Haskell 101

February 4, 2008

Matching items

Filed under: Functional programming, Guards, Haskell, Pattern matching, Recursion — Haskell 101 blogger @ 8:02 pm

So far, we have created a function that determine whether a given item appears in a list, and another that counts how many times a given item appears in a list. Now let’s create one that returns a complete list of all items from a given list that match a given item. In other words, we have a list y and want to get a list of all items in y that match a given item x.We want a function that takes an integer and a list, and returns a list. 

(1) matches::Int->[Int]->[Int]

 As we did with our instances function, we start with the case that ends a recursion. 

(2) matches x [] = []

 For the other cases, we’ll mimic the structure from our instances function. This time, though, instead of adding one to the total, we’ll simply add the matching item to the list we’re collecting. 

(3) matches x (y:ys)
	| x==y = x:(matches x ys)
	| otherwise = matches x ys

 

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