isdigit() c++, probably simple question, but stuck
<p>I'm having trouble with isdigit.  I read the documentation, but when I cout &lt;&lt; isdigit(9), I get a 0.  Shouldn't I get a 1?  </p>

<pre><code>#include &lt;iostream&gt;
#include &lt;cctype&gt;
#include "Point.h"

int main()
{
    std::cout &lt;&lt; isdigit(9) &lt;&lt; isdigit(1.2) &lt;&lt; isdigit('c');
    // create &lt;int&gt;i and &lt;double&gt;j Points
    Point&lt;int&gt; i(5, 4);
    Point&lt;double&gt; *j = new Point&lt;double&gt; (5.2, 3.3);

    // display i and j
    std::cout &lt;&lt; "Point i (5, 4): " &lt;&lt; i &lt;&lt; '\n';
    std::cout &lt;&lt; "Point j (5.2, 3.3): " &lt;&lt; *j &lt;&lt; '\n';

    // Note: need to use explicit declaration for classes
    Point&lt;int&gt; k;
    std::cout &lt;&lt; "Enter Point data (e.g. number, enter, number, enter): " &lt;&lt; '\n' 
        &lt;&lt; "If data is valid for point, will print out new point. If not, will not "
        &lt;&lt; "print out anything.";
    std::cin &gt;&gt; k;
    std::cout &lt;&lt; k;

    delete j;
}
</code></pre>
