Create a boolean array " prime [ 0 . . high ] " and initialize all entries it as true . A value in prime [ i ] will finally be false if i is Not a prime , else true .
bool [ ] prime = new bool [ high + 1 ] ; bool twin = false ; for ( int i = 0 ; i < prime . Length ; i ++ ) { prime [ i ] = true ; } prime [ 0 ] = prime [ 1 ] = false ;