title: Using function as another function input
body: <p>Hi I'm quite new to coding yet I have a quite complex pshysics problem to solve with cpp and I've encountered some problems</p>
  
  <pre><code>#include &lt;stdio.h&gt;
  #include &lt;cmath&gt;
  #include &lt;iostream&gt;
  
  using namespace std;
  
  double delta_x(double x, double n)
  {
      return x/n;
  }
  
  double theta(delta_x(double x,double n),double i, double a)
  {
      return atan(abs(2*a*((delta_x(x,n)*i)-50)-(0.01)));
  }
  </code></pre>
  
  <p>Codeblox givex me following errors:</p>
  
  <pre><code>|9|error: expected primary-expression before 'double'|
  |9|error: expected primary-expression before 'double'|
  |9|error: expected primary-expression before 'double'|
  |9|error: expected primary-expression before 'double'|
  |9|error: expression list treated as compound expression in initializer [-fpermissive]|
  |10|error: expected ',' or ';' before '{' token|
  |20|error: 'theta' cannot be used as a function|
  </code></pre>
  
  <p>Thanks in advance</p>