1. The Daring do Statement

Loops are so important, and are so common in programs, that there are several Java language features for implementing them. You have already seen while loops and for loops. This chapter looks at do loops.

A programming language needs to have only one general looping method. The while statement can be used to implement any type of loop. However, as we have seen, the for statement often is convenient, and also can be used to implement any type of loop.

The do loop is occasionally convenient and can be used to implement any of the three types of loops. However, of the three looping statements, it is used the least. Some programmers prefer not to use it at all.

The do statement is not part of the AP (Advanced Placement) computer science test given to high school students in the US. It is not used in the chapters that follow this one.

Chapter Topics:

• The do statement
• Bottom-driven loops
  
• Dangers of bottom-driven loops

Question 1:
There are several looping statements in Java, although only one (any one of them) is logically necessary.
Are there also several branching statements in Java?          Is only one of them necessary?

Source: Bradley Kjell, http://programmedlessons.org/Java9/chap35/ch35_01.html
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 License.

2. The do Statement

Answer:
Yes — Java has the if statement (both with and without an else), the switch statement, and the conditional statement.
Of these, the if statement (with an else) is by far the most useful. The other two types of 
branches are not logically needed, although they are sometimes useful.

The do Statement

int count = 0;                      // initialize count to 0

do
{
  System.out.println( count );      // loop body: includes code to
  count++  ;                        // change the count
}
while ( count < 10 );               // test if the loop body should be
                                    // executed again.

Question 2:
Does the code fragment include the three things that all loops must do?

3. Bottom-driven Loop

Answer:
Yes.

Bottom-driven Loop

All loops must do three things:

1. The loop must be initialized correctly.
2. The ending condition must be tested correctly.
3. The body of the loop must change the condition that is tested.

The code fragment includes all three, but in a different order than usual. The ending condition test is done after the loop body has executed.

int count = 0;                       // initialize count to 0

do
{
  System.out.println( count );      // loop body: includes code to
  count++  ;                        // change the count
}
while ( count < 10 );               // test if the loop body should be
                                    // executed again.         

Loops with the test at the top of the loop (while and for loops) are called top-driven loops.

Loops implemented with the test at the bottom (a do loop) are called bottom-driven loops. This location for the test has some odd (and sometimes undesirable) effects. Examine the following:

int count = 1000;

do
{
  System.out.println( count );
  count++  ;                  
}
while ( count < 10 );

Question 3:
What is the output of the revised loop?

4. Loop Body is Always Executed at Least Once

Answer:
It prints:
1000

Loop Body is Always Executed at Least Once

int count = 1000;                   // initialize

do
{
  System.out.println( count );
  count++  ;                        // change
}
while ( count < 10 );               // test  

Question 4:
(Thought question: ) Do you think that a do loop is a good choice for a counting loop?

5. User Interaction

Answer:
No. A for loop is the best choice for a counting loop.

User Interaction

import java.util.Scanner ;
public class SqrtCalc
{
  public static void main( String[] args )
  {
    String chars;
    double x;
    Scanner scan = new Scanner(System.in );

    do
    {
      System.out.print("Enter a number-->");
      x = scan.nextDouble(); 
      chars = scan.nextLine();   // flush rest of the line
      System.out.println("Square root of " + x + " is " + Math.sqrt( x ) );
      System.out.print("Do you wish to continue? (yes or no) --> ");
      chars = scan.nextLine();

    }
    while ( chars.equals( "yes" ) );    

  }
}

Question 5:
Examine the code. How would it be written with a while loop?

6. while Loop Version

Answer:
See below.

while Loop Version

import java.util.Scanner;

public class SqrtCalc
{
  public static void main( String[] args )
  {
    String chars;
    double x;
    Scanner scan = new Scanner(System.in );

    chars = "yes" ;        // enable first iteration of the loop

    while ( chars.equals( "yes" ) )
    {
      System.out.print("Enter a number-->");
      x = scan.nextDouble(); 
      chars = scan.nextLine();   // flush rest of the line
      System.out.println("Square root of " + x + " is " + Math.sqrt( x ) );
      System.out.print("Do you wish to continue? (yes or no) --> ");
      chars = scan.nextLine();
    }

  }
}

Question 6:
Examine the code (again.) How would it be written with a for loop?

7. for Loop Version

Answer:
See below.

for Loop Version

import java.io.* ;
public class SqrtCalc
{
  public static void main( String[] args )
  {
    String chars;
    double x;
    Scanner scan = new Scanner(System.in );

    for ( chars = "yes"; chars.equals( "yes" );  )  // last part of "for" omitted
    {                                               // (this is OK)
      System.out.print("Enter a number-->");
      x = scan.nextDouble(); 
      chars = scan.nextLine();   // flush rest of the line
      System.out.println("Square root of " + x + " is " + Math.sqrt( x ) );
      System.out.print("Do you wish to continue? (yes or no) --> ");
      chars = scan.nextLine();
    }

  }
}     

Question 7:
You want the program to continue when the user to enters any of the following:
yes
YES
Y
y
The program should quit for anything else. How (in general) can you do this?

8. Boolean Expression in a Condition

Answer:
The test part of the loop can use the OR operator to allow several choices.

Boolean Expression in a Condition

The test part of a loop (any of the three varieties) is a boolean expression. We want a boolean expression that evaluates to true if the user enters:

• yes
• YES
• Y
• y

and evaluates to false for anything else. Here is a near-complete expression:

while ( chars.equals( "yes" )   chars.equals( "YES" )   
        chars.equals( "y" )     chars.equals( "Y" )  )

Question 8:
Fill in the blanks.

9. Program with Alternatives

Answer:
The || (or operator) is the appropriate choice:
while ( chars.equals( "yes" ) || chars.equals( "YES" ) ||  
        chars.equals( "y" )   ||  chars.equals( "Y" )   )

Program with Alternatives

import java.util.Scanner;

public class SqrtCalc
{
  public static void main( String[] args )
  {
    String chars;
    double x;
    Scanner scan = new Scanner(System.in );

    chars = "yes" ;        // enable first iteration of the loop

    while ( chars.equals( "yes" || chars.equals( "YES" ) ||  
            chars.equals( "y" ) ||  chars.equals( "Y" )  )
    {
      System.out.print("Enter a number-->");
      x = scan.nextDouble(); 
      chars = scan.nextLine();   // flush rest of the line
      System.out.println("Square root of " + x + " is " + Math.sqrt( x ) );
      System.out.print("Do you wish to continue? (yes or no) --> ");
      chars = scan.nextLine();
   }

  }
}

Question 9:
Do you wish to continue?

10. End of the Chapter

Answer:
No.

End of the Chapter

You have reached the bottom of this chapter. Click on a subject that interests you if you wish to loop back to where it is discussed.

• do statement The do statement.
• loop, bottom-driven Bottom-driven loops.
• A bottom-driven loop is always executed loop, bottom-driven quirk how many times?