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Conditional statements

Dept. of Computer Engineering Faculty of Engineering, Kasetsart University

Bangkok, Thailand

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Conditional statement

In the previous lessons, all the statements in the programs are executed sequentially without skipping

Conditional statement Now, some statements may be executed only if certain conditions

are satisfied, otherwise, the statements will be skipped

number = int.Parse(Console.ReadLine());if(number>=0) Console.WriteLine("The number is positive");else Console.WriteLine("The number is negative");

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Data type: bool

Boolean (bool when declared) is a logical data type. A value of a variable whose data type is boolean can be either true or false.

Notes : A variable with type bool can not be read by input statement. However it can be displayed by output statement

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static void Main(string[] args){

bool found; string fact; int year;

Console.Write("Please enter faculty's name : ");fact = Console.ReadLine();fact = fact.ToLower();Console.Write("please enter year studying : ");year = int.Parse(Console.ReadLine());

if (((fact == "engineer")&& (year >= 2)) || ((fact == "sciences") && (year >=3)))found = true;

elsefound = false;

Console.WriteLine(found);

}

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class ClassBoolean{

static void Main(string[] args){

Console.WriteLine(2+3 == 5);Console.WriteLine('A' < 'B');Console.WriteLine("Engineer" == "engineer");Console.WriteLine(3*4 == 7*4/2);

}}

Output : true

true

false

false

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Relational Operators

They are used to compare data items of the compatible type

Relational Operator in Pascal Meaning

== Equal to

> Greater than

< Less than

>= Greater than or equal to

<= Less than or equal to

!= Not equal to

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Boolean Expression- Boolean Constants and Boolean Variable

Boolean constants and Boolean variables are of type bool

static void Main(string[] args){ const bool sunRiseFromEast = true; const bool oneGreaterThanTwo = false; bool boy;

Console.WriteLine("The sun rises from the east is " + sunRiseFromEast); boy = false; Console.WriteLine("It is " + boy + " that Paul is a boy");}

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Boolean Expression- Boolean Expressions with Operands of Arithmetic

Expressions

Boolean Expression Value

2.5 == 5 false

4%10 > 1-3 true

5.7 >= -2 true

5/2 <= 1.1*2 false

999!=999.9 true

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Boolean Expression- Boolean Expressions with Character

Operands

ASCII – American Standard Code for Information Interchange

Contains 128 characters

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Boolean Expression- Boolean Expressions with Character Operands

Characters are compared using their code values according to certain code system.

Boolean Expression

’\’ != ’j’

’9’ > ’4’

’A’ >= ’a’

ASCII of First Operand

92

57

65

ASCII of Second Operand

106

52

97

Boolean Expression

true

true

false

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Compound Boolean Expressions

Simple Boolean Expressions may be joined by logical operators to form compound Boolean expressions

Logical Operators && (and) || (or) ! (not)

Examples (age >= 18) && (age <=20) (grade == 'F') || (grade =='W')

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Compound Boolean Expressions- Logical Operators

Truth Table for ‘and’ operator

Truth Table for ‘or’ operator

Boolean expression P Boolean expression Q P && Q

TRUE TRUE TRUE

TRUE FALSE FALSE

FALSE TRUE FALSE

FALSE FALSE FALSE

Boolean expression P Boolean expression Q P || Q

TRUE TRUE TRUE

TRUE FALSE TRUE

FALSE TRUE TURE

FALSE FALSE FALSE

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Compound Boolean Expressions- Logical Operators

Truth table for ‘not’ operatorBoolean Expression P ! Q

TRUE FALSE

FALSE TRUE

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Precedence

The order of precedence is ‘!’, ‘&&’, ‘||’ The relational operators have lower precedence than logical operators

Operator Precedence Level

( ) 1

++, --, !, cast 2

*, /, % 3

+, - 4

<, <=, >, >= 5

==, != 6

&& 7

|| 8

int x = 3, y = 2, z = 4;Console.WriteLine(++x+3 >= y+6 && z>=4);

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Boolean Expression Evaluation

!('F' != 'M') || (19 > 12) && (19 < 18)

!( true ) || (true) && (false)

( false ) || (true) && (false)

( false ) || ( false )

( false )

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Flowcharts

Graphical representation

Terminator

Process

Input/output

Condition

Connector

Flow line

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Conditional Statements

Two kinds of conditional statements in C# are if statements and switch statements

If statements If-else statements Nested if statements Switch statements