4.5 Comparing Loop Structures

With three loop structures available in Java, how do we decide which to use in a given situation? To some extent, this is a matter of personal taste, but usually one of the statements is more appropriate than either of the others.

The for statement should usually be used if we know, before we start the loop, how many repetitions we want. Problems that call for the reading of a given number of data items or the production of a table of a given size are examples of situations where we would normally use a for statement.

The while and do statements should normally be used when the number of repetitions is not known before we start execution of the loop. If it is convenient to test the condition that controls the loop before entry to the loop body, then a while is the usual choice. If the condition is only established within the body, then a do should be used. If it is possible that the loop body may never be entered, a while statement should be used; if it is certain that the loop body will be executed at least once, then a do might be the better choice.

Example 1

All three program fragments shown here have the same effect. They all find the sum of the series

In each part, the variable sum is of type double while i is of type int. 
  1. sum = 0;
i = 1;
while (i <= 5)
{
  sum += 1.0/i;
  i++;  
}
    

  2. sum = i = 0;
do
{
  i++;
  sum += 1.0/i;
}
while (i < 5);
    

  3. for (sum = 0, i = 1; i <= 5; i++)
  sum += 1.0/i;
The solution using a for statement is the shortest and clearest.

We have suggested that counted loops use for statements while conditional loops use while or do statements. Sometimes, however, loops have both of these elements. For these situations, the choice of structure is very often a matter of personal preference.

Example 2

As part of a program to generate report cards, one segment is required to read and total the marks. Most students take MAX_SUBJECTS subjects and input for them consists of that many marks. Some students take fewer subjects. Input for them consists of their marks followed by a sentinel value of -l.

In this problem, the number of repetitions of the loop is sometimes simply counted (for students taking the maximum number of subjects) but sometimes conditional (for students with fewer subjects). The segment shown below will give the correct values for totalMarks and numberOfMarks for input of either form. We have solved the problem with a do but other solutions are possible.

  totalMarks = numberOfMarks = 0;
  do
  {
    System.out.println("Next mark please");
    mark = In.getInt();
    if (mark != -1)
    {
      totalMarks += mark;
      numberOfMarks++;
    }
  }
  while (mark != -1 && numberOfMarks < MAX_SUBJECTS);

Exercise 4.5

  1. Describe what the fragment does and then rewrite it using a for statement. 
    1. count = 1;
while (count < 10)
{
  System.out.println(count+" "+Math.sqrt(count));
  count++;
}

      2. 

    2. i = 5;
do
{
  i--;
  System.out.println(Math.pow(i,3));
}
while (i > 0); 



      3.
  2. Describe what the fragment does and then rewrite it using a while statement. 
    1. for (i = 0; i < 10; i++)
{
  x = 2 * i - 10;
  y = Math.abs(x / 5 - 2);
  System.out.println (x + " " + y);
}

      2. 

    2. product = 1;
do
{
  System.out.println("Next value please");
  next = In.getInt();
  if (next != TERMINATOR)
    product *= next;
}
while (next != TERMINATOR);

      3. 

    

    

    3. 

  3. Describe what the fragment does and then rewrite it using a do statement.

    1. System.out.println("Enter a value - zero to stop");
value = Math.abs(In.getInt());
biggest = value;
while (value != 0)
{
  if (value > biggest)
    biggest = value;
  System.out.println("Enter a value (0 to stop)");
  value = Math.abs(In.getInt());
}

      2. 

    2. for (i = 0; i == 0; )
{
  System.out.println("Continue? (Y/N)");
  response = In.getChar();
  if (response == 'Y' || response == 'N')
    i = 1;
}


    

  1. State, with reasons, which loop structure you think would be most
appropriate for solving each problem. Do not actually solve the problem.
    


      
   
    1. Find the sum of the cubes of the numbers from 1 to 100.

      2. 
   
    2. Repeatedly prompt a user for a password, rejecting any submissions
until the correct password has been provided.

      3. 
   
    3. Read and sum positive integers until a sentinel value of -1 is
read.

      4. 
   
    4. Determine the number of times that a positive integer can be
divided by two.
      5.  
      
   
    5. Find the amount to which $1 will grow in ten years at a rate of
8%.

      6. 

    

    2.