Module 5 : Statements & Exceptions

Overview
 Introduction to Statements
 Using Selection Statements
 Using Iteration Statements
 Using Jump Statements
 Handling Basic Exceptions
 Raising Exceptions

Introduction to Statements
 Statement Blocks
 Types of Statements

Statement Blocks
 Use braces
As block
delimiters
{
// code
}
{
int i;
...
{
int i;
...
}
}
{
int i;
...
}
...
{
int i;
...
}
 A block and its
parent block
cannot have a
variable with
the same name
 Sibling blocks can
have variables
with
the same name

Types of Statements
Selection Statements
The if and switch statements
Iteration Statements
The while, do, for, and foreach statements
Jump Statements
The goto, break, and continue statements

Using Selection Statements
 The if Statement
 Cascading if Statements
 The switch Statement
 Quiz: Spot the Bugs

The if Statement
 Syntax:
 No implicit conversion from int to bool
int x;
...
if (x) ... // Must be if (x != 0) in C#
if (x = 0) ... // Must be if (x == 0) in C#
if ( Boolean-expression )
first-embedded-statement
else
second-embedded-statement

Cascading if Statements
enum Suit { Clubs, Hearts, Diamonds, Spades }
Suit trumps = Suit.Hearts;
if (trumps == Suit.Clubs)
color = "Black";
else if (trumps == Suit.Hearts)
color = "Red";
else if (trumps == Suit.Diamonds)
color = "Red";
else
color = "Black";

The switch Statement
 Use switch statements for multiple case blocks
 Use break statements to ensure that no fall
through occurs
switch (trumps) {
case Suit.Clubs :
case Suit.Spades :
color = "Black"; break;
case Suit.Hearts :
case Suit.Diamonds :
color = "Red"; break;
default:
color = "ERROR"; break;
}

Quiz: Spot the Bugs
if number % 2 == 0 ...
if (percent < 0) || (percent > 100) ...
if (minute == 60);
minute = 0;
switch (trumps) {
case Suit.Clubs, Suit.Spades :
color = "Black";
case Suit.Hearts, Suit.Diamonds :
color = "Red";
defualt :
...
}
2
3
4
1

Using Iteration Statements
 The while Statement
 The do Statement
 The for Statement
 The foreach Statement
 Quiz: Spot the Bugs

The while Statement
 Execute embedded statements based on Boolean value
 Evaluate Boolean expression at beginning of loop
 Execute embedded statements while Boolean value
Is True
int i = 0;
while (i < 10) {
Console.WriteLine(i);
i++;
}
0 1 2 3 4 5 6 7 8 9

The do Statement
 Execute embedded statements based on Boolean value
 Evaluate Boolean expression at end of loop
 Execute embedded statements while Boolean value
Is True
int i = 0;
do {
i++;
} while (i < 10);
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The for Statement
 Place update information at the start of the
loop
 Variables in a for block are scoped only within
the block
 A for loop can iterate over several values
for (int i = 0; i < 10; i++) {
}
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for (int i = 0; i < 10; i++)
Console.WriteLine(i); // Error: i is no longer in scope
for (int i = 0, j = 0; ... ; i++, j++)

The foreach Statement
 Choose the type and name of the iteration variable
 Execute embedded statements for each element of the
collection class
ArrayList numbers = new ArrayList( );
for (int i = 0; i < 10; i++ ) {
numbers.Add(i);
}
foreach (int number in numbers) {
Console.WriteLine(number);
}
0 1 2 3 4 5 6 7 8 9

Quiz: Spot the Bugs
for (int i = 0, i < 10, i++)
int i = 0;
while (i < 10)
for (int i = 0; i >= 10; i++)
do
...
string line = Console.ReadLine( );
guess = int.Parse(line);
while (guess != answer);
2
3
4
1

Using Jump Statements
 The goto Statement
 The break and continue Statements

The goto Statement
 Flow of control transferred to a labeled statement
 Can easily result in obscure “spaghetti” code
if (number % 2 == 0) goto Even;
Console.WriteLine("odd");
goto End;
Even:
Console.WriteLine("even");
End:;

The break and continue Statements
 The break statement jumps out of an iteration
 The continue statement jumps to the next iteration
int i = 0;
while (true) {
i++;
if (i < 10)
continue;
else
break;
}

Handling Basic Exceptions
 Why Use Exceptions?
 Exception Objects
 Using try and catch Blocks
 Multiple catch Blocks

Why Use Exceptions?
 Traditional procedural error handling is cumbersome
int errorCode = 0;
FileInfo source = new FileInfo("code.cs");
if (errorCode == -1) goto Failed;
int length = (int)source.Length;
char[] contents = new char[length];
// Succeeded ...
Failed: ... Error handling
Core program logic

Exception Objects
Exception
SystemException
OutOfMemoryException
IOException
NullReferenceException
ApplicationException

Using try and catch Blocks
 Object-oriented solution to error handling
 Put the normal code in a try block
 Handle the exceptions in a separate catch block
try {
Console.WriteLine("Enter a number");
int i = int.Parse(Console.ReadLine());
}
catch (OverflowException caught)
{
Console.WriteLine(caught);
}
Error handling
Program logic

Multiple catch Blocks
 Each catch block catches one class of exception
 A try block can have one general catch block
 A try block is not allowed to catch a class that is derived
from a class caught in an earlier catch block
try
{
Console.WriteLine("Enter first number");
int i = int.Parse(Console.ReadLine());
Console.WriteLine("Enter second number");
int j = int.Parse(Console.ReadLine());
int k = i / j;
}
catch (OverflowException caught) {…}
catch (DivideByZeroException caught) {…}

Raising Exceptions
 The throw Statement
 The finally Clause
 Checking for Arithmetic Overflow
 Guidelines for Handling Exceptions

The throw Statement
 Throw an appropriate exception
 Give the exception a meaningful message
throw expression ;
if (minute < 1 || minute >= 60) {
throw new InvalidTimeException(minute +
" is not a valid minute");
// !! Not reached !!
}

The finally Clause
 All of the statements in a finally block are always
executed
Monitor.Enter(x);
try {
...
}
finally {
Monitor.Exit(x);
}
Any catch blocks are optional

Checking for Arithmetic Overflow
 By default, arithmetic overflow is not checked
 A checked statement turns overflow checking on
checked {
int number = int.MaxValue;
Console.WriteLine(++number);
}
unchecked {
int number = int.MaxValue;
Console.WriteLine(++number);
} -2147483648
OverflowException
Exception object is thrown.
WriteLine is not executed.
MaxValue + 1 is negative?

Guidelines for Handling Exceptions
 Throwing
 Avoid exceptions for normal or expected cases
 Never create and throw objects of class Exception
 Include a description string in an Exception object
 Throw objects of the most specific class possible
 Catching
 Arrange catch blocks from specific to general
 Do not let exceptions drop off Main

Custom Exception
 Custom Exception class is derived from System.Exception
or one of the other common base exception classes.
 Provides a type safe mechanism for a developer to detect
an error condition.
 Add situation specific data to an exception.
 No existing exception adequately described my problem.

Review
 Introduction to Statements
 Using Selection Statements
 Using Iteration Statements
 Using Jump Statements
 Handling Basic Exceptions
 Raising Exceptions

Module 5 : Statements & Exceptions

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Module 5 : Statements & Exceptions