CODEWITHSUNDEEP
haskell
Azizul Karim
Azizul Karim

Reputation: 11

Function that query first element in a list of tuples and returns the other two

I would like to have list of tuples [(a,b,c)..] where a is String, b and c are Num. I will have a function, possibly pattern matching comprehension, that will only search for the first element (Unique ID) in any of the tuples in the list and if a match is found, the function will return remaining two elements in the tuple.

I'm new to programming and after much research I thought I would start with Haskell. I'm finding this http://learnyouahaskell.com very useful to start with but stuck at how comprehension and type annotations plays a role in pattern matching. So far what I see consistent in comprehension syntactically is that

{ Manipulate Set B, but mind the predicate | Trim it down further to make Set B | Select set A, Predicate }

I may sound naive and lazy, but honestly looking for some quick aha breakthrough so I can move on. Thanks in advance.

Upvotes: 1

Views: 713

Answers (2)

Hari Krishna
Hari Krishna

Reputation: 3548

getMtach [] _ = []
getMtach ((a, b, c):xs) key
    | (a == key) = (b, c) : getMtach xs key
    | otherwise = getMtach xs key

Main> getMtach [(1, "abc", 12.3), (2, "asg", 4.5), (1, "xyz", 123)] 1
[("abc",12.3),("xyz",123.0)]
*Main> 
*Main> getMtach [(1, "abc", 12.3), (2, "asg", 4.5), (1, "xyz", 123)] 2
[("asg",4.5)]

Upvotes: 0

Daniel Jour
Daniel Jour

Reputation: 16156

First, here's how you can pattern match inside list comprehensions:

[(b,c) | (a,b,c) <- input]
-- (b,c) constructs your results
-- (a,b,c) is the pattern matching

Then, just add your predicate:

[(b,c) | (a,b,c) <- input, a == id]

And, since these ids are unique, you're only interested in the first match, so just take 1 from the above result.

Personally, I like do notation better:

do (a,b,c) <- input
   guard (a == id)
   return (b,c)

Upvotes: 2

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