Creating a simple equation calculator with C#

Question

So this is a bit of homework I have. I have to create a calculator application that asks for user input then calculates it. The input must be in an equation format. For example: " x = 3 + 8 ", " x = 6 - 3 " or x = " 6 - 3 * 9 ". My approach to this problem is to first break down the string user input and store it into an array of char:

private char[] userInput;
string input = Console.ReadLine();
input = input.Replace(" " ,"");
userInput = input.ToCharArray();

At this point, userInput will contain all char from input. Next, I look for the variable of equation by looping through the array, this should give me the first alphabet character it found:

char var = 'x';
for (int i = 0; i < userInput.Length; i++)
{
     char c = userInput[i];
     if (Char.IsLetter(c)){
         var = c;
         break;
     }
}

Next, I will break the equation up with variable one side and all of the number and operator in the other side, separated by '=', then add all number and operator to a new char array:

//get '=' position
int equalPos = 0;
for (int i = 0; i < userInput.Length; i++)
{
    char c = userInput[i];
    if (Char.IsSymbol(c))
    {
       if (c.Equals('='))
          {
              equalPos = i;
              break;
           }
     }
}
//add equation to new array
rightSide = new char[userInput.Length-equalPos];
int a = 0;
for (int i = equalPos + 1; i < userInput.Length; i++)
{
    char c = userInput[i];
    rightSide[a] = c;
    a++;
}

At this point, the rightSide array will contain all of the number and operator as character. I can calculate this part by using System.Data.DataTable().Compute(). However, if I am not allowed to use any library, how could I implement this? The equation should only contain 1 variable(always appear on the left side of the equation), four basic operators (+-/*) and no parenthesis.

It's good that you have provided the setup code, but what code have you written to solve your problem? — Neil, Sep 01 '18 at 07:40
Many existing implementations of such calculator can be already found on Stack Overflow, for example [Expression Evaluator](https://stackoverflow.com/a/4195437/1286670), [C# string calculator](https://stackoverflow.com/questions/47621011/c-sharp-string-calculator), [Making a simple calculator with arrays in C#](https://stackoverflow.com/questions/40174345/making-a-simple-calculator-with-arrays-in-c-sharp) and [many many](https://stackoverflow.com/search?q=C%23+Calculator) more. — Ňuf, Sep 01 '18 at 08:58
@Neil currently I have solved x = a + b by looping through rightSide to find the operator position and then get the a and b by using operator position -1/+1 respectively. However, it only works with single digit number and it cannot calculate a third number if it exists in the expression. — Takashi, Sep 01 '18 at 10:11

Daniel Loudon · Answer 1 · 2018-09-03T05:52:06.030

If you first split the string by the = operator you will get the right and left hand side. So on the right hand side of the equation, if the equation is 'x = 6 * 2 + 1', we have '6 * 2 + 1', so we can compute that and follow standard BIDMAS rules using a loop and switch:

i have removed all error checking, this solution is for when a user inputs a perfect equation in the form 'x = {equation}' or '{equation} = x'

Also to note, a string is a char[]

//get user input
Console.Write("Enter equation:");
string input = Console.ReadLine();
string[] splitInput = input.Split('=');
int index = char.IsLetter(splitInput[0].Replace(" ", "")[0]) ? 1 : 0;
string sideWithEquation = splitInput[index];

//Compute right hand side
string[] equation = sideWithEquation.Split(' ');

Using BIDMAS, and ignoring brackets and indices, we compute divison and multiplication first.

    //compute for * and /
    for (int i = 1; i < equation.Length - 1; i++)
    {
        string item = equation[i];
        int num = 0;
        switch (item)
        {
           case "*":
                num = Convert.ToInt32(equation[i - 1]) * Convert.ToInt32(equation[i + 1]);
                break;
            case "/":
                num = Convert.ToInt32(equation[i - 1]) / Convert.ToInt32(equation[i + 1]);
                break;
        }
        if (num > 0)
        {
             equation[i - 1] = "";
             equation[i] = "";
             equation[i + 1] = num.ToString();
         }
     }

And then we comoute for addition and subtraction

//Now compute for + and -
 equation = string.Join(" ", equation).Split(' ');
 for (int i = 1; i < equation.Length - 1; i++)
 {
      string item = equation[i];
      int num = 0;
      switch (item)
      {
           case "+":
              num = Convert.ToInt32(equation[i - 1]) + Convert.ToInt32(equation[i + 1]);
              break;
           case "-":
              num = Convert.ToInt32(equation[i - 1]) - Convert.ToInt32(equation[i + 1]);
              break;
      }
      if (num > 0)
      {
            equation[i - 1] = "";
            equation[i] = "";
            equation[i + 1] = num.ToString();
      }
  }

and then to display the value of x to the user again

  string total = string.Join("", equation);     
  //display what x is
  Console.WriteLine($"x = {int.Parse(total)}" );

Thanks for answering, but in the case "/" and '-", shouldn't it be: equation[i + 1] instead of equation[1 + 1]? — Takashi, Sep 03 '18 at 00:18

score 0 · Answer 2 · answered Sep 01 '18 at 12:56

0

Your answer can be divided into two parts First How to convert char array to a string type Second How to convert a string to a executable code block For the first part use this method:

char[] chars;
string s = new string(chars);

For the Second part IT IS TOO DIFFiCULT to find a way without any pre-written code to do so then you must use Microsoft.CSharp.CSharpCodeProvider to compile code on-the-fly. In particular, search for CompileAssemblyFromFile.

answered Sep 01 '18 at 12:56

Pvria Ansari

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4
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I like this but he doesn't want to use any libraries – Daniel Loudon Sep 01 '18 at 13:00
Yes, But I think It is needed to be clarified. – Pvria Ansari Sep 01 '18 at 13:08

Creating a simple equation calculator with C#

2 Answers2