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    <p>I check if a number is prime or not with the following code( of course using sqrt ):</p> <pre><code>bool IsPrime(const unsigned int x) { const unsigned int TOP = static_cast&lt;int&gt;( std::sqrt( static_cast&lt;double&gt;( x ) ) ) + 1; for ( int i=2; i != TOP; ++i ) { if (x % i == 0) return false; } return true; } </code></pre> <p>I use this method to determine the primes:</p> <pre><code>#include &lt;iostream&gt; using std::cout; using std::cin; using std::endl; #include &lt;cmath&gt; void initialize( unsigned int *, const unsigned int ); void show_list( const unsigned int *, const unsigned int ); void criba( unsigned int *, const unsigned int ); void setItem ( unsigned int *, const unsigned int, const unsigned int ); bool IsPrime(const unsigned int x) { const unsigned int TOP = static_cast&lt;int&gt;( std::sqrt( static_cast&lt;double&gt;( x ) ) ) + 1; for ( int i=2; i != TOP; ++i ) { if (x % i == 0) return false; } return true; } int main() { unsigned int *l; unsigned int n; cout &lt;&lt; "Ingrese tope de criba" &lt;&lt; endl; cin &gt;&gt; n; l = new unsigned int[n]; initialize( l, n ); cout &lt;&lt; "Esta es la lista" &lt;&lt; endl; show_list( l, n ); criba( l, n ); cout &lt;&lt; "Estos son los primos" &lt;&lt; endl; show_list( l, n ); } void initialize( unsigned int *l, const unsigned int n) { for( int i = 0; i &lt; n - 1; i++ ) *( l + i ) = i + 2; } void show_list( const unsigned int *l, const unsigned int n) { for( int i = 0; i &lt; n - 1; i++ ) { if( *( l + i ) != 0) cout &lt;&lt; l[i] &lt;&lt; " - "; } cout &lt;&lt; endl; } void setItem( unsigned int *l, const unsigned int n, const unsigned int p) { unsigned int i = 2; while( p * i &lt;= n) { *( l + (i * p - 2) ) = 0; i++; } } void criba( unsigned int *l, const unsigned int n) { for( int i = 0; i * i &lt;= n ; i++ ) if( IsPrime ( *( l + i) ) ) setItem( l, n, *(l + i) ); } </code></pre>
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