CODEWITHSUNDEEP
haskellconcatenation
Nomics
Nomics

Reputation: 716

concatenation - 2 lists

I need some help :/

I have the fowlling list:

xs = [("a",1,[(11,"aa","aa"),(22,"bb","bb")]),("b",2,[(33,"cc","cc"),(44,"dd","dd")])]

And I need to make a function that allows me to change the values of "a" and 1 (the first 2 values from the first 3-tuple on the list).

I tried to solve this by spliting my list:

xs1 = ("a",1,[(11,"aa","aa"),(22,"bb","bb")])
xs2 = ("b",2,[(33,"cc","cc"),(44,"dd","dd")])

then:

changeXs (_, _, z) x y = (x,y,z)
newXs x y = [changeXs xs1 x y]

this works for change xs1. I thought that now it would be easy concatenate xs2 to my newXs function, using ++ ... But it wont work (I believe I cant use ++ whit pattern matching, am I right?)

Any ideas about how can I concatenate xs2 to my newXs function ? Or any other approach to solve the problem?

Thanks in advance.

Upvotes: 0

Views: 778

Answers (3)

lbolla
lbolla

Reputation: 5411

You cannot concatenate newXs x y, that is a list, with xs2, that is a tuple.

Upvotes: 0

amindfv
amindfv

Reputation: 8448

Your problem is with ++.

++ has the type [a] -> [a] -> [a]
In other words, it takes two lists and "puts them together"
What you want is to add some a to a [a]

Try
(:) :: a -> [a] -> [a]

Upvotes: 0

Cat Plus Plus
Cat Plus Plus

Reputation: 130004

insert_new_xy x y ((_, _, z) : rest) = (x, y, z) : rest

Test:

*Main> insert_new_xy "foo" 42 xs
[("foo",42,[(11,"aa","aa"),(22,"bb","bb")]),("b",2,[(33,"cc","cc"),(44,"dd","dd")])]

The ((_, _, z) : rest) pattern matches a list, whose head is a three-element tuple and binds last element in that tuple and the tail of the list. Then it's just a matter of recombining the list, by creating a new tuple with given x, y and extracted z, and prepending it to the tail.

Upvotes: 4

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