Humn... I agree with both of you, Albert and Tom. I started it from the beginning, using map and don't using reverse anymore. But the C program is still 7x faster than the Haskell one. Here is the code of the Haskell program: main :: IO () main = do bmFile <- openFileEx "in.txt" (BinaryMode ReadMode) bmString <- hGetContents bmFile writeFile "out.txt" (map inc bmString) hClose bmFile inc :: Char -> Char inc a = toEnum ((fromEnum a) + 1) And now the C program: #include void main() { FILE *in, *out; char ch; in = fopen("in.txt","rb"); out = fopen("out.txt","wb"); while ( (ch=getc(in)) != EOF) putc(ch + 1,out); fclose(out); fclose(in); } As you noticed, the main idea here is to increment the character by "one". Is the Haskell program now a nice translation of the C program? If it is (I think so...) why is the C program so much faster than it? Thanks again, -- Andre ----- Original Message ----- From: Albert Lai To: Sent: Thursday, October 18, 2001 9:42 PM Subject: Re: Working character by character in Haskell

"Andre W B Furtado" writes:

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For example, suppose function doSomeStuffWith returns its own parameter. Using a 1.5MB file in this case, the Haskell program ends in almost 5 seconds, while the C program ends in less than 0.5 seconds... Is my Haskell program too bad implemented? (I'm using GHC 4.08-1 under a Windows 98 platform.)

I indeed think that your Haskell program is inefficient and is not a translation of your C program. It reverses a huge string, which takes not only execution time but also garbage collection time and memory, which entails even more time for OS overhead and swapping out other things in the physical memory.

(Many programmers complain "my linear-time algorithm takes just 1 second on a 100M list, so why is it taking 5 seconds on a 200M list?" They forget that because of issues such as cache locality and virtual memory, their computers do not scale.)

I am wondering that if doSomeStuffWith is pure-functional, why are you writing and using copyFile instead of just using map? I mean:

main :: IO () main = do bmFile <- openFileEx "in.txt" (BinaryMode ReadMode) bmString <- hGetContents bmFile writeFile "out.txt" (map doSomestuffWith bmString) hClose bmFile

Because both hGetContents and map are lazy and use O(1) memory, this program behaves exactly as your C program plus occasional garbage collections that don't hurt too much. In partcular, the OS will see no difference (although the cache does).

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