Testing and Debugging Loops

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Review

Desk Checking:
- Verifying that a program satisfies specifications by carefully reading the code
Testing:
- Verifying that a program satisfies specifications by executing the code
Debugging:
- Locating and repairing logic errors

Desk Checking Loops

Ensure that you don't compare floating point numbers with the == operator

  double   total;

  total = 50.0;
  while (total != 0.0)
  {
      // ...
      total = total - 0.01;
  }

Desk Checking Loops (cont.)

Ensure that you don't change the variable(s) used in the boolean expression within the body of for loops

  int   i;

  for (i=0; i<10; i++)
  {
      // ...
      i = i + 1;
  }

Desk Checking Loops (cont.)

Ensure that you do change the variable(s) used in the boolean expression within the body of while and do-while loops

  int   i;

  i = 1;
  while (i < 10)
  {
      // ...
  }

Desk Checking Loops (cont.)

Ensure that the correct variable is updated by the correct amount within the body of the loop

  int   i, j;

  i = 1;
  while (i < 10)
  {
      // ...
     j++;
  }

Desk Checking Loops (cont.)

Ensure that the loop doesn't terminate early/late (especially "off by one")

  int   i;

  i = 1;

  // Loop ten times
  while (i < 10)
  {
      // ...
      i = i + 1;
  }

Desk Checking Loops (cont.)

Ensure that you don't have a semicolon at the end of for and/or while loops

  int i;

  for (i=0; i<10; i++);
  {
      // ...
  }

Desk Checking Loops (cont.)

Ensure that loop counters are initialized and/or re-initialized properly

  int   i;

  i = 1;
  while (i < 10)
  {
      // ...
  }

  while (i < 10)
  {
      // ...
  }

Testing Loops

An Observation:
- A loop that is supposed to iterate n times might actually iterate 0 times, 1 time, n-1 times, n+1 times, or "forever"
Good Test Cases:
- Will allow you to conclude that none of these "errors" can arise

Testing Loops (cont.)

An Observation:
- for and while loops might have 0 iterations

An Example:

while (i < n)
{
  // ...
  i = i + 1;
}

Test Cases:
- i = 0; and n = 10; ensures that the body is entered
- i = 20; and n = 10; ensures that the body is not entered

Instrumenting Loops During Debugging

The Basics:
- Include print() calls before while loops to see if the code will/should enter the body
- Include print() calls (that include the variables that change) in the body of all loops

An Example:

javaexamples/basics/DebuggingLoops.java

(Fragment: 0)

        // Prompt for 10 numbers and find the total
        i = 1;
        n = 10;

        // For debugging:
        JMUConsole.printf("i: %d   n: %d", i, n);
        while (i < n)
        {
            // For debugging:
            JMUConsole.printf("Iteration: %d", i);

            JMUConsole.printf("Enter a number: ");
            number = JMUConsole.readInt();

            // For debugging:
            JMUConsole.printf("  BEFORE total:%d number:%d", total, number);

            total = total + number;

            // For debugging:
            JMUConsole.printf("  AFTER total:%d number:%d", total, number);

            ++i;
        }