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How to list other lists?
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How to list other lists?
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I'm trying to make a list contains several smaller lists, and double the first element of each sub-lists.Here is my code:
double_the_first::(Num a)=>[[a]]->[a]
double_the_first list = case list of
[] ->[]
[[x]] ->[x*2]
[x:xs] ->map(*2)(map(head)[x:xs])
How can I represent a list like [[2,3],[4,5]]? I know my code is wrong, but haskell does not show any error, how? It will not work when I input a list like this:
double_the_first [[2,3],[2,4],[4,3]]
*** Exception: test.hs:(3,25)-(6,51): Non-exhaustive patterns in case
Can anyone help me?
First write a function that does what you want to each element of the list. In your case, each element is itself a list.
doubleFirst :: Num a => [a] -> [a]
doubleFirst [] = []
doubleFirst (x:xs) = (2*x:xs)
Then use map to apply that function to the list of elements:
map doubleFirst [[2,3],[2,4],[4,3]]
Tags
haskell
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