Re: [Haskell-cafe] Comments and/or Criticisms

On Mon, 2007-09-10 at 15:47 +1000, Stuart Cook wrote:

On 9/10/07, PR Stanley wrote:

...

Hi Any comments and/or criticisms would be most appreciated: --count the occurrences of char in string countC :: Char -> [Char] -> Int countC x xs = sum [1 | c <- xs, c == x]

That's a clever implementation, but I think there are clearer ways of achieving the same goal. You could replace sum with length and get the same answer, at which point the list comprehension isn't buying you much.

How about this?

countC ch xs = length $ filter (ch ==) xs

...

--count occurrences of chars in string countCS :: [Char] -> [(Char, Int)] countCS xs = [(x, (countC x xs)) | x <- [' '..'z'], (countC x xs) > 0]

A few things to note:

* Those extra parens around countC are unnecessary; function application has highest precedence. * [' '..'z'] has a few issues: it misses a few ASCII characters (including all the control codes), and won't catch non-ASCII characters. * Duplicating the call to countC will probably result in redundant evaluation, so it's best to avoid that if possible -- difficult to avoid cleanly inside a comprehension though. * Scanning the string repeatedly for each potential character [' '..'z'] seems excessive, especially if you want to expand it to include non-ASCII. Better just to deal with the characters actually in the string as they appear, I think.

You can use 'let' in a list comprehension, [(x,count) | x <- [' '..'z'], let count = countC x xs, count > 0]