Transcript int x = 1

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Chapter 5
Control Structures: Part II
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Counter-Controlled Repetition
• 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.
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Counter-Controlled Repetition
• 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.
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Counter-Controlled Repetition
// WhileCounter.java
import java.awt.Graphics;
import javax.swing.JApplet;
public class WhileCounter extends JApplet
{
public void paint( Graphics g )
{
int counter;
// 1.) count variable
counter = 1;
// 2.) starting value
while( counter <= 10 )
// 4.) condition, final value
{
g.drawLine( 10, 10, 250, counter * 10 );
++counter;
// 3.) increment
}
}
}
•
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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
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Counter-Controlled Repetition
// ForCounter.java
import 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.)
2.)
3.)
4.)
count variable
starting value
increment
condition, final value
• When appropriate, the 1for is quick and easy.
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Counter-Controlled Repetition
• 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. )
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1.
2.
int counter = 1;
3.
counter <= 10
?
TRUE
FALSE
{ body }
counter++
4.
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Counter-Controlled Repetition
• All Three Sections are Optional
• Effects of Omitting Sections: condition
for(for(
int int
x = x1;= x1;;
< 10;
x++ x++
) )
• If you omit the condition, Java
assumes the statement is true, and
you have an infinite loop.
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Counter-Controlled Repetition
• Effects of Omitting Sections: initialization
int x = 1;
for(
intx x< =10;
1; x++
x < )10; x++ )
for(;
• You can omit the initialization if
you have initialized the control
variable someplace else.
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Counter-Controlled Repetition
• Effects of Omitting Sections: increment
for(
for(int
intx x= =1;1;x x< <10;
10;)
x++ )
{
other stuff
x++;
}
• You can omit the increment of
the variable if you are doing so
within the body of1 the loop.
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// Calculate Compound Interest
import javax.swing.JOptionPane;
import java.text.DecimalFormat;
import javax.swing.JTextArea;
public class Interest
{
Introducing two new class objects:
DecimalFormat
and the
JTextArea.
JTextArea is one of many GUI classes
that we use to put text onto a window
} // end of class Interest
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// Calculate Compound Interest
import javax.swing.JOptionPane;
import java.text.DecimalFormat;
import javax.swing.JTextArea;
w
public class Interest
{
public static void main( String args[] )
{
double amount, principle = 1000.0, rate = 0.05;
DecimalFormat twoDig =
This is the common style of creating objects.
ObjectType instanceName.
After this statement executes, twoDig is a
complete example, or instance, of the
DecimalFormat class.
Actually, since it hasn’t been initialized,
System.exit( 0 );
twoDig is still only a reference.
end of main()
} //
} // end of class Interest
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// Calculate Compound Interest
import javax.swing.JOptionPane;
import java.text.DecimalFormat;
import javax.swing.JTextArea;
w
public class Interest
{
public static void main( String args[] )
{
double amount, principle = 1000.0, rate = 0.05;
DecimalFormat twoDig = new DecimalFormat( “0.00” );
Every class has a default method—a Constructor—whose only
purpose is to initialize a fresh instantiation of the class.
The new keyword fires the default “Constructor” method.
The Constructor always has the exact same name as the class.
( Chapter 6 covers this in greater detail.)
System.exit( 0 );
} // end of main()
} // end of class Interest
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// Calculate Compound Interest
import javax.swing.JOptionPane;
import java.text.DecimalFormat;
import javax.swing.JTextArea;
public class Interest
{
public static void main( String args[] )
{
double amount, principle = 1000.0, rate = 0.05;
DecimalFormat twoDig = new DecimalFormat( “0.00” );
JTextArea output = new JTextArea( 11, 20 );
}
Once again, we have the same pattern. The name of a
class—JTextArea—followed by the name of an
instantiation of that class—output.
Next, we fire off the default Constructor using the
new keyword.
In this case, we are making a JTextArea
System.exit( 0 );
}
//columns
end of main()
11
wide by 20 columns
tall.
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// end of class Interest
JTextArea—a brief sidebar
• A JTextArea is a multi-line area that displays plain text.
• This is the actual
documentation for the
JTextArea. You
must become
comfortable
looking up and
interpreting this
information.
Here, you should
notice that there are several Constructors.
The top one—which you notice takes no arguments—is
the “default” Constructor.
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// Calculate Compound Interest
import javax.swing.JOptionPane;
import java.text.DecimalFormat;
import javax.swing.JTextArea;
public class Interest
{
public static void main( String args[] )
{
double amount, principle = 1000.0, rate = 0.05;
DecimalFormat twoDig = new DecimalFormat( “0.00” );
JTextArea output = new JTextArea( 11, 20 );
output.append( “Year\tAmount on deposit\n” );
Since output is an object of type JTextArea, it possesses all the
methods of that object. Method append says to add the String data to
the JTextArea in the order that we wish it to appear within the
JTextArea.
System.exit( 0 );
Notice,
we
either pass it a String object (a variable that contains a
} // end can
of main()
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} // end of String
class Interest
object), or we can
just pass it a String.
// Calculate Compound Interest
import javax.swing.JOptionPane;
import java.text.DecimalFormat;
import javax.swing.JTextArea;
public class Interest
{
public static void main( String args[] )
{
double amount, principle = 1000.0, rate = 0.05;
DecimalFormat twoDig = new DecimalFormat( “0.00” );
JTextArea output = new JTextArea( 11, 20 );
output.append( “Year\tAmount on deposit\n” );
for( int year = 1; year <= 10; year++ )
{
} // end of for
System.exit( 0 );
} // end of main()
} // end of class Interest
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// Calculate Compound Interest
import javax.swing.JOptionPane;
import java.text.DecimalFormat;
import javax.swing.JTextArea;
Since Java has no exponentiation operator, we
calling on the Math library’s method (pow for
power).
public class Interest
{
Notice,
it expects
public static void main(
String
args[] ) as arguments two
{
b
doubles.
These
will
be
used
in
this
way:
a
double amount, principle = 1000.0, rate = 0.05;
DecimalFormat twoDig or
= (1.0
new DecimalFormat(
“0.00” );
+ rate)year
JTextArea output = new JTextArea( 11, 20 );
The return value is a double.
output.append( “Year\tAmount on deposit\n” );
for( int year = 1; year <= 10; year++ )
{
amount = principle * Math.pow( 1.0 + rate, year );
output.append( year + “\t” + twoDig.format(amount) + “\n”);
} // end of for
System.exit( 0 );
} // end of main()
} // end of class Interest
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// Calculate Compound Interest
import javax.swing.JOptionPane;
import java.text.DecimalFormat;
import javax.swing.JTextArea;
public class Interest
{
public static void main( String args[] )
{
double amount, principle = 1000.0, rate = 0.05;
DecimalFormat twoDig = new DecimalFormat( “0.00” );
JTextArea output = new JTextArea( 11, 20 );
output.append( “Year\tAmount on deposit\n” );
for( int year = 1; year <= 10; year++ )
{
amount = principle * Math.pow( 1.0 + rate, year );
output.append( year + “\t” + twoDig.format(amount) + “\n”);
} // end of for
JOptionPane.showMessageDialog( null, output,
“Compound Interest”, JOptionPane.INFORMATION_MESSAGE);
System.exit( 0 );
} // end of main()
} // end of class Interest
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// Calculate Compound Interest
import javax.swing.JOptionPane;
import java.text.DecimalFormat;
import javax.swing.JTextArea;
public class Interest
{
public static void main( String args[] )
{
double amount, principle = 1000.0, rate = 0.05;
DecimalFormat twoDig = new DecimalFormat( “0.00” );
JTextArea output = new JTextArea( 11, 20 );
output.append( “Year\tAmount on deposit\n” );
for( int year = 1; year <= 10; year++ )
{
amount = principle * Math.pow( 1.0 + rate, year );
output.append( year + “\t” + twoDig.format(amount) + “\n”);
} // end of for
JOptionPane.showMessageDialog( null, output,
“Compound Interest”, JOptionPane.INFORMATION_MESSAGE);
System.exit( 0 );
} // end of main()
} // end of class Interest
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Counter-Controlled Repetition
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Multiple-Selection Structure
• 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.
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Multiple-Selection Structure
int x = 0;
switch( x )
{
case
case
case
case
case
• The integer expression x is
evaluated. If x contains a 1,
1:
do stuff;
then the case 1 branch is
break;
performed. Notice the
2:
do stuff;
‘break;’ statement.
break;
This is required. Without it,
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do stuff;
every line after the match
break;
will be executed until it
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299:
reaches a break;
do stuff okay for both;
break;
default:
if nothing else do this stuff;
break;
}
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Multiple-Selection Structure
• 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):
byte
short
int
char
but they will be reduced to an integer and that value
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will be used in the comparison.
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Multiple-Selection Structure
• 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.
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Multiple-Selection Structure
• 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.
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Multiple-Selection Structure
• Question: if only integer values can appear in the
switch( x ) statement, then how is it possible for
a char to be the expression?
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Statements
break; and continue;
• Both of these statements alter the flow of control.
• The break statement can be executed in a:
while
do/while
for
switch
• break causes the immediate exit from the structure
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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.
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Statements break; and continue;
• 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.
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Statements break; and continue;
• 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”;
1
}
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JScrollPane—a brief sidebar
• We are all familiar with the scrollable pane—it means
the amount of text available is larger than the screen.
• Java provides the JScrollPane, which is an API that is
capable of scrolling text in this manner.
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JScrollPane—a brief sidebar
• Bizarre construction, because it uses composition—
where several smaller tools are combined to form a larger
tool.
• A String object feeds into a JTextArea, which feeds into
a JScrollPane object.
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JScrollPane—a brief sidebar
String
JTextArea(
)
JScrollPane(
)
JOptionPane(
JOptionPane( null, JScrollPane( JTextArea( String) ) )
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)
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JScrollPane—a brief sidebar
// ExploreJScrollPane.java
import javax.swing.*;
public class ExploreJScrollPane
{
public static void main( String args[] )
{
String output = "Test\nTest\nTest\n";
System.exit( 0 );
}
}
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JScrollPane—a brief sidebar
// ExploreJScrollPane.java
import javax.swing.*;
public class ExploreJScrollPane
{
public static void main( String args[] )
{
String output = "Test\nTest\nTest\n";
JTextArea outputArea = new JTextArea( 10, 10 );
outputArea.setText( output );
System.exit( 0 );
}
}
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JScrollPane—a brief sidebar
// ExploreJScrollPane.java
import javax.swing.*;
public class ExploreJScrollPane
{
public static void main( String args[] )
{
String output = "Test\nTest\nTest\n";
JTextArea outputArea = new JTextArea( 10, 10 );
outputArea.setText( output );
JScrollPane scroller = new JScrollPane( outputArea );
System.exit( 0 );
}
}
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JScrollPane—a brief sidebar
// ExploreJScrollPane.java
import javax.swing.*;
public class ExploreJScrollPane
{
public static void main( String args[] )
{
String output = "Test\nTest\nTest\n";
JTextArea outputArea = new JTextArea( 10, 10 );
outputArea.setText( output );
JScrollPane scroller = new JScrollPane( outputArea );
JOptionPane.showMessageDialog( null, scroller,
"Test JScrollPane",
JOptionPane.INFORMATION_MESSAGE );
System.exit( 0 );
}
}
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JScrollPane—a brief sidebar
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Logical Operators
• So far, all of the conditions we have tested were simple.
• It is possible to construct complex conditions using the
Java Logical Operators, which—again—were inherited
form C/C++.
&&
Logical AND
||
Logical OR
!
Logical NOT
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Logical Operators
• Logical AND—two ampersands together.
&&
if( gender == ‘F’ && age >= 65 )
• Condition is true only if both halves are true.
• Java will short-circuit the process—skipping the 2nd
half of the expression—if the first half is false.
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Logical Operators
• Logical OR—two “pipes” together. (Shift of key
underneath backspace.)
||
(no space between)
if( gender == ‘F’ || age >= 65 )
• Entire condition is true if either half is true.
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Logical Operators
• Logical NOT—single exclamation mark.
!
if(
!(age <= 65)
)
• Negates the expression—not many opportunities to
use.
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Logical Operators
• Logical Boolean AND—one ampersand.
&
if( gender == ‘F’ & ++age >= 65 )
• A Logical Boolean AND [ & ] works exactly like
a Logical AND [ && ] with one exception.
• A Logical Boolean AND [ & ] will always check
both halves of the equation, 1even if the first is false.
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Logical Operators
• Logical inclusive Boolean OR—one pipe.
|
if( gender == ‘F’ | age >= 65 )
• Again, this works just like the Logical OR, but you
are guaranteed that both sides of the expression will
always be executed.
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• If either half is true, the entire
‘if’ is true.
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Logical Operators
• Logical exclusive Boolean OR.
^
( shift 6 )
if( gender == ‘F’ ^ age >= 65 )
• This Logical exclusive Boolean OR is true only if
one side is true and the other false.
• If both sides are true, the entire expression is false.
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