Haskell – How to get a List of Tuples with certain combinations
Problem Description:
Is there a way to create a list of tuples with a certain pattern?
I have following inputs for example:
a = 100
b = [10,20,30]
c = ['a','b','c','d','e','f','g']
And I would like to have following Output:
[(100,10,'a'),(100,10,'b'),(100,10,'c'),(100,10,'d'),(100,10,'e'),(100,10,'f'),(100,10,'g'),
(100,20,'a'),(100,20,'b'),(100,20,'c'),(100,20,'d'),(100,20,'e'),(100,20,'f'),(100,20,'g'),
(100,30,'a'),(100,30,'b'),(100,30,'c'),(100,30,'d'),(100,30,'e'),(100,30,'f'),(100,30,'g')]
I think if I only had a and b for example I would do:
makeTuple :: Int -> [Int] -> [(Int,Int)]
makeTuple _ [] = []
makeTuple i (x:xs) = [(i,x)] ++ makeTuple i xs
This would give me:
[(100,10),(100,20),(100,30)]
Im not sure how I could make it work with 3 Inputs .
Solution – 1
You can use applicative composition:
ghci> (,,) <$> [a] <*> b <*> c
[(100,10,'a'),(100,10,'b'),(100,10,'c'),(100,10,'d'),(100,10,'e'),(100,10,'f'),(100,10,'g'),
(100,20,'a'),(100,20,'b'),(100,20,'c'),(100,20,'d'),(100,20,'e'),(100,20,'f'),(100,20,'g'),
(100,30,'a'),(100,30,'b'),(100,30,'c'),(100,30,'d'),(100,30,'e'),(100,30,'f'),(100,30,'g')]
Here’s an explanation of how that works, with examples in F#, Haskell, and C#: An applicative password list.
Solution – 2
List comprehensions can do it nicely:
[(a, x, y) | x <- b, y <- c]