update to the table are normally constant-time operations. Keys are compared using equal?, eqv?, or eq?, depending on whether the hash table is created with make-hash, make-hasheqv, or make-hasheq.

Examples:
> (define ht (make-hash))
> (hash-set! ht "apple" '(red round))
> (hash-set! ht "banana" '(yellow long))
> (hash-ref ht "apple")

'(red round)

> (hash-ref ht "coconut")

hash-ref: no value found for key

  key: "coconut"

> (hash-ref ht "coconut" "not there")

"not there"

The hash, hasheqv, and hasheq functions create immutable hash tables from an initial set of keys and values, in which each value is provided as an argument after its key. Immutable hash tables can be extended with hash-set, which produces a new immutable hash table in constant time.

Examples:
> (define ht (hash "apple" 'red "banana" 'yellow))
> (hash-ref ht "apple")

'red

> (define ht2 (hash-set ht "coconut" 'brown))
> (hash-ref ht "coconut")

hash-ref: no value found for key

  key: "coconut"

> (hash-ref ht2 "coconut")

'brown

A literal immutable hash table can be written as an expression by using #hash (for an equal?-based table), #hasheqv (for an eqv?-based table), or #hasheq (for an eq?-based table). A parenthesized sequence must immediately follow #hash, #hasheq, or #hasheqv, where each element is a dotted key–value pair. The #hash, etc. forms implicitly quote their key and value sub-forms.

Examples:
> (define ht #hash(("apple" . red)
                   ("banana" . yellow)))
> (hash-ref ht "apple")

'red

+Reading Hash Tables in The Racket Reference documents the fine points of the syntax of hash table literals.

Both mutable and immutable hash tables print like immutable hash tables, using a quoted #hash, #hasheqv, or #hasheq form if all keys and values can be expressed with quote or using hash, hasheq, or hasheqv otherwise:

Examples:
> #hash(("apple" . red)
        ("banana" . yellow))

'#hash(("banana" . yellow) ("apple" . red))

> (hash 1 (srcloc "file.rkt" 1 0 1 (+ 4 4)))

(hash 1 (srcloc "file.rkt" 1 0 1 8))

A mutable hash table can optionally retain its keys weakly, so each mapping is retained only so long as the key is retained elsewhere.

Examples:
> (define ht (make-weak-hasheq))
> (hash-set! ht (gensym) "can you see me?")
> (collect-garbage)
> (hash-count ht)

0

Beware that even a weak hash table retains its values strongly, as long as the corresponding key is accessible. This creates a catch-22 dependency when a value refers back to its key, so that the mapping is retained permanently. To break the cycle, map the key to an ephemeron that pairs the value with its key (in addition to the implicit pairing of the hash table).

+Ephemerons in The Racket Reference documents the fine points of using ephemerons.

Examples:
> (define ht (make-weak-hasheq))
> (let ([g (gensym)])
    (hash-set! ht g (list g)))
> (collect-garbage)
> (hash-count ht)

1

> (define ht (make-weak-hasheq))
> (let ([g (gensym)])
    (hash-set! ht g (make-ephemeron g (list g))))
> (collect-garbage)
> (hash-count ht)

0

+Hash Tables in The Racket Reference provides more on hash tables and hash-table procedures.

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