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Arithmetic Expression Calculator

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This post describes implementation of arithmetic expression calculator in C#.

I have below specification in the form of Backus–Naur Form expression. It is mainly used to define syntax of languages in compiler design world. You can read more about it on wikipedia.

There are certain readymade solutions are available to calculate expressions.  Microsoft also introduced Expression tree through which we can compile complex expressions using Lambda expressions

Basically this expression is defining arithmetic calculation with some validations. Following defines list of validations and rule.

  • Expression should contain arithmetic operations and operands only.
  • Expression can only have +, -, * operators.
  • Numbers can be negative or positive.
  • Decimals numbers can have “.” as decimal.
  • Digits should be non zero.
    Expression Example

cmd::= expression* signed_decimal

expresion::= signed_decimal ‘ ‘* operator ‘ ‘* eg. 2.3 * + * 2.3

operator::= ‘+’ | ‘-‘ | ‘*’

signed_decimal::= ‘-‘? decimal_number

decimal_number::= digits | digits ‘.’ digits

digits::= ‘0’ | non_zero_digit digits*

non_zero_digit::= ‘1’|’2’|’3’|’4’|’5’|’6’|’7’|’8’|’9′

Let’s start Implementation

Here I’ll take simple approach to evaluate this expression without use of .Net Expression Tree with possible failure and pass test cases. I tried to put relevant comments in code itself to make it self explanatory.

 

using System;
using System.Linq;
using NUnit.Framework;

namespace Calculator
{
    /// 
    /// Basic calc engine for handling +, -, * operations.
    /// 
    public class CalcEngine
    {

        char[] operatorList = new char[3] { '+', '-', '*' };

        /// 
        /// Calculate arithmatic Expression
        /// 
        /// 
        /// 
        public decimal Calculate(string expression)
        {
            foreach (var oper in operatorList)
            {
                if (expression.Contains(oper))
                {
                    var parts = expression.Split(oper);
                    int i = 1;
                    decimal result = Calculate(parts[0].Trim());

                    while (i < parts.Length)
                    {
                        switch (oper)
                        {
                            case '+':
                                result += Calculate(parts[i].Trim());
                                break;
                            case '-':
                                result -= Calculate(parts[i].Trim());
                                break;
                            case '*':
                                result *= Calculate(parts[i].Trim());
                                break;
                        }
                        i++;
                    }
                    return result;
                }

            }
            decimal value = 0;
   //Note: we can also use decimal.Parse and can catch exception in catch block 
   but it is expensive task to wait for system exception
   //better to use TryParse and then throw custom exception
   if (expression.Trim().Length > 0 && 
         !decimal.TryParse(expression, System.Globalization.NumberStyles.Float, 
	  System.Globalization.CultureInfo.InvariantCulture, out value))
            {
                throw new FormatException("Expression is wrong! 
			Please removed un-allowed characters\n 
			please follow following validations:\n" +
                    "Expression should contain arithmetic operations
			 and operands only \n" +
                    "Expression can only have +, -, * operators \n" +
                    "Numbers can be negative or positive \n" +
                    ". as decimal point");
            }

            return value;
        }

    }

Unit Tests

[TestFixture]
    public class CalcEngineTest
    {
        CalcEngine engine;

        [Setup]
        public void Setup()
        {
            engine = new CalcEngine();
        }

        [Test]
        [ExpectedException(typeof(FormatException))]
        public void TestValidationFailur_Nondigit()
        {
            Assert.AreEqual(-14, engine.Calculate("1+2+3-4d"));
        }

        [Test]
        [ExpectedException(typeof(FormatException))]
        public void TestValidationFailure_NonDecimalNotation()
        {
            Assert.AreEqual(0, engine.Calculate("1+2+3,4"));
        }

        [Test]
        public void TestBlankStringShouldZero()
        {
            Assert.AreEqual(0, engine.Calculate(" "));


        }


        [Test]
        public void TestMultiplication()
        {
            Assert.AreEqual(60, engine.Calculate("5*6*2"));

            Assert.AreEqual(469.929812m, engine.Calculate("5.5*2.456*34.789"));
        }

        [Test]
        public void TestSubtraction()
        {

            Assert.AreEqual(45, engine.Calculate("100-35-20"));

            Assert.AreEqual(469.929812m, engine.Calculate("5.5*2.456*34.789"));
        }


    [Test]
    public void TestSummation()
    {
    CalcEngine engine = new CalcEngine();
    Assert.AreEqual(10, engine.Calculate("1+2+3+4"));
    Assert.AreEqual(20.20m, engine.Calculate("1.4+4.5+8.90+5.4"));
    Assert.AreEqual(20 + 20, engine.Calculate("20+20"));
    }

 [Test]
 public void TestAdd_Multiply_Subtraction()
 {
  Assert.AreEqual(-5563.4541m, 
		engine.Calculate("-2*4.5+3+30-20*278.9+30.5459-40"));
  Assert.AreEqual(-5563.4541m, 
		engine.Calculate("-2*4.5+3+30-20*278.9+30.5459-40"));
 }
    [Test]
    public void TestDecimalNumberWithoutOperators()
    {
       Assert.AreEqual(4.5m, engine.Calculate("4.5"));
     }
  }
}
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