-(IBAction)equalButton:(id)sender {  // Interface Builder action for equal (calculation)
    // perform calculation
    [self calculateAnswer];

    // set and display result
    [self setCalcAreaNumber:[NSString stringWithFormat:@"%f", calcAnswer]];  // float (double) to string
    [self setTextCalcAreaLabel];

    // set values to support continued calculations, but wipe if you type a number
    [self saveValueofAnswer];                   // answer -> arg1
    [self setInitialCalcAreaInputState:true];   // number key typing will wipe value
}