.NET - Branching and Flow Control by LearnNowOnline

.NET - Branching and Flow Control

Learn More @ http://www.learnnowonline.com Copyright ÂŠ by Application Developers Training Company

Objectives • Make choices using conditional statements • Manage flow control using branching statements • Repeat blocks of code using looping statements • Break out of loops when necessary

Agenda • Branching in Code • Repeating Code Blocks • Unconditional Branching

Branching in Code • Any programming language must provide means of branching in code • Procedure may need to execute one statement if a condition is true  Optionally, execute a second statement if the condition is false

• .NET provides several different techniques for making decisions as code executes  Conditional branching covered here  Unconditional branching covered as necessary Learn More @ http://www.learnnowonline.com Copyright © by Application Developers Training Company

Conditional Branching • Code can "make decisions" as it's executing      

If statements Single-Line If statements If/Else statements Nested If statements Testing for multiple conditions Comparing a condition to a single value

If Statements • Allows you to execute a block of code if a condition is true  Skips the code if the condition isn't true

• Use the If statement different ways  Depending on whether you want to execute a single statement or block of statements

Select Case Statements (VB) • Select Case is simple to use, but notice specific details:  Select Case statement can match on literal, property, or calculated expression  Include a Case statement for each value you'd like to compare  You can use any "value-type" expression in the Select Case statement o

If you can use = to compare it, it will work

 Case statements can include comparisons other than equality (the default comparison)  Case statements can include comma-delimited lists of values  Visual Basic evaluates Case statements from top to bottom o

As soon as it finds one that matches, it looks at no others

Switch Statements (C#) • Switch statement is simple to use, but note these issues:  The break statement causes C# to jump out of the switch to the line immediately following the switch  If you don't include break, execution falls through to next case  You cannot fall through unless the case includes no code—otherwise, code won't compile  You can use a string literal as the comparison value  If no other cases match, C# executes the code in the default case Learn More @ http://www.learnnowonline.com Copyright © by Application Developers Training Company

Agenda • Branching in Code • Repeating Code Blocks • Unconditional Branching

Repeating Code Blocks • Several different code constructs that allow you to run code repeatedly • Can execute code while or until a condition is/becomes true • Can execute code a fixed number of times • Can execute a block of code once for each element of a collection

Unbounded Looping • Several different options available for repeating a block of code while a condition remains true  While loop  Do loop

The While Loop • Simplest loop:  Executes a block indefinitely, while a condition remains true

• What if condition never changes?  Code must exit loop on demand

• More likely to exit when the condition changes state  Examples show both techniques

The Do Loop (VB) • Offers greater flexibility than While loop  Perhaps too much flexibility

• Allows you to check the condition at the top or bottom of the loop  Allows you to loop while or until the condition remains true or changes

• Placing check for the condition at the bottom of the loop guarantees that the code runs at least once Learn More @ http://www.learnnowonline.com Copyright © by Application Developers Training Company

The do…while Loop (C#) • While loop checks the condition at the top of the loop  It's possible you'll never enter the loop

• Do…While loop checks the condition at the end of the loop  Guarantees that you'll run the code within the loop at least once

Looping a Specific Number of Times • Sometimes code can determine the number of times to repeat a block of code • In some cases, repeat block as an integer variable iterates between one value and another • In other cases, repeat code for each element of a collection of objects

The For Loop • Repeat a block of code as a variable takes on all the values between 1 and 100? • Add a separate line for each case?  Awfully tedious…

• Like most languages, Visual Basic and C# provide a way to loop through a range of integer values  Can increment or decrement the looping variable  By 1 or some other value Learn More @ http://www.learnnowonline.com Copyright © by Application Developers Training Company

The For Loop • For loop allows you to:  Run code repeatedly as integer variable takes on a value between two endpoints  Increment the looping variable by 1, -1, or any other integer value  Skip looping variable values by incrementing the looping value by 2, 3, or some other value  Loop backwards, by setting the loop increment to -1 or any negative integer  Nest loops, providing support for multi-dimensional data

Using Loops with the .NET Framework • Can use loops to iterate through all the objects returned by a .NET Framework method  DriveInfo.GetDrives returns an array of DriveInfo objects

• Array represents multiple objects of the same type  Can index into list by position, using an index

The For Each Loop • Can use For loop to iterate through all the elements of a data structure • Requires you to keep track of the end points, and manage the index yourself • Can also use a for each loop  Language handles details for you

Comparing For and For Each • Two important reasons to choose For:  If you need a nested loop  If you need to traverse the data in reverse order (from the end back to the beginning) o

For Each only moves forward

• Why traverse backwards?  Good example: Remove each item from a collection  Think about numbering o o

As you remove items, they renumber Removing from front to back would break the collection

Agenda • Branching in Code • Repeating Code Blocks • Unconditional Branching

Unconditional Branching (VB) • Language includes a few more ways to jump about in code  Exit For/Exit While/Exit Do  Goto  Continue

Exit Statements (VB) • Can explicitly exit an executing loop  Exit For, Exit While, Exit Do

• If you have nested loops, you can exit from any of the loops, at any level  Loops must be of different types for this to work

Unconditional Branching (C#) • Language includes a few more ways to jump about in code  break  goto  continue

The break Statement (C#) • Breaks out of the current block, and jumps immediately to the line following the block  Can use break in switch statements, but also in loops

The Goto Statement • Allows code to jump unconditionally to a label within the current procedure • Useful for exiting a deeply nested loop • Otherwise, it’s best to avoid Goto • Can never use Goto to branch:  Into a For or For Each loop  From a Catch statement into a Try statement other than its most local Try statement  Out of a Finally block  Into a Catch or Finally block Learn More @ http://www.learnnowonline.com Copyright © by Application Developers Training Company

Continue Statement • What if you want to jump back to the top of a loop without executing the rest of the statements in the loop?  Continue statement solves this problem

Learn More! • This is an excerpt from a larger course. Visit www.learnnowonline.com for the full details! • Learn more about .NET on SlideShare: • Getting Started with .NET • .NET Variables and Data Types • Using the .NET Framework