How to write a program?

Program accepts input, performs the function and produces the output.

Accept inputs → Perform the function (logic) → Produce output

Let us try to write a simple calculator. The calculator has to support addition, subtraction, multiplication and division and also display the result.
Try to write the calculator program on a paper by following the steps below
Step #1 - Identify the user inputs.

number1
number2
operation

Step #2 - Identify the output.

result

Step #3 - Write the logic.

if operation is "add"
{
    number1+number2=result
}
if operation is "sub"
{
    number1-number2=result
}
if operation is "mul"
{
    number1*number2=result
}
if operation is "div"
{
    number1/number2=result
}

Step #4 - Display the output (result).
Now try to execute the program on the paper by following the steps
Step #1 - Accept user inputs

number1=8
number2=7
operation="sub"

Step #2 - Nothing to do in this step
Step #3 - Execute the logic

if operation is "add"
{
    number1+number2=result
}
if operation is "sub"
{
    number1-number2=result
    8 - 7 = 1(result)
}
if operation is "mul"
{
    number1*number2=result
}
if operation is "div"
{
    number1/number2=result
}

Step #4 - Display the result

Result is 1

The program works fine on the paper.
C# example- Now open Microsoft Visual Studio
or Microsoft Visual C# 2008 Express Edition and create a
console application (File → New Project → Console Application) and try to write the program as below.

using System;

namespace calculator
{
    class Program
    {
        static void Main(string[] args)
        {
            //Identify the user inputs
            float Number1;
            float Number2;
            string Operation;

            //Identify the output
            float Result=0;

            //Accept user inputs
            //can write code to get the input through the console or user interface
            Number1=8;
            Number2=7;
            Operation = "sub";

            //Write the logic
            if ("add" == Operation)
            {
            Result = Number1+Number2;
            }
            if ("sub" == Operation)
            {
            Result = Number1 - Number2;
            }
            if ("mul" == Operation)
            {
            Result = Number1 * Number2;
            }
            if ("div" == Operation)
            {
            Result = Number1 / Number2;
            }

            //Display the output
            Console.Write("Result is {0}",Result);

            //To make the console wait for the user to read the result
            Console.ReadLine();
        }
    }

}

Output:
Result is 1

The above program works fine. So now try to write the logic
or function part as class as below.

using System;

namespace ConsoleApplication6
{
    class Calc//also called as type
    {
        public Calc()//constructor
        {

        }
        //Logic as function
        public float Add(float Num1, float Num2)//function with arguments/inputs
        {
            float Result;//local variable
            Result = Num1 + Num2;
            return Result;//return Result/output
        }
        //Logic as function
        public float Sub(float Num1, float Num2)//function with arguments/inputs
        {
            float Result;//local variable
            Result = Num1 - Num2;
            return Result;//return Result/output

        }
        //Logic as function
        public float Mul(float Num1, float Num2)//function with arguments/inputs
        {
            float Result;//local variable
            Result = Num1 * Num2;
            return Result;//return Result/output
        }
        //Logic as function
        public float Div(float Num1, float Num2)//function with arguments/inputs
        {
            float Result;//local variable
            Result = Num1 / Num2;
            return Result;//return Result/output
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            //Identify the user inputs
            float Number1;
            float Number2;
            string Operation;

            //Identify the output
            float Result = 0;

            //Accept user inputs
            //can write code to get the input through the console or user interface
            Number1 = 8;
            Number2 = 7;
            Operation = "sub";

            Calc Calc1 = new Calc();//create new Calc

            //Display the inputs
            Console.WriteLine("Number1 is {0},Number2 is {1}", Number1, Number2);
            //Write the logic
            if ("add" == Operation)
            {
                //Display the operation
                Console.WriteLine("Operation is Add");
                Result=Calc1.Add(Number1, Number2);
            }
            if ("sub" == Operation)
            {
                //Display the operation
                Console.WriteLine("Operation is Sub");
                Result = Calc1.Sub(Number1, Number2);
            }
            if ("mul" == Operation)
            {
                //Display the operation
                Console.WriteLine("Operation is Mul");
                Result = Calc1.Mul(Number1, Number2);
            }
            if ("div" == Operation)
            {
                //Display the operation
                Console.WriteLine("Operation is Div");
                Result = Calc1.Div(Number1, Number2);
            }

            //Display the output
            Console.Write("Result is {0}", Result);

            //To make the console wait for the user to read the result
            Console.ReadLine();



        }
    }
}

Output:
Number1 is 8,Number2 is 7
Operation is Sub
Result is 1

JavaScript example- Now let us write the same
program in JavaScript and dhtml, so open a notepad and try to
write the program as below and save it as .html file.

<html>
    <head>
        <title>Calculator</title>
        <style>
        body{
        background:blue;
        color:white;
        }
        td{
        background:white;
        color:maroon;
        font-family:verdana;
        font-size:15px;
        }
        #cssresult{
        background:fuchsia;
        color:lime;
        }
        .cssbutton{
        background:yellow;
        }
        </style>
        <script>
        function calc(op)
        {
            //Identify the user inputs
            var number1;
            var number2;
            var operation;
            //Identify the output
            var result;
            //Accept user inputs
            number1 = (parseFloat)(document.getElementsByName("txtnum1")[0].value);
            number2 = (parseFloat)(document.getElementsByName("txtnum2")[0].value);
            operation=op;
            //Write the logic
            if(operation == "add")
            {
                result = number1 + number2;
                //Display the result
                document.getElementsByName("txtresult")[0].value=result;
                //Display the result (dhtml)
                document.getElementById("divResult")
                .innerHTML="<b>Result is "+result+"</b>";
                alert("Result is "+result);
            }
            if(operation == "sub")
            {
                result = number1 - number2;
                //Display the result
                document.getElementsByName("txtresult")[0].value=result;
                //Display the result (dhtml)
                document.getElementById("divResult")
                .innerHTML="<b>Result is "+result+"</b>";
                alert("Result is "+result);
            }
            if(operation == "mul")
            {
                result = number1 * number2;
                //Display the result
                document.getElementsByName("txtresult")[0].value=result;
                //Display the result (dhtml)
                document.getElementById("divResult")
                .innerHTML="<b>Result is "+result+"</b>";
                alert("Result is "+result);
            }
            if(operation == "div")
            {
                result = number1 / number2;
                //Display the result
                document.getElementsByName("txtresult")[0].value=result;
                //Display the result (dhtml)
                document.getElementById("divResult")
                .innerHTML="<b>Result is "+result+"</b>";
                alert("Result is "+result);
            }
            if(operation == "clear")
            {
                document.getElementsByName("txtnum1")[0].value="";
                document.getElementsByName("txtnum2")[0].value="";
                document.getElementsByName("txtresult")[0].value="";
                document.getElementById("divResult").innerHTML="";
            }
        }
        </script>
    </head>
    <body >
        Calculator
        <table border="2">
        <tr>
        <td>Number 1</td>
        <td><input type="textbox" name="txtnum1"></td>
        <td class="cssbutton">
            <input type="button" value="Add" onclick="calc('add')">
        </td>
        </tr>
        <tr>
        <td>Number 2</td>
        <td><input type="textbox" name="txtnum2"></td>
        <td class="cssbutton">
            <input type="button" value="Sub" onclick="calc('sub')">
        </td>
        </tr>
        <tr >
        <td>Result</td>
        <td id="cssresult"><input type="textbox" name="txtresult"></td>
        <td class="cssbutton">
            <input type="button" value="Mul" onclick="calc('mul')">
        </td>
        </tr>
        <tr>
        <td colspan="2" class="cssbutton">
            <input type=button value="Clear" onclick="calc('clear')">
        </td>
        <td class="cssbutton">
            <input type="button" value="Div" onclick="calc('div')">
        </td>
        </tr>
        </table>
        <div id="divResult"></div>
    </body>
</html>

Output:
Result is 1

Note: Extend the above program to accept input through the user interface and support more features.

Happy programming!

Note: Please read the guidelines listed in this website for efficient programming.