Continue to calculate with the outcome

Question

I am creating a calculator in c#. Something's not working for me. A standard calculation works for me. For example 13+15. The result is then 28. But if I want to continue calculating with 28, it goes wrong. For example 28 (the result of the previous sum) + 3. You would say it is 31, but my calculator comes out at 21. This is with every math problem, if I want to calculate further with it. How can I make sure it's correct? Do I need a loop? I do want to be able to continue calculating with the result 31. Does anyone have an idea how I can take care of this? Btw: I have asked this question earlier, but the answers were all wrong:(.

This is my code (feel free to put it in there ;))

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace test
{
    public partial class Form1 : Form
    {
        string first = "";
        string second = "";
        string userInput = "";
        string space = " ";
        char function;
        int times;
        double result;

        public Form1()
        {
            InitializeComponent();
        }
        private void number1_Click(object sender, EventArgs e)
        {
            calculatorDisplay.Text = "";
            userInput += "1";
            calculatorDisplay.Text += userInput;
        }
        private void number2_Click(object sender, EventArgs e)
        {
            calculatorDisplay.Text = "";
            userInput += "2";
            calculatorDisplay.Text += userInput;
        }

        private void number3_Click(object sender, EventArgs e)
        {
            calculatorDisplay.Text = "";
            userInput += "3";
            calculatorDisplay.Text += userInput;
        }

        private void number4_Click(object sender, EventArgs e)
        {
            calculatorDisplay.Text = "";
            userInput += "4";
            calculatorDisplay.Text += userInput;
        }

        private void number5_Click(object sender, EventArgs e)
        {
            calculatorDisplay.Text = "";
            userInput += "5";
            calculatorDisplay.Text += userInput;
        }

        private void number6_Click(object sender, EventArgs e)
        {
            calculatorDisplay.Text = "";
            userInput += "6";
            calculatorDisplay.Text += userInput;
        }

        private void number7_Click(object sender, EventArgs e)
        {
            calculatorDisplay.Text = "";
            userInput += "7";
            calculatorDisplay.Text += userInput;
        }

        private void number8_Click(object sender, EventArgs e)
        {
            calculatorDisplay.Text = "";
            userInput += "8";
            calculatorDisplay.Text += userInput;
        }

        private void number9_Click(object sender, EventArgs e)
        {
            calculatorDisplay.Text = "";
            userInput += "9";
            calculatorDisplay.Text += userInput;
        }

        private void clearButton_Click(object sender, EventArgs e)
        {
            first = "";
            second = "";
            userInput = "";
            result = 0.0;
            calculatorDisplay.Text = "0";
            feedback.Text = "";
            times = 0;
        }

        private void divideButton_Click(object sender, EventArgs e)
        {
            function = '/';
            first = userInput;
            userInput = "";
            feedback.Text = first + space + "÷";     
        }

        private void multiplyButton_Click(object sender, EventArgs e)
        {
            function = '*';
            first = userInput;
            userInput = "";
            feedback.Text = first + space + "*";
        }

        private void plusButton_Click(object sender, EventArgs e)
        {
            function = '+';
            first = userInput;
            userInput = "";
            feedback.Text = first + space + "+";
        }

        private void minusButton_Click(object sender, EventArgs e)
        {
            function = '-';
            first = userInput;
            userInput = "";
            feedback.Text = first + space + "-";
        }

        private void equalButton_Click(object sender, EventArgs e)
        {
            times++;
            second = userInput;
            double firstNum, secondNum;
            firstNum = Convert.ToDouble(first);
            secondNum = Convert.ToDouble(second);
            
            if (function =='+')
            {
                result = firstNum + (secondNum * times);
                calculatorDisplay.Text = result.ToString();
                feedback.Text = (result - secondNum) + space + "+" + space + second + space + "=";
            }
            else if(function == '-')
            {
                result = firstNum - (secondNum * times);
                calculatorDisplay.Text = result.ToString();
                feedback.Text = (result+secondNum) + space + "-" + space + second + space + "=";
            }
            else if (function == '/')
            {
                if(secondNum == 0)
                {
                    calculatorDisplay.Text = "Error";
                }
                else
                {
                    result = firstNum / Math.Pow(secondNum, times);
                    calculatorDisplay.Text = result.ToString();
                    feedback.Text = result*secondNum + space + "÷" + space + second + space + "=";
                }
            }
            else if (function == '*')
            {
                result = firstNum * Math.Pow(secondNum, times);
                calculatorDisplay.Text = result.ToString();
                feedback.Text = result/secondNum + space + "*" + space + second + space + "=";
            }
        }
        
        private void decimalButton_Click(object sender, EventArgs e)
        {
            calculatorDisplay.Text += ".";
        }

        private void zeroButton_Click(object sender, EventArgs e)
        {
            calculatorDisplay.Text = "";
            userInput += "0";
            calculatorDisplay.Text += userInput;
        }
    }
}

Answer 1

As has been commented on the original post, learning to use the debugger is key here.

---

Following the state of first, second and userInput, this is what I see (manual debugging for the sake of trying to guide you a little bit):

After having performed the first calculation (13 + 15 =), this is what you will have:

first = "13"
second = "15"
userInput = "15"

(None of these values are reset at the end of equalButton_Click(), which may be expected?)

Then, you click the + button, and your state is updated to:

first = "15" // userInput value
second = "15"
userInput = "" // reset

Then, you click the 3 button, and your state is updated to:

first = "15"
second = "15"
userInput = "3"

Then, you click the = button, which updates the state to:

first = "15"
second = "3"
userInput = "3"

Seeing as you are using your times counter, which is now equal to 2 (after having clicked the = button twice in total), your calculation will therefore be 15 + (3 * 2) => 15 + 6 => 21.