Control statements

Chapter 4

Control Structures

http://www.java2all.com

“ There is No goto in Java ”

• Structured programming: the building blocks

• There are 3 different kinds of operations in a program:

perform a sequence of actions,

perform a selection between alternative actions, or

perform a repetition or iteration of the same action.

Sequence, Selection, Iteration

Structured Programming


• Sequence: one thing after another


task1

task2

task3


• Selection: making choices


taskA

taskB

?YES

NOStructured

programming,

only one entrance,

only one exit.


• Repetition, Part I: doing the same thing again until there’s a reason to stop.


taskATRUE

FALSE

Do while: maybe won’t ever do taskA even once.

“A while loop repeats as long as a condition is true.”

expression?


• Repetition, Part II: doing the same thing again until there’s a reason to stop.


taskA

?TRUE

FALSEDo until: will always do taskA at least once.

“A Do Until loop repeats as long as a condition is false.”http://www.java2all.com


Procedural Structured Programming

• Begin at the top, move to the bottom.

• Each program unit has only one entrance and only one exit.


Selection in JavaObject Oriented Programming

• Within a method, procedural code.

• Simple ‘if’ with or without brackets.

if( expression )statement;

if( expression ){ statement;}


Object Oriented Programming

• Simple ‘if’ with or without brackets.


if( expression ){ statement;}

• Within brackets, a “block.”

Selection in Java



• Simple ‘if’ / ‘else’ without brackets.


elsestatement;

• Without brackets, limit of only one statement per branch.

Selection in Java


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• Simple ‘if’ / ‘else’ with brackets.

if( expression ){

statement;statement;

}else

{statement;statement;

}

Selection in Java


• Compound ‘if’ / ‘else if’ / ‘else if’ / ‘else’.

if( expression ){

statement;}

else if( expression ){

statement;}

else{

statement;}

Selection in Java


If Statement Syntax• Decision Making

• The if exactly mirrors C/C++, and it has three variants:1.) if( expression )

statement;

2.) if( expression )statement;

else statement;

3.) if( expression ) statement;

else if( expression ) statement;

else statement;



if( expression ){ statement1;

statement2;

}

If Statement Syntax

• Simple If• The “expression” must be something that uses the

comparison operators and resolves to either true or false.

• The statement is executed if the expression is true.

• Only one statement can be made conditional without brackets. If you wish to conditionally execute more than

one statement, you use brackets to create a block.http://www.java2all.com

if( expression )statement;else

statement;

If Statement Syntax

• Simple if/else• If the “expression” is true, the if branch executes, if not,

the else branch executes.


if( expression ){

statement1;statement2;

} else{

statement3;statement4; }

If Statement Syntax

• Simple if/else• If the “expression” is true, the if branch executes, if not,

the else branch executes.

if( expression ){

statement1;statement2;} //end of if

else{statement3;

statement4;

} // end of else

Don’t bother to label the closing brackets unless you have a really long if.

Still you should always lineup your brackets.


If Statement Syntax

if( expression )statement;else if( expression )

statement;else

statement;

• Compact if/else if/ else

• To prevent your nested ‘if’s from marching across the page, you can use this nested if. You can go on nesting

them as long as you like, and the last one is just an else.


Multiple-Selection Structure

(switch selection statement)• Once you start nesting many ‘if’s, it becomes a nuisance.

• Java—like C and C++ before it—provides the switch structure, which provides multiple selections.

• Unfortunately—in contrast to Visual Basic’s Select Case and even COBOL’s Evaluate—you cannot use any of type of argument in the switch statement other than an integer.


int x = 0;switch( x )

{case 1:

do stuff;break;

case 2:do stuff; break;

case 55:do stuff; break;

case 102:case 299:

do stuff okay for both; break;

default:if nothing else do this stuff; break;

}


• The integer expression x isevaluated. If x contains a 1,then the case 1 branch isperformed. Notice the‘break;’ statement. This is required. Without it,every line after the matchwill be executed until it

reaches a break;

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• The expression within the switch( expression )section must evaluate to an integer.• Actually, the expression can evaluate to any of these types (all numeric but long):

byteshortint char

but they will be reduced to an integer and that value will be used in the comparison.



• The expression after each case statement can only be a

constant integral expression

—or any combination of character constants and integer constants that evaluate to a constant integer value.



• The default: is optional.• If you omit the default choice, then it is possible for none of your choices to find a match and that nothing will be executed.

• If you omit the break; then the code for every choice after that—except the default!—will be executed.



• Question: if only integer values can appear in the switch( x ) statement, then how is it possible for a char to be the expression?


• Used when you know in advance how many times you want the loop to be executed.

4 Requirements:

1. Variable to count the number of repetitions

2. Starting value of counter

3. Amount in increment the counter each loop

4. The condition that decides when to stop looping.

Counter-Controlled Repetition


• The for Loop

• A common structure called a for loop is specially designed to manage counter-controlled looping.


for( int x = 1; x < 10; x++ )

1.) count variable,2.) starting value

3.) Increment

4.) condition, final value


// ForCounter.javaimport java.awt.Graphics;import javax.swing.JApplet;

public class ForCounter extends JApplet{ public void paint( Graphics g ) { 1. 2. 4. 3.

for( int counter=1 ; counter <= 10 ; counter++ ){

g.drawLine( 10, 10, 250, counter * 10 ); }

}} 1.) count variable 2.) starting value 3.) increment 4.) condition, final value

• When appropriate, the for is quick and easy.




• The for loop is a do-while.

• It tests the condition before it executes the loop for the first time.

( • Note: since the variable int counter was declared within the for , it vanishes after the for is finished. )


counter <= 10?

int counter = 1;

{ }

counter++

TRUE

FALSE

1. 2.

3. body

4.


• All Three Sections are Optional

• Effects of Omitting Sections: condition


for( int x = 1; x < 10; x++ )

• If you omit the condition, Java assumes the statement is true, and you have an infinite loop.

for( int x = 1;; x++ )


• Effects of Omitting Sections: initialization


for( int x = 1; x < 10; x++ )

• You can omit the initialization if you have initialized the control variable someplace else.

int x = 1; for(; x < 10; x++ )


• Effects of Omitting Sections: increment


for( int x = 1; x < 10; x++ )

• You can omit the increment of the variable if you are doing so within the body of the loop.

for( int x = 1; x < 10;){

other stuff

x++;}


• Can Use the while Loop

• Although the while is usually used when we don’t know how many times we’re going to loop, it works just fine.

• Still must supply the 4 Requirements.



while—“the Do While”

• The Test is First

Repetition: while Part I

while( expression )statement;


while—“the Do While”

• The Test is First

Repetition: while Part I

while( expression ){


}

The while { “Do While” } is used when you can’t predict exactly how many times your

loop will be executed.

The while may not be executed even once. It executes the loop while the expression is

still true.

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// WhileTest.java// Since "c" is already false when it reaches the// test, the loop never executes.

public class DoWhile{ public static void main( String args[] ) { boolean c = false

while( c ) {

System.out.println( ”Execute DoWhile while c is true" ); }

System.exit( 0 ); }}


while—“the Do Until”

• The Test is Last

Repetition: while Part II

do{


}

while( expression );

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while—“the Do Until”

• The Test is Last

Repetition: while Part II

do{


}

while( expression );

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This do/while {“Do Until”} is also used when you can’t predict exactly how many times

your loop will be executed.

It executes at least once. It executes Until the expression becomes

false.


// DoUntil.java// Even though "c" begins the loop false,// it still executes at least once.

public class DoUntil{ public static void main( String args[] ) { boolean c = false; do { System.out.println( ”Execute DoUntil at least once " ); } while( c ); System.exit( 0 ); }}


Statements break; and continue;

• Both of these statements alter the flow of control.

• The break statement can be executed in a:

whiledo/whileforswitch

• break causes the immediate exit from the structure


Statements break; and continue; • After a break exits the “structure”—whatever that is—execution resumes with the first statement following the structure.

• If you have nested structures—be they a while, do/while/ for or switch—the break will only exit the innermost nesting.

• break will not exit you out of all nests. To do that, you need another break*

* There is a variant of the break called a labeled break—but this is similar to a goto and is frowned upon.



• The continue statement, when used in a while or do/while or a for, skips the remaining code in the structure and returns up to the condition.

• If the condition permits it, the next iteration of the loop is permitted to continue.

• So, the continue is a “temporary break.”

• The continue is only used in iterative structures, such as the while, do/while and for.



• The “Labeled” continue and break statements send execution to the label to continue execution.• Note: using the labeled break and continue is bad code. Avoid using them!

stop:

for(row = 1; row <= 10; row++){ for(col=1; col <=5; col++) { if( row == 5) {

break stop; // jump to stop block }

output += “* “; } output += “\n”;}


Control statements

Technology

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