[Haskell-cafe] Re: Nice way to calculate character frequency in a
Chad.Scherrer at pnl.gov
Tue Oct 25 11:58:36 EDT 2005
> Hello, I need to calculate the frequency of each character in a
> String. And if I can do this really well in C, I don't find a nice
> fast) answer in haskell. I tried several functions, listed below, and
> even the fastest do a lot of unnecessary things :
A while back, I was trying to build a "table" function similar to what
you describe, and Dean Herrington helped me speed it up and stop the
stack overflows I had been getting. Here it is, in case this is useful
table :: (Ord a) => [a] -> [(a,Int)]
table xs = Map.assocs $! foldl' f Map.empty xs
where f m x = let m' = Map.insertWith (+) x 1 m
Just v = Map.lookup x m'
in v `seq` m'
I would be interested in whether this is much slower than the other
approaches you've tried. If a faster approach could be made polymorphic,
I'd have a faster way to build tables!
Computational Mathematics Group
Pacific Northwest National Laboratory
"Time flies like an arrow; fruit flies like a banana." -- Groucho Marx
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