Parser

[carstenht]

Hi
Im taking a Language and Compiler course as a part of my software engineering degree. In that part, i tought it could be fun developing my own mini calculator to make something practical. I have learnt the needed tools such as Automatons, Lexers,Scanners,EBNF and the like. I understand the theory perfectly well, the problem is that I have problems translating this theory to actual code. My fellow students didnt have any troubles with making their own parser, took them a weekend and they where done. But I dont know where to start...very frustrating.

I have made a small grammar of the calculator, so I have that

Command ::= Expression =
Expresstion ::= Numeral((+ | - | *) Numeral)*
Numeral ::= Digit Digit*
Digit ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

I would really hope that you guys could help me do the actual coding part, some hints to get started, maybe some examples.

Thanks in advance

[carstenht]

I have made this small code snippet, but there is some problems in the Eqauls(operator)...

using System;
using System.Collections.Generic;
using System.Text;

namespace minicalc
{
    class Program
    {
        public static String input, tmpexp;       // Expression
        public static int length;         // Lenght of input
        public static int curPos;         // Current token
        public static int value, temp = 0;    // value holders
        public static String Op;
        static void Main(string[] args)
        {
           
            

            System.Console.WriteLine("Please enter expression to calculate: ");
            input = System.Console.ReadLine();
            

            length = input.Length;
            curPos = 0;

            while (curPos < length)
            {
                if ((!input[curPos].Equals("+")) | (!input[curPos].Equals( "-")) | (!input[curPos].Equals("*")))
                {
                    tmpexp += input[curPos];
                    System.Console.WriteLine(input[curPos]);

                }
                temp += Convert.ToInt32(tmpexp);
                //System.Console.WriteLine(tmpexp);
                Op = Convert.ToString( input[curPos + 1]);

                // Determine which operator we have
                switch (Op)
                {
                    case "+": value += temp;
                        break;
                    case "-": value -= temp;
                        break;
                    case "*": value *= temp;
                        break;
                }
                
                curPos++;
                


            }

            System.Console.WriteLine(value);
            System.Console.ReadLine();

        }
    }
}

It reads the operators in the value string even tough it shouldn't

[daveStudent]

have you thought about using regular expressions?

here is a good starting point

i have a degree in computer engineering and we did a decent sized study of regex when we had to write our own compiler.

[carstenht]

Sounds interesting ill take a look

[carstenht]

but would really like this simple thing to work first

[carstenht]

converted it to this:

using System;
using System.Collections.Generic;
using System.Text;
using System.Text.RegularExpressions;

namespace minicalc
{
    class Program
    {
        public static String input, tmpexp;       // Expression
        public static int length;         // Lenght of input
        public static int curPos;         // Current token
        public static int value, temp = 0;    // value holders
        public static String Op;
        
        static void Main(string[] args)

        {
           
            

            System.Console.WriteLine("Please enter expression to calculate: ");
            input = System.Console.ReadLine();
            

            length = input.Length;
            curPos = 0;
            
            while (curPos < length)
            {
                if (input[curPos] == 0 |input[curPos] == 1 |input[curPos] == 2 |input[curPos] == 3 | input[curPos] == 4 |input[curPos] == 5 |input[curPos] == 6 |input[curPos] == 7 |input[curPos] == 8 |input[curPos] == 9 )
                {
                    tmpexp += input[curPos];
                    System.Console.WriteLine(input[curPos]);

                }
               
                temp += Convert.ToInt32(tmpexp);
               
                
                Op = Convert.ToString( input[curPos + 1]);

                // Determine which operator we have and use it on the result
                switch (Op)
                {
                    case "+": value += temp;
                        break;
                    case "-": value -= temp;
                        break;
                    case "*": value *= temp;
                        break;
                }
                
                curPos++;
                


            }

            System.Console.WriteLine("Result of calculation: " + value);
            System.Console.ReadLine();

        }
    }
}

But now I get and array index out of bounds at:

 Op = Convert.ToString( input[curPos + 1]);

[daveStudent]

try something like this:

i made this using txtInput as my input text box, and txtOutput as my output text box

            string strInput = txtInput.Text;

            //define your operands
            string strOps = "[+]|[*]|[-]";
            
            string[] myNumerals = new string[2];
            
            //determines which operand is present
            Match myMatch = Regex.Match(strInput, strOps);

            //gets the two numerals on each side of the operand
            myNumerals = Regex.Split(strInput, strOps);
            
            //myMatch.ToString() will return which operand was in the expression
            switch (myMatch.ToString())
            {
                case "+":
                    txtOutput.Text = (Convert.ToDouble(myNumerals[0]) + Convert.ToDouble(myNumerals[1])).ToString();
                    break;
                case "*":
                    txtOutput.Text = (Convert.ToDouble(myNumerals[0]) * Convert.ToDouble(myNumerals[1])).ToString();
                    break;
                case "-":
                    txtOutput.Text = (Convert.ToDouble(myNumerals[0]) - Convert.ToDouble(myNumerals[1])).ToString();
                    break;
            }

[carstenht]

hi
i just tried your suggestion, but i get 3 errors in the Switch:
Error 1 Cannot implicitly convert type 'string' to 'int'
Error 2 Cannot implicitly convert type 'string' to 'int'
Error 3 Cannot implicitly convert type 'string' to 'int'

[carstenht]

Im still quite determined to get my own code working, so i still get the something educational from it, and it appears that i never enter the if statement:

if (input[curPos].Equals("0") || input[curPos].Equals("1") || input[curPos].Equals("2") || input[curPos].Equals("3") || input[curPos].Equals("4") || input[curPos].Equals("5") || input[curPos].Equals("6") || input[curPos].Equals("7") || input[curPos].Equals("8") || input[curPos].Equals("9"))
                {
                    System.Console.WriteLine("in if");
                    tmpexp += input[curPos];
                    System.Console.WriteLine(input[curPos]);

                }