How to find the frequency of characters in a string in Haskell?

Question

How can I count the frequency of characters in a string and then output them in sort of a table?

For example, if I input the word \"happy\" the result would be

Answer 1

Use list comprehension, no need for any imports or sorting.

[ (x,c) | x<-['A'..'z'], let c = (length.filter (==x)) "happy", c>0 ]

Result:

[('a',1),('h',1),('p',2),('y',1)]

Above is the filtered and rewritten (only character with count > 0) from:

[(x,(length.filter (==x)) "happy" ) | x<-['A'..'z']]

Explanation:

• Make a list of all characters that match a given character (A..z).
• For each character, count this list (==length)
• Put this count in a tuple with the character