How do you not pop from an empty stack?

[ovaltine99]

Solved! Thanks!

[keskiverto]

Your compute() assumes (lines 74-78) that the stack has at least two values in it. Either the caller (i.e. main) must ensure that this is true, and/or the function has to check it self.

[JLBorges]

void compute(const string& variable, stack<double>& rpnStack)
{
    double left, right, result;
  
    if( !rpnStack.empty() )
    {
          right = rpnStack.top();
          rpnStack.pop();
    }
    else 
    {
          // stack is empty
          // report error
    }

    if( !rpnStack.empty() )
    {
          left = rpnStack.top();
          rpnStack.pop();
    }
    else 
    {
          // stack is empty
          // report error
    }
    
    // ...