Data.Array

The Ix class is used to map a contiguous subrange of values in a type onto integers. It is used primarily for array indexing (see the array package).

The first argument (l,u) of each of these operations is a pair specifying the lower and upper bounds of a contiguous subrange of values.

An implementation is entitled to assume the following laws about these operations:

• inRange (l,u) i == elem i (range (l,u))

• range (l,u) !! index (l,u) i == i, when inRange (l,u) i

• map (index (l,u)) (range (l,u))) == [0..rangeSize (l,u)-1]

• rangeSize (l,u) == length (range (l,u))

The type of immutable non-strict (boxed) arrays with indices in i and elements in e.

Construct an array with the specified bounds and containing values for given indices within these bounds.

The array is undefined (i.e. bottom) if any index in the list is out of bounds. The Haskell 2010 Report further specifies that if any two associations in the list have the same index, the value at that index is undefined (i.e. bottom). However in GHC's implementation, the value at such an index is the value part of the last association with that index in the list.

Because the indices must be checked for these errors, array is strict in the bounds argument and in the indices of the association list, but non-strict in the values. Thus, recurrences such as the following are possible:

a = array (1,100) ((1,1) : [(i, i * a!(i-1)) | i <- [2..100]])

Not every index within the bounds of the array need appear in the association list, but the values associated with indices that do not appear will be undefined (i.e. bottom).

If, in any dimension, the lower bound is greater than the upper bound, then the array is legal, but empty. Indexing an empty array always gives an array-bounds error, but bounds still yields the bounds with which the array was constructed.

Construct an array from a pair of bounds and a list of values in index order.

The accumArray function deals with repeated indices in the association list using an accumulating function which combines the values of associations with the same index.

For example, given a list of values of some index type, hist produces a histogram of the number of occurrences of each index within a specified range:

hist :: (Ix a, Num b) => (a,a) -> [a] -> Array a b
hist bnds is = accumArray (+) 0 bnds [(i, 1) | i<-is, inRange bnds i]

accumArray is strict in each result of applying the accumulating function, although it is lazy in the initial value. Thus, unlike arrays built with array, accumulated arrays should not in general be recursive.

The value at the given index in an array.

The bounds with which an array was constructed.

The list of indices of an array in ascending order.

The list of elements of an array in index order.

The list of associations of an array in index order.

Constructs an array identical to the first argument except that it has been updated by the associations in the right argument. For example, if m is a 1-origin, n by n matrix, then

m//[((i,i), 0) | i <- [1..n]]

is the same matrix, except with the diagonal zeroed.

Repeated indices in the association list are handled as for array: Haskell 2010 specifies that the resulting array is undefined (i.e. bottom), but GHC's implementation uses the last association for each index.

accum f takes an array and an association list and accumulates pairs from the list into the array with the accumulating function f. Thus accumArray can be defined using accum:

accumArray f z b = accum f (array b [(i, z) | i <- range b])

accum is strict in all the results of applying the accumulation. However, it is lazy in the initial values of the array.

ixmap allows for transformations on array indices. It may be thought of as providing function composition on the right with the mapping that the original array embodies.

A similar transformation of array values may be achieved using fmap from the Array instance of the Functor class.