mukesh tiwari mukeshtiwari.iiitm at gmail.com
Tue Nov 8 12:50:42 CET 2011

```I am  not sure about Int overflow. There is no case of Int overflow in
prime , pList and divPrime function however lets assuming Int overflow in
main but then still answer should be outputted.

Regards
Mukesh Tiwari

On Tue, Nov 8, 2011 at 5:08 PM, Lyndon Maydwell <maydwell at gmail.com> wrote:

> Could Int be overflowing?
>
> On Tue, Nov 8, 2011 at 7:21 PM, mukesh tiwari
> <mukeshtiwari.iiitm at gmail.com> wrote:
> > 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 Data.Array.ST
> > import Data.Array.Unboxed
> >
> > 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
> >
> > _______________________________________________