Hash Table and Hash Set

In hashtable: Hash Table and Hash Set

The hashtable packages provides three implementations of hash tables and hash maps:

1. using std::unordered_map and std::unordered_set from C++, in functions hash_table() and hash_set(),
2. using the fastmatch package, in functions hash_fm_table() and hash_fm_set(),
3. using environment, in functions hash_env_table() and hash_env_set().

They share the same user interface.

Hash table

Three ways to create hash tables:

library(hashtable)
h1 = hash_table(letters, 1:26)
h2 = hash_fm_table(letters, 1:26)
h3 = hash_env_table(letters, 1:26)
h1
h2
h3

The user interfaces for the two methods are the same. We only demonstrate using h1.

Get all keys:

hash_keys(h1)

Get all values:

hash_values(h1)

Get a subset of values by specifying keys:

hash_values(h1, c("a", "b", "c"))

Get a single value using $, [[ or [:

h1$a
h1[["a"]]
h1[c("a", "b")]

Test whether keys are in the hash table:

hash_exists(h1, c("a", "b", "foo"))

Delete key-value pairs:

hash_delete(h1, c("a", "b"))
hash_exists(h1, c("a", "b"))

Insert new key-value pairs, or modify key-value pairs if they already exist:

hash_insert(h1, "c", 100L); h1$c
hash_insert(h1, "c", -1L); h1$c
hash_insert(h1, "foo", 0L); h1$foo

Insert or modify key-value pairs using $<-, [[<- or [<-:

h1$a = 20L; h1$a
h1[["bar"]] = -100L; h1$bar
h1[c("c", "d", "e")] = c(-1L, -2L, -3L); h1[c("c", "d", "e")]

In previous examples, values are atomic vectors. It is basically the same if values are more general list.

h1 = hash_table(c("a", "b"), list(1L, "text"))
h1
h1$c = 3.14
h1$d = lm(1~1) # an lm object
h1

Convert between hash table and named vector or list:

h1 = hash_table(c("a", "b"), 1:2)
as.vector(h1)

h1 = hash_table(c("a", "b"), list(1L, "text"))
as.vector(h1)

vec = structure(1:2, names = c("a", "b"))
as.hash_table(vec)

lt = structure(list(1L, "text"), names = c("a", "b"))
as.hash_table(lt)

Hash set

Hash sets are hash tables with no value associated. There are also three ways to create hash sets.

h1 = hash_set(letters)
h2 = hash_fm_set(letters)
h3 = hash_env_set(letters)
h1

Get all keys:

hash_keys(h1)

Hash set has no value associated, so calling hash_value() throws an error.

hash_values(h1)

Test whether keys are in the hash set:

hash_exists(h1, c("a", "foo"))

Add new keys:

hash_insert(h1, "foo")
hash_exists(h1, "foo")

Delete keys:

hash_delete(h1, "foo")
hash_exists(h1, "foo")

Hash sets are basically used to test whether keys exist, so we let $, [[ and [ to return TRUE or FALSE to represent whether keys exist.

h1$a
h1[["a"]]
h1$foo
h1[c("a", "b", "foo")]

If assigning TRUE, new keys are added to the hash set (if they do not exist), and if assigning FALSE, corresponding keys are deleted.

h1$a = FALSE
hash_exists(h1, "a")
h1$foo = TRUE
hash_exists(h1, "foo")

Convert between vectors (where elements are unique) and hash sets:

as.vector(h1)
as.hash_set(letters)

hash_fm_table and hash_fm_set

Although sharing the same user interface, the hash table created by hash_fm_table() allows to modify the values if corresponding keys already exist.

h = hash_fm_table(letters, 1:26)
h$a = 100L

But it does not allow to delete or create key-value pairs.

h$foo = 1L  # insert new key foo
hash_delete(h, "a")

The hash set created by hash_fm_set() does not allow to delete or create keys.

h = hash_fm_set(letters)
h$a = FALSE
h$foo = TRUE

Session info

sessionInfo()

hashtable documentation built on July 27, 2026, 5:09 p.m.