How to add values from two lists (+extra condition) in Haskell

Question

I got a problem with this exercise. I've been trying to solve it for a long time searching for stuff, but I am unable to.

Define functions:

addLnat :: [Int] -> [Int] -> [Int]
mulLnat :: [Int] -> [Int] -> [Int]

addLnat adds numbers from two arrays eg.

addLnat [4,5,6] [8,5,2] ->  [2,1,9]

as [4+8 gives 2 carry 1, 5+5+1 gives 1 carry 1, 6+2+1 = 9]

Lnat, is a "list natural number", represented as a list of base-10 digits, least significant first. So the number 654 is [4,5,6].

What I got is:

addLnat :: [Int] -> [Int] -> [Int]
addLnat _ [] = []
addLnat [] _ = []
addLnat (x:xs) (y:ys) = (if (x+y) > 9 then x+y-10 else (x+y)):(addLnat xs ys)

I adding number and ignoring carry. Not sure how to solve it. Any help would be much appreciated.

I have improved the solution as per user5402 comment, so created addLnat' cr xs ys, but when I what to pass carry as a parameter it fails to load - most probable that I am getting the syntax wrong:( (cr is 0 only for now and it will be replaced by maths).

addLnat' c (x:xs) (y:ys) = d : addLnat' cr xs ys 
where d = if c+x+y < 9 then x+y else c+x+y-((quot (c+x+y) 10)*10) 
cr = 0

Any ideas?

Here's a hint: try defining a helper function addLnatCarry :: Int -> [Int] -> [Int] -> [Int] which takes as a first argument a carry digit (or a Bool telling whether there's a carry if you prefer). — Daniel Wagner
– Daniel Wagner, Commented Jan 11, 2015 at 23:42
Thanks - changed array to list in the topic. It's what I thought , but that was what I was given by someone who knows the stuff. — Lutka
– Lutka, Commented Jan 12, 2015 at 12:20

ig-melnyk · Accepted Answer · 2015-01-11 23:57:58Z

2

I am not very good at haskell but maybe this can help ;

add::[Int]->[Int]->[Int]
add x y = add' 0 x y

There we define a function add that will use add' to add two lists The main idea is to "save" carry and carefully work with corner cases. Here carry is saved in "variable" rest

    add'::Int->[Int]->[Int]->[Int]
    add' 0 x []      = x 
    add' rest (x:[]) (y:[]) = [(r `mod` 10),(r `div` 10)]
       where r = x+y+rest
    add' y (x:xs) [] = add' (r `div` 10) ((r `mod` 10):xs) [] 
       where r = x+y
    add' rest (x:xs) (y:ys) = (r `mod` 10) : (add' (r `div` 10) xs ys)
       where r = x+y+rest

List x must be bigger than list y but that's not a problem

add [5,7,8] [4,3,2]  => [9,0,1,1] (correct)
add [1,2,3] [4,5,6]  => [5,7,9,0] (correct)

edited Jan 11, 2015 at 23:57

answered Jan 11, 2015 at 23:52

ig-melnyk

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Comments

ErikR · Accepted Answer · 2015-01-11 23:43:37Z

2

You need to write a version of addLnat which accepts a carry parameter:

addLnat' c (x:xs) (y:ys) = d : addLnat c' xs ys
  where d = if c+x+y > 9 then ... else ...
        c' = ... the next carry bit ...

There are a lot more details and corner cases to consider, but this is the basic idea. Finally,

addLnat xs ys = addLnat' 0 xs ys  -- initially the carry is 0

answered Jan 11, 2015 at 23:43

ErikR

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1 Comment

Lutka Over a year ago

Thanks a lot for your help :-) That's the way I took when I was starting with it. It's what I got working (what doesn't include real carry) addLnat' c (x:xs) (y:ys) = d : addLnat' 0 xs ys where d = if c+x+y < 9 then x+y else c+x+y-((quot (c+x+y) 10)*10) When I want to include real carry as a variable, it doesn't work addLnat' c (x:xs) (y:ys) = d : addLnat' cr xs ys where d = if c+x+y < 9 then x+y else c+x+y-((quot (c+x+y) 10)*10) cr = 0 My guess is wrong syntax, but I cannot get it right :-(

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