haskell: convert one member of a list of lists from char to int

Question

Suppose I have a list like this one:

list= [["a","123","b"],["h","435","t"],["w","234","j"]]

What I need is to convert each second member of that list of lists into an Integer because it will serve as a size indicator after that so I can sort my list by size.

I've came up with a function for conversion:

charToInt c = ord (chr c)

but I dont know how to convert each second member of the list.

Answer 1

To expand on what Paul Johnson said, you need to define a datatype for the data you're trying to hold.

data MusicFile = MusicFile {music :: String,
                            size :: Integer,
                            artist :: String}
    deriving Show

musicFileFromStrings :: [String] -> MusicFile
musicFileFromStrings [sMusic, sSize, sArtist]
   = MusicFile sMusic (read sSize) sArtist

Then if you have

list = [["a","123","b"],["h","435","t"],["w","234","j"]]

you can say

anotherList = map musicFileFromStrings list

and then

map music anotherList   -- ["a", "h", "w"]
map artist anotherList  -- ["b", "t", "j"]

(EDIT)

If you want to sort the list by a particular field you can use "sortBy" and " comparing ".

import Data.List
import Data.Ord

sizeOrderList = sortBy (comparing size) anotherList

The "comparing" function turns a function on a value (in this case "size") into a comparison function between two values. The only requirement is that the output of "size" (in this case) be a type that is an instance of "Ord".

If you want descending order than use

sizeOrderList = sortBy (comparing (flip size)) anotherList

Answer 2

You cannot convert the second element of the inner lists to an Integer without changing the type from a list to something like a tuple. The reason is that lists are homogeneous in Haskell, so you need a tuple to represent mixed types.

Converting a String into an Integer is done like this:

read "123" :: Integer

You need to add the type directly since the type of read is Read a => String -> a , meaning that it'll return something of a type that can be "read". Luckily, Integer is a member of that type class, so we can convert String into Integer .

Now it's just a simple matter of converting the second element of each inner list:

convert :: [[String]] -> [(String, Integer, String)]
convert lists = map (\[a, b, c] -> (a, read b, c)) lists