Question

I am trying to do the fractional knapsack problem in haskell so far I have

Code:

{- Input "how much can the knapsack hole <- x" "Possible items in sack [(label, value, weight), ...]" -}
knap x [] = []
knap x y = if length y == 1 then 

The input list is in the form [([Char], Integer, Integer), … ] a list of lists of (list of chars, integer, and integer).

My problem is trying to pull out the label, value, and weight of each item that could possibly be put in the knapsack. (pulling values out of the list of lists)

In my prelude> prompt I am doing some trying by doing

ghci output:

Prelude> let x = [("label 1", 2, 14), ("label 2", 1, 15)]
Prelude> :t x
x :: [([Char], Integer, Integer)]
Prelude> length x
2
Prelude> x !! 0
("label 1",2,14)
Prelude> x !! 0 !! 1

<interactive>:1:1:
    Couldn't match expected type `[a0]'
                with actual type `([Char], Integer, Integer)'
    Expected type: [[a0]]
      Actual type: [([Char], Integer, Integer)]
    In the first argument of `(!!)', namely `x'
    In the first argument of `(!!)', namely `x !! 0'

As you can see I am trying to do list !! index !! index to try and pull a weight off of a “item”. What is the proper syntax to do this?

Answer 1

Well, the !! operator only works on lists, as you can see from its type signature: [a] -> Int -> a.

If you’d like to stick with tuples, you could define your own functions for 3-tuples in the style of fst and snd. You could do this with pattern matching; something like:

first :: (a,b,c) -> a
first (a,_,_) = a

However, it might be better to design a datatype for an item, and use records to extract the field you need.

data Item = Item { label  :: String
                   value  :: Int
                   weight :: Int
                 }

Then to make a new item x you can use let x = Item {label = "label 1", value = 2, weight = 14}

Now you could model your knapsack as a list of items of type [Item], and to get the value of the first item, you could use value $ knapsack !! 0, where knapsack is your list of items.