asp.net programming with c# and sql server first edition chapter 3 using functions, methods, and...

ASP.NET Programming with C# and SQL Server

First Edition

Chapter 3Using Functions, Methods,

and Control Structures

Objectives

In this chapter, you will:

• Learn how to use functions to organize your C# code

• Work with the Request object

• Use if statements, if…else statements, and switch statements to make decisions

• Use while statements, do…while statements, and for statements to execute code repeatedly

ASP.NET Programming with C# and SQL Server, First Edition 2

Working with Functions

• Most programming languages allow you to group programming statements into logical units

• Decision-making and flow-control statements allow you to determine the order in which statements execute in a program

• Function: a procedure, used to organize a related group of C# statements as a single unit


Creating Code Declaration Blocks

• Code declaration block: contains ASP.NET functions and global variables

• <script> element: used to create a code declaration block– Must specify the scripting language and where to run

the code

• Syntax: <script language =“C#” runat=“server”>

…

</script>


Defining Functions

• Must define a function before using it

• Function definition: the lines that make up a function

• Syntax:

returnDataType functionName(parameters)

{

statements;

}


Defining Functions (cont’d.)

• Return data type: indicates the type of data that the function will provide when it completes– void keyword: specifies that a function or method

does not return a value

• Parameter: a variable used within a function– Placed within the parentheses of a function definition– Must declare the parameter’s data type– Multiple parameters are separated by commas

• Parameters are used like local variables within the body of the function


Calling Functions

• A function definition does not execute the function

• Call: to invoke, or execute, the function

• Function call: the code that calls the function

• Syntax: functionName(parameters);

• Arguments: values or variables to be assigned to the function parameters– Also called actual parameters

• Passing arguments: sending arguments to the parameters


Returning Values

• If a function returns a value, the return value can be assigned to a variable

• Syntax: variable = functionName(parameters);

• Return statement: a statement that returns a value to the statement that called the function

• Syntax:

return variable;


Understanding Variable Scope

• Variable scope: where the variable can be used– Determined by where the variable was declared

• Scope can be global or local

• Global variable: a variable declared within a code declaration block but outside a function– Available to all parts of the page

• Local variable: a variable declared inside a function or code render block– Available only within the function or code render

block in which it is declared


Understanding Variable Scope (cont’d.)

• Local variables cease to exist when the function or code render block ends

• If a program contains both a global and a local variable with the same names, the local variable takes precedence when its function is called– Considered poor programming practice to use the

same name for both local and global variables


Working with the Request Object

• Request object: a built-in core object that represents the current URL request from a client– Request object uses collections

• Collection: a data structure similar to an array that stores object properties– To access an object collection: object.collection(“property”)

• Form collection: contains variables representing form elements from a Web page that was submitted with a method of “post”– Example: Request.Form[“name”]


Working with the Request Object (cont’d.)


Table 3-1: Common collections of the Request object

• QueryString collection: contains variables representing form elements from a Web page that was submitted with a method of “get”

• Example: Request.QueryString[“name”]• A query string on a URL will also assign the name=value pairs to the Request.QueryString collection

• Count property: returns the number of variables in a collection

• Example: Request.Form.Count


Working with the Request Object (cont’d.)

Making Decisions

• Decision making (or flow control): the process of determining the order in which statements execute in a program or determining which sets of statements execute

• if statement: most common type of decision-making statement


if Statements

• if statement: used to execute specific programming code based on the result of a conditional expression– If the conditional expression is true, the statements

will be executed

• Syntax: if (conditional expression)

statement;

• Command block: a set of statements contained within a set of braces– Can be used anywhere a single statement is used


if…else Statements

• if…else statement: executes one set of statements if the conditional expression is true or a different set of statements if the conditional expression is false– Only one set of statements will be executed

• Syntax:

if (conditional expression)

statement;

else

statement;


if…else Statements (cont’d.)

• With command blocks:

if (conditional expression)

{

statements;

}

else

{

statements;

}


Nested if and if…else statements

• Nested decision-making structure: when one decision-making structure is contained within another decision-making structure– Allows the evaluation of more than one conditional

expression

• Example:if (condition-1)

if (condition-2)

statement;


switch Statement

• switch statement: executes a specific set of statements depending on the value of an expression– Compares the value of an expression with the value

in a case label

• case label: represents a specific value and contains code that executes if the switch’s expression matches in value

• Can be used instead of a set of if or if…else statements


switch Statement (cont’d.)• Syntax:

switch (expression) {

case label:

statement(s);

break;

case label:

statement(s);

break;

default:

statement(s);

}ASP.NET Programming with C# and SQL Server, First Edition 20

switch Statement (cont’d.)

• default label: contains statements that execute when no match is found in the case labels

• Must include a break statement to stop processing after the desired statements have been executed– Without a break statement, the rest of the code in

the following case labels will also be executed


Repeating Code

• Loop statement: a control structure that repeatedly executes a statement or series of statements while a condition holds

• Three types of loop statements:– while– do…while– for


while Statements

• while statement: repeats a set of statements as long as a given conditional expression evaluates to true– When condition becomes false, the loop ends

• Syntax:

while (conditional expression)

{

statement(s);

}• Iteration: a repetition of the looping statement


while Statements (cont’d.)

• Counter: a variable that increments or decrements with each iteration of a loop statement

• To ensure that the while statement will eventually end, you must have code that will change the value of the conditional expression

• Infinite loop: a loop statement that never ends because its conditional expression is never false

• A while statement may not execute at all if the expression is initially false


do…while Statements

• do…while statement: executes its statements once, and then repeats the execution as long as a conditional expression evaluates to true

• Syntax:

do {

statement(s);

} while (conditional expression);• In the do…while statement, the statements

execute before the conditional expression is evaluated– A do…while loop always executes at least once


for Statements

• for statement: used for repeating a set of statements as long as a given conditional expression evaluates to true– Utilizes a counter whose value changes with each

iteration

• Syntax:for (counter declaration and

initialization; condition; update

statement) {

statement(s);

}ASP.NET Programming with C# and SQL Server, First Edition 26

for Statements (cont’d.)

• Counter variable in a for statement is updated automatically with each iteration of the loop

• for statement is usually used when you know (or can programmatically determine) exactly how many iterations are required


Using continue Statements to Restart Execution

• continue statement: stops the current iteration of the loop, but allows the loop to continue with a new iteration– Allows you to skip some statements in the loop body

in a single iteration without stopping the looping


Summary

• Use <script> elements to create code declaration blocks to contain ASP.NET functions and global variables

• Functions organize a group of related C# statements as a single unit

• Global variables are declared with a code declaration block but outside of a function; are available to all parts of the page

• Local variables are declared inside of a function or code render block; are available only within the function or block in which it was declared


Summary (cont’d.)

• Request object represents the current URL request from the client

• Collections are data structures that store object properties

• Flow control is the process of determining the order in which statements execute

• if statement executes specific code if a conditional expression evaluates to true

• if…else statement executes the if clause if the conditional expression is true or executes the else clause if the expression is false


Summary (cont’d.)

• Decision-making structures can be nested within each other

• switch statement controls program flow by executing specific statements depending on the value of an expression

• Loop statement repeatedly executes a set of statements while a condition is true or until a condition becomes true

• while statement repeats a set of statements as long as the condition is true


Summary (cont’d.)

• do…while statement repeats a set of statements as long as the condition is true

• for statement repeats a set of statements as long as the condition is true

• continue statement is used to terminate a single loop iteration early but continue the loop with the next iteration


asp.net programming with c# and sql server first edition chapter 3 using functions, methods, and...

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