Counting unique elements in a list

Question

Is there a straight-forward combination of standard higher-order functions to count the unique elements in a list?

For example the result for

[1, 1, 4, 0, 4, 4]

would be something like

[(1,2), (4,3), (0,1)]

Answer 1

If order is not important this works:

map (\xs@(x:_) -> (x, length xs)) . group . sort

group . sort will give you a list of lists where all elements that are equal to each other are grouped into the same sublist (without sort, only consecutive equal elements would be grouped together). The map then turns each sublist into a (element, lengthOfSublist)-tuple.

If you want to order the result by first occurrence, you can use zip before the sort to add an index to each element, then, after grouping, sort again by that index and then remove the index.

Answer 2

The simplest thing would be to sort the items into order, use "group" to put them into sub-lists of equal elements, and then count the items in each sub-list.

map (\xs -> (head xs, length xs)) . group . sort