Introduction to Programming - (CPTR125) - Lectures  

Lecture Outline

    - Essentials of Counter-Controlled Repetition
    - for Repetition Statement
    - Examples Using the for Statement
    - do…while Repetition Statement
    - break and continue Statements
    - Labeled break and continue Statements
 

Essentials of Counter-Controlled Repetition

• Counter-controlled repetition requires:
    – Control variable (loop counter)
    – Initial value of the control variable
    – Increment/decrement of control variable through each loop
    – Condition that tests for the final value of the control variable

 

for Repetition Statement

                                               for statement header components


for (  initialization;  loopContinuationCondition;  increment  ) 
         statement;

can usually be rewritten as:

initialization;
while (loopContinuationCondition) 

{
         statement;
         increment;
}



1 // Fig. 5.1: ForCounter.java
2 // Counter-controlled repetition with the for statement.
3 import java.awt.Graphics;
4 import javax.swing.JApplet;
6 public class ForCounter extends JApplet {
8     
9 	// draw lines on applet’s background
10 	public void paint( Graphics g )
11	 {
12 		super.paint( g );       // call paint method inherited from JApplet
13                   
14 		// for statement header includes initialization,  
15 		// repetition condition and increment
16 		for ( int counter = 1; counter <= 10; counter++ ) 
17 		g.drawLine( 10, 10, 250, counter * 10 );
18    
19	 } // end method paint
20   
21 } // end class ForCounter                               

Examples Using the for Statement
    •Varying control variable in for statement
        – Vary control variable from 1 to 100 in increments of 1
    • for ( int i = 1; i <= 100; i++ )
        – Vary control variable from 100 to 1 in increments of –1
    • for ( int i = 100; i >= 1; i-- )
        – Vary control variable from 7 to 77 in increments of 7
    • for ( int i = 7; i <= 77; i += 7 )

1      // Fig. 5.2: Sum.java
2      // Summing integers with the for statement.
3 import javax.swing.JOptionPane;
4      
5 public class       Sum {
6      
7     public static void main( String args[] )
8     {
9       int total = 0; // initialize sum
10   
11     // total even integers from 2 through 100
12     for ( int number = 2; number <= 100; number += 2 )
13         total += number; // end of for loop
14    
15         // display results
16         JOptionPane.showMessageDialog( null, "The sum is " + total, 
17         "Total Even Integers from 2 to 100", JOptionPane.INFORMATION_MESSAGE ); 
18             
19    
20         System.exit( 0 );  // terminate application
21   
22     } // end main
23    
24 } // end class Sum

do…while Repetition Statement

    - do…while structure
    - Similar to while structure
    - Tests loop-continuation after performing body of loop
    - i.e., loop body always executes at least once

 

1 // Fig. 5.3: DoWhileTest.java
2 // Using the do...while statement.
3 import java.awt.Graphics;
4
5 import javax.swing.JApplet;
6
7     public class DoWhileTest extends JApplet {
8
9         // draw lines on applet
10       public void paint( Graphics g )
11     {
12         super.paint( g ); // call paint method inherited from JApplet
13
14         int counter = 1; // initialize counter
15
16         do {
17             g.drawOval( 110 - counter * 10, 110 - counter * 10, counter * 20, counter * 20 );
18              ++counter;
19            
20         }  while ( counter <= 10 ); // end do...while
21
22     } // end method paint
23
24 } // end class DoWhileTest

 

 

 

break and continue Statements

    • break/continue
        – Alter flow of control
    • break statement
        – Causes immediate exit from control structure
    • Used in while, for, do…while or switch statements
    • continue statement
        – Skips remaining statements in loop body
        – Proceeds to next iteration
    • Used in while, for or do…while statements

 

Labeled break and continue Statements

    • Labeled block
        – Set of statements enclosed by {}
        – Preceded by a label
    • Labeled break statement
        – Exit from nested control structures
        – Proceeds to end of specified labeled block

1 // Fig. 5.13: BreakLabelTest.java
2 // Labeled break statement.
3 import javax.swing.JOptionPane;
4
5 public class BreakLabelTest {
6
7     public static void main( String args[] )
8     {
9         String output = "";
10
11         stop: {   // labeled block
12
13        // count 10 rows
14         for ( int row = 1; row <= 10; row++ ) {
15
16             // count 5 columns
17             for ( int column = 1; column <= 5 ; column++ ) {
18
19                 if ( row == 5 ) // if row is 5,
20                 break stop; // jump to end of stop block
21
22                 output += "* ";
23
24             } // end inner for
25
26         output += "\n";
27
28         } // end outer for
29
30     // following line is skipped
31     output += "\nLoops terminated normally";
32
33     } // end labeled block
34
35     JOptionPane.showMessageDialog( null, output, "Testing break with a label",
36     JOptionPane.INFORMATION_MESSAGE );
37
38
39     System.exit( 0 ); // terminate application
40
41     } // end main
42
43 } // end class BreakLabelTest

 

                    Source: Java How to Program – 5th Edition – Deitel & Deitel