Numeric Haskell: A Vector Tutorial

Quick Tour

Vector is a Haskell library for working with arrays. The main data types are boxed and unboxed arrays, and arrays may be immutable (pure), or mutable. Arrays are indexed by non-negative Int values.

The vector library has an API similar to the famous Haskell list library, with many of the same names.

Importing the library

Download the vector package:

$ cabal install vector

and import it as, for boxed arrays:

 import qualified Data.Vector as V

or:

 import qualified Data.Vector.Unboxed as V

for unboxed arrays. The library needs to be imported qualified as it shares the same function names as list operations in the Prelude.

Generating Vectors

New vectors can be generated in many ways:

$ ghci
GHCi, version 6.12.1: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.

Prelude> :m + Data.Vector

-- Generating a vector from a list:
Prelude Data.Vector> let a = fromList [10, 20, 30, 40]

Prelude Data.Vector> a
fromList [10,20,30,40] :: Data.Vector.Vector

-- Or filled from a sequence
Prelude Data.Vector> enumFromStepN 10 10 4
fromList [10,20,30,40] :: Data.Vector.Vector

-- A vector created from four consecutive values
Prelude Data.Vector> enumFromN 10 4
fromList [10,11,12,13] :: Data.Vector.Vector

You can also build vectors using operations similar to lists:

-- The empty vector
Prelude Data.Vector> empty
fromList [] :: Data.Vector.Vector

-- A vector of length one
Prelude Data.Vector> singleton 2
fromList [2] :: Data.Vector.Vector

-- A vector of length 10, filled with the value '2'
-- Note  that to disambiguate names,
-- and avoid a clash with the Prelude,
-- with use the full path to the Vector module
Prelude Data.Vector> Data.Vector.replicate 10 2
fromList [2,2,2,2,2,2,2,2,2,2] :: Data.Vector.Vector

In general, you may construct new vectors by applying a function to the index space:

Prelude Data.Vector> generate 10 (^2)
fromList [0,1,4,9,16,25,36,49,64,81] :: Data.Vector.Vector

Vectors may have more than one dimension:

-- Here we create a two dimensional vector, 10 columns,
-- each row filled with the row index.
Prelude Data.Vector> let x = generate 10 (\n -> Data.Vector.replicate 10 n)

-- The type is "Vector of Vector of Ints"
Prelude Data.Vector> :t x
x :: Vector (Vector Int)

Vectors may be grown or shrunk arbitrarily:

Prelude Data.Vector> let y = Data.Vector.enumFromTo 0 11
Prelude Data.Vector> y
fromList [0,1,2,3,4,5,6,7,8,9,10,11] :: Data.Vector.Vector

-- Take the first 3 elements as a new vector
Prelude Data.Vector> Data.Vector.take 3 y
fromList [0,1,2] :: Data.Vector.Vector

-- Duplicate and join the vector
Prelude Data.Vector> y Data.Vector.++ y
fromList [0,1,2,3,4,5,6,7,8,9,10,11,0,1,2,3,4,5,6,7,8,9,10,11] :: Data.Vector.Vector