[Haskell-cafe] Project Euler Problem 357 in Haskell

[mukesh tiwari]

Hello all
Being a Haskell enthusiastic , first I tried to solve this problem in
Haskell but it running for almost 10 minutes on my computer but not getting
the answer. A similar C++ program outputs the answer almost instant so
could some one please tell me how to improve this Haskell program.

import Control.Monad.ST
import Data.Array.ST
import Data.Array.Unboxed
import Control.Monad

prime :: Int -> UArray Int Bool
prime n = runSTUArray $ do
    arr <- newArray ( 2 , n ) True :: ST s ( STUArray s Int Bool )
    forM_ ( takeWhile ( \x -> x*x <= n ) [ 2 .. n ] ) $ \i -> do
        ai <- readArray arr i
        when ( ai  ) $ forM_ [ i^2 , i^2 + i .. n ] $ \j -> do
            writeArray arr j False

    return arr

pList :: UArray Int Bool
pList = prime $  10 ^ 8

divPrime :: Int -> Bool
divPrime n = all ( \d -> if mod n d == 0 then pList ! ( d + div  n  d )
else True )  $  [ 1 .. truncate . sqrt . fromIntegral  $ n ]


main = putStrLn . show . sum  $ [ if and [ pList ! i , divPrime . pred $ i
] then pred  i else 0 | i <- [ 2 .. 10 ^ 8 ] ]


C++ program which outputs the answer almost instant.

#include<cstdio>
#include<iostream>
#include<vector>
#define Lim 100000001
using namespace std;

bool prime [Lim];
vector<int> v ;

void isPrime ()
     {
		for( int i = 2 ; i * i <= Lim ; i++)
		 if ( !prime [i]) for ( int j = i * i ; j <= Lim ; j += i ) prime [j] = 1 ;

		for( int i = 2 ; i <= Lim ; i++) if ( ! prime[i] ) v.push_back( i ) ;
		//cout<<v.size()<<endl;
		//for(int i=0;i<10;i++) cout<<v[i]<<" ";cout<<endl;

     }

int main()
	{
		isPrime();
		int n = v.size();
		long long sum = 0;
		for(int i = 0 ; i < n ; i ++)
		 {
			int k = v[i]-1;
			bool f = 0;
			for(int i = 1 ; i*i<= k ; i++)
				if ( k % i == 0 && prime[ i + ( k / i ) ] )  { f=1 ; break ; }

			if ( !f ) sum += k;
		 }
		cout<<sum<<endl;
	}


Regards
Mukesh Tiwari
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