Fundamentals of Variables

Introduction

A variable is a value used in the code of a webpage. Before using a variable, you must let the compiler know. This is referred to as declaring a variable. You can declare a variable in the body of the code delimiters, @{ and }. To declare a variable, you must provide two pieces of information. The first indicates the type of value of the variable. This is called a data type. In C# (and many other languages, but not all languages), a data type is represented by a word referred to as a reserved word or a keyword (in some languages such as C and C++, a data type can be specified using a combination of keywords).

Practical LearningPractical Learning: Introducing Values

  1. Start Microsoft Visual Studio
  2. To create a new website, on the main menu, click File -> New -> Project...
  3. In the left list, click Visual C# and click Web
  4. In the middle list, click ASP.NET Web Application (.NET Framework)
  5. Change the Name to DepartmentStore03
  6. Click OK
  7. In the New ASP.NET Web Application dialog box, click Empty
  8. Click OK
  9. To create a file for the code, in the Solution Explorer, right-click DepartmentStore03 -> Add -> New Item...
  10. In the left list of the Add New Item dialog box, under Visual C# and under the Web node, click Razor
  11. In the middle list, click Web Page (Razor v3)
  12. Change the Name to Index
  13. Click Add
  14. Change the document as follows:
    <!DOCTYPE html>
    <html>
    <head>
    <title>Fun Department Store</title>
    </head>
    <body>
    <h2>Fun Department Store - Inventory</h2>
    
    @{ 
    
    }
    
    </body>
    </html>

The Name of a Variable

A variable must have a name. This is the second piece of information you must provide. There are strict rules you must follow to specify the name of a variable. In our lessons, here are the rules we will follow to name things:

Besides these rules, you can also create your own rules but that follow the above restrictions. For example:

When naming your things such as variables, you must avoid reserved words, also called keywords. These keywords are:

abstract as async await base bool  
continue break byte case catch char checked
class const   is override stackalloc ulong
fixed delegate for lock params static unchecked
internal finally interface float out (generic)    
out (methods) do foreach long private string unsafe
sizeof double goto namespace protected struct ushort
uint else if new (generic) public switch using
decimal enum implicit new (LINQ) readonly this virtual
default event in (foreach) new (variable) ref throw void
  explicit in (generic) null return true volatile
  extern int object sbyte try while
  false   operator sealed short typeof

There are other names that are not considered C# keywords but should be avoided because they may cause a conflict in your code. They are referred to as contextual keywords. They are:

add descending global let remove var
alias dynamic group orderby select where (generic)
ascending from into partial (method) set where (query)
async get join partial (type) value yield
await          

 

There are situation when you want to, you need to, or in fact you must, use a keyword. To do that, preceede the name of the variable with the @symbol.

C# is case-sensitive. This means that the names Case, case, and CASE are different.

Based on our introduction to data types and variables names, the basic formular to declare a variable is:

data-type variable-name;

If you must use a keyword as the name of a variable, the formula to use is:

data-type @variable-name;

Initializing a Variable

Initializing a variable consists of giving it an initial value when the variable is declared. The formula you use is the same for all types but the value you want to use depends on the data type. To initialize a variable, start with its data type, followed by a name for the variable, followed by =, followed by the desired but appropriate value, and end with a semicolon. The formula to follow is:

data-type variable-name = desired-value

The Value of a Web Control

To get the value of a webcontrol of a webpage, use the following formula:

@{
    Request["control-name"]
}

You can assign that expression to a variable.

Values on a Webpage

The values of a webpage display on a browser. To display the value held by a variable, somewhere in the code of a webpage, type @ followed by the name of the variable.

Introduction to Strings

Fundamentals of Strings

A string is an empty space, a character, a word, or a group of words. Primarily, the value of a string starts with a double-quote and ends with a double-quote. An example of a string is "Welcome to the wonderful world of web development!".

To declare a variable for a string, use the string keyword. You can initialize it with an empty space, a character, a symbol, a word, or a group of words. The value of the variable must be included in double-quotes.

Practical LearningPractical Learning: Introducing Strings

  1. Change the code of the webpage as follows:
    <!DOCTYPE html>
    <html>
    <head>
    <title>Fun Department Store</title>
    </head>
    <body>
    <h2>Fun Department Store - Inventory</h2>
    
    @{ 
        string itemName = "Petite Jersey Matte Dress";
        string itemSize = "Small";
    }
    
    <p>Item Name: @itemName</p>
    <p>Sizee: @itemSize</p>
    </body>
    </html>
  2. To execute the project, on the main menu, click Debug -> Start Without Debugging:

    Introducing Variables - Strings

  3. After viewing the result, close the webpage and return to your programming environment
  4. To use a table, change the code as follows:
    <!DOCTYPE html>
    <html>
    <head>
    <title>Fun Department Store</title>
    </head>
    <body>
    <h2>Fun Department Store - Inventory</h2>
    
    @{ 
        string itemName = "Petite Jersey Matte Dress";
        string itemSize = "Small";
    }
    
    <table>
      <tr>
        <td><b>Item Name:</b></td>
        <td>@itemName</td>
      </tr>
      <tr>
        <td><b>Size:</b></td>
        <td>@itemSize</td>
      </tr>
    </table>
    
    </body>
    </html>
  5. To execute the project, on the main menu, click Debug -> Start Without Debugging:

    Introducing Variables - Strings

  6. After viewing the result, close the webpage and return to your programming environment
  7. To create and use a form, change the code as follows:
    <!DOCTYPE html>
    <html>
    <head>
    <title>Fun Department Store</title>
    </head>
    <body>
    <h2>Fun Department Store - Inventory</h2>
    
    @{ 
        string itemName = "Petite Jersey Matte Dress";
        string itemSize = "Small";
    }
    
    <form name="fromDepartmentStore" method="post">
    <table>
      <tr>
        <td><b>Item Name:</b></td>
        <td><input type="text" name="txtItemName" value="@itemName" /></td>
      </tr>
      <tr>
        <td><b>Size:</b></td>
        <td><input type="text" name="txtItemSize" value=@itemSize /></td>
      </tr>
    </table>
    </form>
    </body>
    </html>
  8. To execute the project, on the main menu, click Debug -> Start Without Debugging
  9. After viewing the result, close the webpage and return to your programming environment

Converting a Value to a String

Many of the values with which you will deal are not string. Sometimes, to use one of those values where a string would be used, you should (sometimes must) convert that value to a string. To do this, after the value or the name of the variable, type .ToString().

Introduction to Integers

Introduction to Natural Numbers

An integer is a natural number that is made of one or a combination of digits that are 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9 in the sequence of your choice.

To declare a variable for an integer, use a keyword named int. To initialize the variable or to assign a value to it, provide a number that uses only digits.

If the value of a webcontrol is an integer, you must first convert that value from a string. To do this, after Request["control-name"], type .AsInt(). You can then assign that expression to an integer variable.

Converting an Integral Value to a String

As mentioned already, to convert a natural number to a string, after the value or the name of the variable, type .ToString().

Practical LearningPractical Learning: Introducing Integers

  1. To use an integer, change the code as follows:
    <!DOCTYPE html>
    <html>
    <head>
    <title>Fun Department Store</title>
    </head>
    <body>
    <h2>Fun Department Store - Inventory</h2>
    
    @{
        int itemNumber = 284075;
        string itemName = "Petite Jersey Matte Dress";
        string size = "Small";
    }
    
    <form name="fromDepartmentStore" method="post">
    <table>
      <tr>
        <td><b>Item #:</b></td>
        <td><input type="text" name="txtItemNumber" value="@itemNumber" /></td>
      </tr>
      <tr>
        <td><b>Item Name:</b></td>
        <td><input type="text" name="txtItemName" value="@itemName" /></td>
      </tr>
      <tr>
        <td><b>Size:</b></td>
        <td><input type="text" name="txtItemSize" value=@size /></td>
      </tr>
    </table>
    </form>
    </body>
    </html>
  2. To execute the program, press Ctrl + F5
  3. Close the webpage and return to your programming environment
  4. To format using cascading style sheet, change the code as follows:
    <!DOCTYPE html>
    <html>
    <head>
    <title>Fun Department Store</title>
    <style>
    #main-title {
        font-size: 22px;
        font-weight: bold;
        text-align: center;
        font-family: Garamond, 'Times New Roman', serif
    }
    
    .sub-title {
        font-size: 18px;
        text-align: center;
        font-family: Garamond, 'Times New Roman', serif
    }
    
    .container {
        margin: auto;
        width: 365px;
    }
    
    .grave { font-weight: bold }
    .left-column { width: 100px; }
    .tbl-content-holder { width: 100% }
    </style>
    </head>
    <body>
    <div class="container">
       <p id="main-title">Fun Department Store</p>
       <p class="sub-title alignment grave">Inventory Creation</p>
    
       @{
            int itemNumber = 284075;
            string itemName = "Petite Jersey Matte Dress";
            string size = "Small";
        }
    
        <form name="fromDepartmentStore" method="post">
            <table class="tbl-content-holder">
                <tr>
                    <td class="left-column grave">Item #:</td>
                    <td><input type="text" name="txtItemNumber" value="@itemNumber" style="width: 60px" /></td>
                </tr>
                <tr>
                    <td class="grave">Size:</td>
                    <td><input type="text" name="txtItemSize" value=@size /></td>
                </tr>
                <tr>
                    <td class="grave">Item Name:</td>
                    <td><input type="text" name="txtItemName" value="@itemName" style="width: 200px" /></td>
                </tr>
            </table>
        </form>
    </div>
    </body>
    </html>
  5. To execute the project, on the main menu, click Debug -> Start Without Debugging:

    Introducing Variables - Integers

  6. After viewing the result, close the webpage and return to your programming environment

Introduction to Decimal Numbers in C#

Overview

A floating-point number is one made of one or two sections, with one section required. The required section is one or a combination of digits, as we saw for an integer.

To declare a variable for a floating-point number, use a data type named decimal. Here is an example:

@{
    int @int = 128;
    
    decimal salary;
}

Like an integer, a floating-point number can consist of a natural number. If you are initializing the variable with a natural number, simply assign that number to the variable. Here is an example:

@{
    decimal = 428;
}

Unlike an integer, a floating-point number can include a second part that represents a fraction of 1. That is, a number divided by 100, as in number / 100. That second part is referred to as the precision portion.

When specifying the value of a decimal number, if the value includes a precision portion, the value must be followed by either m or M (on its right side). Here is an example:

@{
    decimal distance = 296.73M;
}

Even if the value is provided as a natural number, you can still include the letter. Here is an example:

@{
    decimal = 296M;
}

If the decimal part of a number is 0 or includes only 0s, you can omit that part. If the integral part is 0, you can omit that 0. Here is an example:

@{
    decimal distance = 296.73M;
    decimal salary = 64800m;
    decimal portion = .8804M;
}

If the value of a webcontrol is a decimal number, you must first convert that value from a string. To do this, after Request["control-name"], type .AsDecimal(). You can then assign that expression to a decimal variable.

Practical LearningPractical Learning: Introducing Decimal Numbers

  1. To use decimal numbers, change the code as follows:
    <!DOCTYPE html>
    <html>
    <head>
    <title>Fun Department Store</title>
    <style>
    #main-title {
        font-size: 22px;
        font-weight: bold;
        text-align: center;
        font-family: Garamond, 'Times New Roman', serif
    }
    
    .sub-title {
        font-size: 18px;
        text-align: center;
        font-family: Garamond, 'Times New Roman', serif
    }
    
    .container {
        margin: auto;
        width: 365px;
    }
    
    .grave { font-weight: bold }
    .left-column { width: 100px; }
    .tbl-content-holder { width: 100% }
    </style>
    </head>
    <body>
    <div class="container">
       <p id="main-title">Fun Department Store</p>
       <p class="sub-title alignment grave">Inventory Creation</p>
    
       @{
           int itemNumber = 284075;
           string itemName = "Petite Jersey Matte Dress";
           string size = "Small";
           decimal unitPrice = 138.95m;
        }
    
        <form name="fromDepartmentStore" method="post">
            <table class="tbl-content-holder">
                <tr>
                    <td class="left-column grave">Item #:</td>
                    <td><input type="text" name="txtItemNumber" value="@itemNumber" style="width: 60px" /></td>
                </tr>
                <tr>
                    <td class="grave">Size:</td>
                    <td><input type="text" name="txtItemSize" value=@size /></td>
                </tr>
                <tr>
                    <td style="font-weight: bold">Unit Price:</td>
                    <td><input type="text" name="txtItemSize" value="@unitPrice" /></td>
                </tr>
                <tr>
                    <td class="grave">Item Name:</td>
                    <td><input type="text" name="txtItemName" value="@itemName" style="width: 200px" /></td>
                </tr>
            </table>
        </form>
    </div>
    </body>
    </html>
  2. To execute the program, press Ctrl + F5:

    Introducing Variables - Decimal Numbers

  3. Close the webpage and return to your programming environment

Converting a Floating-Point Value to a String

To convert a decimal value to a string, after the value or the name of the variable, type .ToString("F"). There are many other options or techniques but this will be enough for now.

Primary Topics on C# Variables

C# as an Inferred Language

As mentioned previously, initializing a variable is the process of assigning an initial value to it immediately when it is declared. When declaring a variable, if you plan to initialize it, in other words if you already know the initial and type of value you want it to hold, you can use a type named var. The formula to follow is:

var data-type variable-name = desired-value

If you decide to use this keyword, you must initialize the variable (on the same line). Here is an example:

@{
    var salary = 64800m;
}

After initializing the variable, the compiler would become aware of the data type that must be applied to the variable. For this reason, C# is referred to as an inferred language.

Updating a Variable

Assigning a value to a variable consists of giving it a (new) value. This is done after the variable has been declared, whether it was initialized or not. In this case, you can first declare the variable by specifing its data type, its name and a semicolon. Then, on another line, type the name of the variable, followed by =, and the value. This can be done as follows:

@{
    data-type variable-name;

    variable-name = desired-value;
}

Here is an example:

@{
    decimal distance;
    var salary = 64800;
    distance = 296;
}

Another technique is to change the value of an initialized variable. In this case, after declaring and initializing a variable, on another line, assign a different value to the variable. This can be done as follows:

@{
    data-type variable-name = initial-value;

    variable-name = desired-value; Error
}

Declaring Many Variables

You can declare each variable on its own line as we have done so far. Different variables can be declared with the same data type. Here is an example:

@{
    string firstName;
    string lastName;
}

When various variables use the same data type, instead of declaring each on its own line, you can declare two or more of these variables on the same line. Here is an example:

@{
    string firstName, lastName;
}

The variables can also be initialized. In this case, a variable must have its own initialization. You don't have to initialize each variable if it is not required. Here are examples:

@{
    string firstName = "Daniel", middleInitial, lastName = "Ndo"
}

Introduction to the Nullity of a Variable

A value is referred to as null when it cannot be clearly determined.

When declaring a variable for a primitive data type, to indicate that a variable can hold a null value, add a question mark to it. Here are examples:

<!DOCTYPE html>
<html>
<head>
<title>Exercise</title>
</head>
<body>
<h1>Exercise</h1>

@{ 
    int? number = 0;
    decimal ? nbr = 0;
    int ?expression = 0;
}

<p>Number = @number</p>
</body>
</html>

Managing Variables in Microsoft Visual Studio

Accessing a Variable

The Code Editor of Microsoft Visual Studio provides many tools to assist you in managing your code. If you are using Microsoft Visual Studio and if you want to find different occurrences of a known character, symbol, word, or group of words, first select that item. Then:

In the same way, if you have a variable that is used more than once in your code and you want to see all places where that variable is used, simply click the name (and wait one second) and all of its occurrences would be highlighted.

To get a list of all sections where the variable is used, if you are using Microsoft Visual Studio:

Accessing a Variable's Declaration

If you create a long document that has many lines of code, in a certain section you may encounter a variable but you want to find out where it was declared. If you are using Microsoft Visual Studio, to access the place where a variable was declared:

Accessing a Line of Code by its Index

If you are using the Code Editor of Microsoft Visual Studio, if you create a long document that has many lines of code, if you want to jump to a certain line of code:

This would display a dialog box. Enter the line number and click OK or press Enter.

Constants

Introduction

A constant is a value that never changes. Examples are 244, "ASEC Mimosa", 39.37. These are constant values you can use in your program any time. You can also create your own constants. To create a constant, type the const keyword to the left of a variable. You must immediately initialize that variable with an appropriate value. Here is an example:

@{
  const decimal ConversionFactor = 39.37M;
}

Once a constant has been created and it has been appropriately initialized, you can use its name where the desired constant is needed. Here is an example:

@{
    const decimal conversionFactor = 39.37m;
}

With this feature, if you mistype the name of the variable in an operation, you would receive an error, giving you the time to fix it.

To initialize a constant variable, the value on the right side of the assignment operator "=" must be a constant or a value that the compiler can determine as constant. Instead of using a known constant, you can also assign it another variable that has already been declared as constant.

Built-in Constants

There are two main categories of constants you will use in your programs. You can create your own constants as we saw above. The C# language, or rather the .NET Framework, also provides various constants.

PI: PI is a constant used as the ratio of the circumference of a circle to its diameter. To use the PI constant, type it as Math.PI.

Practical LearningPractical Learning: Ending the Lesson


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