1. ASP.NET Programming with C# and SQL Server First Edition Chapter 3 Using Functions, Methods, and Control Structures

2. 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

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

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

5. Defining Functions • Must define a function before using it • Function definition: the lines that make up a function • Syntax: returnDataType functionName(parameters) { statements; } ASP.NET Programming with C# and SQL Server, First Edition

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

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

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

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

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

11. 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”] ASP.NET Programming with C# and SQL Server, First Edition

12. Working with the Request Object (cont’d.) Table 3-1:Common collections of the Request object ASP.NET Programming with C# and SQL Server, First Edition

13. Working with the Request Object (cont’d.) • 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.QueryStringcollection • Count property: returns the number of variables in a collection • Example: Request.Form.Count ASP.NET Programming with C# and SQL Server, First Edition

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

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

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

17. if…else Statements (cont’d.) • With command blocks: if (conditional expression) { statements; } else { statements; } ASP.NET Programming with C# and SQL Server, First Edition

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

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

20. 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

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

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

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

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

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

26. 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

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

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

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

30. 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 ifclause if the conditional expression is true or executes the elseclause if the expression is false ASP.NET Programming with C# and SQL Server, First Edition

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

32. 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