[Haskell-beginners] Convert String to List/Array of Numbers
lorenzo.isella at gmail.com
Wed Sep 8 13:24:12 EDT 2010
Thanks for your help.
I have a couple of questions left
(1) The first one is quite down to earth.
The snippet below
main :: IO ()
main = do
txt <- readFile "mydata.dat"
let dat = convert txt
print dat -- this prints out my chunk of data
convert x = lines x
pretty much does what it is supposed to do, but if I use this definition
of convert x
convert x = map (map read . words) . lines x
I bump into compilation errors. Is that the way I am supposed to deal
with your function?
(2) This is a bit more about I/O in general. I start an action with "do"
to read some files and I define outside the action some functions which
are supposed to operate (within the do action) on the read data.
Is this the way it always has to be? I read something about monads but
did not get very far (and hope that they are not badly needed for simple
I/O). Is there a way in Haskell to have the action return to the outside
world e.g. the value of dat and then work with it elsewhere?
That is what I would do in Python or R, but I think I understood that
Haskell's philosophy is different...
Am I on the right track here? And what is the benefit of this?
On 09/08/2010 04:06 PM, Daniel Fischer wrote:
> On Wednesday 08 September 2010 15:31:19, Lorenzo Isella wrote:
>> Dear All,
>> I must be stuck on something pretty basic (I am struggling badly with
>> I/O). Let us assume you have a rather simple file mydata.dat (3 columns
>> of integer numbers), see below.
>> 1246191122 1336 1337
>> 1246191142 1336 1337
>> 1246191162 1336 1337
>> 1246191182 1336 1337
>> 1246191202 1336 1337
>> 1246191222 1336 1337
>> 1246191242 1336 1337
>> 1246191262 1336 1337
>> 1246191282 1336 1337
>> 1246191302 1336 1337
>> 1246191322 1336 1337
>> 1246191342 1336 1337
>> 1246191362 1336 1337
>> 1246191382 1336 1337
>> 1246191402 1336 1337
>> 1246191422 1336 1337
>> Now, my intended pipeline could be
>> read file as string--> convert to list of integers-->pass it to hmatrix
>> (or try to convert it into a matrix/array).
>> Leaving aside the last step, I can easily do something like
>> let dat=readFile "mydata.dat"
>> in the interactive shell and get a string,
> Not quite. `dat' is the IO-action that reads the file, of type (IO String)
> and not a String.
> In a programme, you'd do something like
> main = do
> ... -- argument parsing perhaps
> txt<- readFile "mydata.dat"
> let dat = convert txt
> doSomething with dat
>> but I am having problems in
>> converting this to a list or anything more manageable (where every entry
>> is an integer number i.e. something which can be summed, subtracted
>> etc...). Ideally even a list where every entry is a row (a list in
>> itself) would do.
> Depending on what the reult type should be, different solutions are
> The simplest solutions for such a file format are built from
> read -- to convert e.g. "135" to 135
> lines :: String -> [String]
> words :: String -> [String]
> map :: (a -> b) -> [a] -> [b]
> If you want a flat list of Integers from that file,
> convert = map read . words
> will do. First, `words' splits the String on whitespace (spaces and
> newlines), producing a list of digit-strings, those are then read as
> If you want a list of lists, each line its own list inside the top level
> convert = map (map read . words) . lines
> is what you want.
> If you want to convert each line into a different data structure, say
> (Integer, Double, Int64), the general form would still be
> convert = map parseLine . lines
> and parseLine would depend on the structure you want. For the above,
> parseLine str
> = case words str of
> (a : b : c : _) -> (read a, read b, read c)
> _ -> error "Bad line format"
> would be a solution.
> For any but the simplest formats, you should write a real parser to deal
> with possible bad formatting though (writing parsers is fun in Haskell).
>> I found online this suggestion
>> but I am not sure if it really applies to this case.
>> Many thanks
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