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Section 7.1 Worked Example: Writing Loops

Subgoals for Writing a Loop.

  1. Determine purpose of loop
    1. Pick a loop structure (while, for, do_while)
  2. Define and initialize variables
  3. Determine termination condition
    1. Invert termination condition to continuation condition
  4. Write loop body
    1. Update loop control variable to reach termination

Subsection 7.1.1

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Problem 1: Write a loop that will prompt the user to continue to enter numbers until a sentinel value is entered and find the maximum value entered.

Subsection 7.1.2 SG1: Determine purpose of loop

Generally, while-loops are a good choice for sentinel loops.

Subsection 7.1.3 SG2: Define and initialize variables

We will need variables to store the most recent input value, as well as the maximum value so far.
We will also need a Scanner to accept user input, as well as a priming read.
Scanner get = new Scanner(System.in);
int value;
int maxSoFar;
System.out.println("Enter a value, enter -1 to end");
value = get.nextInt();
while (                )
{

}

Subsection 7.1.4 SG3: Define termination condition invert to continuation condition

Scanner get = new Scanner(System.in);
int value;
int maxSoFar;
System.out.println("Enter a value, enter -1 to end");
value = get.nextInt();
while ( value != -1  )
{

}

Subsection 7.1.5 SG4a: Update condition (LCV) to reach termination

Scanner get = new Scanner(System.in);
int value;
int maxSoFar;
System.out.println("Enter a value, enter -1 to end");
value = get.nextInt();

while ( value != -1  )
{
   System.out.println("Enter a value, enter -1 to end");
   value = get.nextInt();
}

Subsection 7.1.6 SG4: Write loop body

Scanner get = new Scanner(System.in);
int value;
int maxSoFar = -1;
System.out.println("Enter a value, enter -1 to end");
value = get.nextInt();

while ( value != -1  )
{
    if (value > maxSoFar)
        maxSoFar = value;
    System.out.println("Enter a value, enter -1 to end");
    value = get.nextInt();
}
if (maxSoFar > -1)
    System.out.println("The maximum is " + maxSoFar);
else
    System.out.println("No values entered");
Problem 2: Write a loop that will add up 1 + 1/2 + 1/4 + 1/8 + 1/16 + … + 1/1024

Subsection 7.1.7 SG1: Determine purpose of loop

Because we know the number of iterations, we could choose a for-loop, but for this exercise we will go with the general purpose while-loop.

Subsection 7.1.8 SG2: Define and initialize variables

double sum = 0;
double term = 1.0;
while (        )
{

}
System.out.println(sum);

Subsection 7.1.9 SG3: Define termination condition invert to continuation condition

The denominators are increasing powers of 2. The first denominator, if you think of 1 as 1/1, is 2 raised to the power of 0. The final denominator, 1024, is 2 raised to the power of 10, so we will iterate from zero to ten.
double sum = 0;
double term = 1.0;
int i = 0;
while ( i <= 10 )
{

}
System.out.println(sum);

Subsection 7.1.10 SG4: Write loop body

double sum = 0;
double term = 1.0;
int i = 0;

while ( i <= 10 )
{
   sum = sum + term;
   term = 0.5 * term; //same as divide term in half
   i++;
}
System.out.println(sum);

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