Comparison of programming languages (associative array): Difference between revisions

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Revision as of 02:24, 8 January 2009

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=== Haskell ===

The [[Haskell (programming language)|Haskell]] programming language provides three different associative containers. The simplest is a list of pairs:

<pre>

m = [("Sally Smart", "555-9999"), ("John Doe", "555-1212"), ("J. Random Hacker", "553-1337")]

main = print (lookup "John Doe" m)

</pre>

output:

Just "555-1212"

Note that the lookup function returns a "Maybe" value, which is "Nothing" if not found, or "Just ''result''" when found.

The second is polymorphic functional maps (represented as immutable balanced binary trees):

<pre>

import qualified Data.Map as M

m = M.insert "Sally Smart" "555-9999" M.empty

m' = M.insert "John Doe" "555-1212" m

m'' = M.insert "J. Random Hacker" "553-1337" m'

main = print (M.lookup "John Doe" m'' :: Maybe String)

</pre>

output:

Just "555-1212"

A specialized version for integer keys also exists as Data.IntMap.

Finally, a polymorphic hash table:

<pre>

import qualified Data.HashTable as H

main = do m <- H.new (==) H.hashString

H.insert m "Sally Smart" "555-9999"

H.insert m "John Doe" "555-1212"

H.insert m "J. Random Hacker" "553-1337"

foo <- H.lookup m "John Doe"

print foo

</pre>

output:

Just "555-1212"

Lists of pairs and functional maps both provide a purely functional interface, which is more idiomatic in Haskell. In contrast, hash tables provide an imperative interface in the IO monad.

=== Java ===