Extract variables from string math expression with parenthesis

[Robertina Pene]

I want to evaluate an a string math expression , so before i want to extract variables so i have to remove all special characters my code only can remove a special character like `a+b*c` and the result i get is `a=;b=;c=`, now if i write two successive special characters like `(a+b)*c` and `a++b` results will be `=;a=;b=;c=` and `a=;=;b=` always there is a sign `=;` added for no reasons so how to get rid of them because after that i will use mathematic functions so that will be a real problem for me please help me ! here is my code
Hide   Copy Code      public string RemoveSpecialCharacters(string str)       {           StringBuilder sb = new StringBuilder();           for(int i=0; i<str.Length;i++)           {               if((str[i] >= '0' && str[i] <='9')                   ||( str[i] >='A' &&  str[i]<='z'|| (str[i]=='-' || str[i] == '+' || str[i] == '*' || str[i] == '/' || str[i] == '%'|| str[i] ==')' || str[i] == '(')))               {                   sb.Append(str[i]);               }           }           return sb.ToString();       }       private void button8_Click(object sender, EventArgs e)       {           string refine_string = RemoveSpecialCharacters(textBox1.Text);           string[] split_var = refine_string.Split('+', '-', '/', '%', '^', '*', '=',')','(');           double value;           string str = "";           for (int i = 0; i < split_var.Length; i++)           {               if (double.TryParse(split_var[i], out value))               {                   split_var[i] = " ";               }           }           split_var = split_var.Where(s => s != " ").ToArray();           string[] variables = split_var.Distinct().ToArray();           for (int j = 0; j < variables.Length; j++)           {               str = str + variables[j] + '=' + ";";           }           textBox2.Text = str.TrimEnd(';');       }

[Swastik Dey]

a+b*c

 So what is the expected output for this expression?

[Robertina Pene]

exactly it's the expected output, i want the output of a+b*c expression in (a+b)*c expression !

[Swastik Dey]

Do you mean except the arithmetic operators (+,-,*./) and operands it will take out everything from the given string?  To add more to this are you looking for bodmas evaluation?

[Robertina Pene]

No i mean if i want to type two splits like `a++b` i want the output Always look like `a=;b= ` but my output is `a=;=;b=` why??  i have a mistake in my code!

[Swastik Dey]

string[] variables = split_var.Distinct().ToArray();

What value are you getting in variables array after execution of the above line?

[Swastik Dey]

The output you are getting seems to because of any array element is a space. for a++b the length should be 2.  Print the length of that array and see what length are you getting.

[Robertina Pene]

it's used to remove duplicate varibles for example if i have a+a+b (input) the (output) will be `a=;b=` not `a=;a=;b=`

[Robertina Pene]

Sorry i didn't understand what you said please can you explain a little?

[Swastik Dey]

If you take a++b as an example.

string[] variables = split_var.Distinct().ToArray();

Can you check the length of variables array?

i.e. value of variables.Length

[Robertina Pene]

Ahh it's 3 ! so why it's not 2?

[Swastik Dey]

Yes that's why for a++b you are getting output as a=;=;b; because the array is now somewhat like {a,' ',b}

What are you doing in this line?

split_var[i] = " ";

 it seems to be adding an extra space.

[Robertina Pene]

Nothing , but it's not the line that cause the problem even if i put it in comment the length is Always 3

[Dave Tolls]

Check whether variables[j] is an empty string (ie the entry caused by something like '++').
If it is then don't append anything to your output string.

[Swastik Dey]

Can you try printing the elements of the array using a loop and see what it prints.

[Robertina Pene]

Yes i put a condition if   variable[j]=="" and it works now , thank you all  for help , i want to ask now another question if someone can answer me i'll be happy , i want to add a func_var which remove strings like 'cos' or 'sin' or 'tan' from string expression , did someone have an idea how to do that ?

[Swastik Dey]

To be honest I am mainly into java and don't know much about c#.  But the logic should be somewhat like search for the position of the string and remove from there till the length of the string.

[Stephan van Hulst]

I think you're going down the wrong path trying to replace characters. I would write a tokenizer and a parser to solve this problem.

I wrote an example project for you to take a look at: https://github.com/Nibsi/MathExpressions

Start by running the MathExpressions.CLI.Program. If you have questions, let me know.