if - Statements

• if Statements are Decision control structures
  -Java statements that allows us to select and execute specific blocks of code while skipping other sections
• Types:
  -if-statement
  -if-else-statement
  -If-else if-statement
• if-statement
  -specifies that a statement (or block of code) will be executed if and only if a certain boolean statement is true.
• if-statement has the form:if( boolean_expression )statement; or if( boolean_expression ){ statement1; statement2; }
  -where,
• boolean_expression is either a boolean expression or boolean variable.
• Eg -
• int grade = 68;
• if( grade > 60 )
• System.out.println("Congratulations!");

Coding Guidelines

• The boolean_expression part of a statement should evaluate to a boolean value. That means that the execution of the condition should either result to a value of true or a false.
• Indent the statements inside the if-block.
  For example,
  if( boolean_expression )

{ //statement1;

//statement2;
}

if-else statement

• if-else statement
  -used when we want to execute a certain statement if a condition is true, and a different statement if the condition is false.
• if-else statement has the form:

if( boolean_expression )

{ statement1;

statement2; . . . }

else

{ statement3;

statement4; . . .}

• Example:

int grade = 68;
if( grade > 60 )
System.out.println("Congratulations!");
else
System.out.println("Sorry you failed");

Coding Guidelines

• To avoid confusion, always place the statement or statements of an if or if-else block inside brackets {}.
• You can have nested if-else blocks. This means that you can have other if-else blocks inside another if-else block.
• For example,
  if( boolean_expression ){

if( boolean_expression )

{ //some statements here }
}
else{ //some statements here
}

if-else-else if statement

• The statement in the else-clause of an if-else block can be another if-else structures.
• This cascading of structures allows us to make more complex selections.
• The statement has the form:

if( boolean_expression1 )

statement1;

else if( boolean_expression2 )

statement2;

else statement3;

• Example

int grade = 68;
if( grade > 90 ){
System.out.println("Very good!");
}
else if( grade > 60 ){
System.out.println("Very good!");
}
else{
System.out.println("Sorry you failed");
}

Common Errors

• The condition inside the if-statement does not evaluate to a boolean value. For example,

//WRONG

int number = 0;

if( number )

{ //some statements here }

The variable number does not hold a boolean value

• Writing elseif instead of else if.
  
  Using = instead of == for comparison.

For example,

//WRONG

int number = 0;

if( number = 0 )

{ //some statements here }

This should be written as,

//CORRECT

int number = 0;

if( number == 0 )

{ //some statements here}