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.