On Wed, Jul 20, 2005 at 02:27:36PM +0800, Sun Yi Ming wrote:
Hello, I have two txt file,and i want to mix the two files line by line, e.g. $ cat url1.txt url1_1.line url1_2.line $ cat url2.txt url2_1.line url2_2.line and i want this file as result: $ cat aha.txt url1_1.line url2_1.line url1_2.line url2_2.line
i first write this snippet of code: --- import System.IO
mix :: [a] -> [a] -> [a] mix [] ys = ys mix xs [] = xs mix (x:xs) (y:ys) = [x,y] ++ mix xs ys
f1 = do contents1 <- readFile "url1.txt" contents2 <- readFile "url2.txt" let urls1 = lines contents1 urls2 = lines contents2 urls = mix urls1 urls2 writeFile "aha.txt" (unlines urls) -- this works fine, but i think if the two file are very big, and the readFile will consume too many mem.
Ah, but this is exactly where lazyness wins bigtime: a smart implementation of readFile will lazily read the actual file for as far as needed. Thus, reading with readFile will not read the entire file into memory at once. What will happen is that writeFile starts writing, and upon discovery of needing the value of urls it will then start reading. Any value already written in this case obviously turns into garbage and will be garbage collected. I would be slightly surprised if this code uses more than constant memory.
so i need to read the file line by line but stunned by the loop in IO Monad: --- main = do h1 <- openFile "url1.txt" ReadMode h2 <- openFile "url2.txt" ReadMode line1 <- hGetLine h1 line2 <- hGetLine h2 print $ line1 : line2 : [] -- i don't howto do hClose h1 hClose h2 -- any ideas? thank you all.
Yes. You need to split the lines line1 <- hGetLine h1 line2 <- hGetLine h2 print $ line1 : line2: [] into a separate function that will then recurse over the file. Doei, Arthur. -- /\ / | arthurvl@cs.uu.nl | Work like you don't need the money /__\ / | A friend is someone with whom | Love like you have never been hurt / \/__ | you can dare to be yourself | Dance like there's nobody watching