java i lecture_4


Chapter 4

Control Structures: Part I


“ 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

FALSE

Do until: will always do taskA at least once.“A Do Until loop repeats as long as a condition is false.”



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


• Special “Ternary” ? : Operator—shorthand ‘if’ / ‘else’

1.) expression must evaluate to True or False.

2.) If expression is True, execute the command before the colon.

3.) If expression is False, execute the command after the colon.

Do thisif expression

is True

Do this if expression

is False

Selection in Java

System.out.print(expression ? “True” : “False”)


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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// 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 ); }}


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.


// 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 ); }}


// SelectionTest.javaimport javax.swing.*;

public class SelectionTest{

public static void main( String args[] ){

int b, s;String big, small, out;

big = JOptionPane.showInputDialog( "Big Number" );small = JOptionPane.showInputDialog( "Small Number" );b = Integer.parseInt( big );s = Integer.parseInt( small );

out = ( b > s ? "Big was larger" : "Small was larger" );

JOptionPane.showMessageDialog( null, out, "Results",JOptionPane.INFORMATION_MESSAGE);

System.exit( 0 );}

}

• Test Trinary Operator


// DoWhileTest.javaimport javax.swing.*;

public class DoWhileTest{

public static void main( String args[] ){ int b = 2, s = 1; String big, small, out = “Big is still Bigger”;

while( b > s ) { JOptionPane.showMessageDialog( null,

out, "Results",JOptionPane.INFORMATION_MESSAGE);

big = JOptionPane.showInputDialog( "Big Number" ); small = JOptionPane.showInputDialog( "Small Number" ); b = Integer.parseInt( big ); s = Integer.parseInt( small ); }

System.exit( 0 );}

}

• Test DoWhile


// DoUntilTest.javaimport javax.swing.*;

public class DoUntilTest{

public static void main( String args[] ){ int b = 2, s = 1; // preload variables. String big, small, out = “Big is still Bigger”;

do{ JOptionPane.showMessageDialog( null,

out, "Results",JOptionPane.INFORMATION_MESSAGE);

big = JOptionPane.showInputDialog( "Big Number" ); small = JOptionPane.showInputDialog( "Small Number" ); b = Integer.parseInt( big ); s = Integer.parseInt( small );}

while( b > s );

System.exit( 0 );}

}

• Test DoUntil


Repetition: while Loops

• The majority of applications use the plain while loop.

• Choose either while loop when you can’t know in advance how many times the loop will be executed.

• The loop is repeated until it encounters a sentinel value,

that announces that the loop has finished.


Assignment Operators• We are already familiar with this statement:

int x;

x = 15;

• This means the value 15 is placed into the variable x.

• We say, “15 is assigned to x.”

• In Java, a single equals sign is the assignment operator.


Assignment Operators

• Another common bit of code is this:int x; Declares x as an int.

x = 15;After this assignment,x contains 15.

x = x + 5;After this assignment,x contains 20.

First the addition on the right is done. Then, the result is assigned to the the variable on the left.



• Java offers a shortcut to the statement below:

int x = 15;


x += 5;

After this assignment,x contains 20.

+=




int x = 15;

x = x - 5;After this assignment,x contains 10.

x -= 5;


-=




int x = 15;

x = x * 5;After this assignment,x contains 75.

x *= 5;


*=




int x = 15;

x = x / 5;After this assignment,x contains 3.

x /= 5;


/=




int x = 15;

x = x % 5;After this assignment,x contains 0.

x %= 5;


%=


Increment/Decrement Operators

• There is one addition statement in Java that is so common, it gets its own operator.

• It means, simply, “Add one to the variable.”

x++ x--




int x = 2;


x++;

After this assignment,x contains 3.x++




int x = 2;

x = x - 1;After this assignment,x contains 1.

x--;After this assignment,x contains 1.

x--



• You can do a pre-increment, or a post-increment.

Pre Post

++x; x++;

--x; x--;

• If each of these statements is on a line by itself, there is no difference in the effect of doing a pre- or post-increment.



• If each of these statements is on a line by itself, there is no difference in the effect of doing a pre- or post-increment.

• However, if the variable—which is having the pre- or post-increment applied to it—is used within another statement, your choice of pre- or post-increment can alter your results.


// PrePostIncrement.java

public class PrePostIncrement{

public static void main( String args[] ){ int x=0;

System.out.println( " Baseline, x = " + x );

System.out.println( "\n Pre-increment = ++x = " + ++x );

System.out.println( "\n After increment, x = " + x );

x = 0; System.out.println( "\n 2nd Baseline, x = " + x );

System.out.println( "\n Post-increment = x++ = " + x++ );

System.out.println( "\n After increment, x = " + x );

System.exit( 0 );}

}

Baseline, x = 0

Pre-increment = ++x = 1

After increment, x = 1

2nd Baseline, x = 0

Post-increment = x++ = 0

After increment, x = 1

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