• Introduce loops with examples
• Examine fence post problems
• Define flow of control
• Study the flow of control for nested loops
• Design methods to replace inner loops

Loops have four sections

• Initialization:
  This step occurs prior to the loop. Variables that need to be initialized are given values prior to the first time the loop test is evaluated.
• Loop test:
  The test determines whether the loop body will be executed. When the loop test is false, the loop body is not executed. If the loop test is always true, an infinite loop results, unless the loop is exited with a break or return statement.
• Loop body:
  The statements that are executed each time the loop test evaluates to true.
• Loop update:
  The statements that affect values in the loop test. These statements ensure that the loop will eventualy terminate. Values of variables in the loop test will be changed by the update statements.

• while
  • particularly good for loops controlled by flags or complex logic
  • test occurs before loop body is executed
  • loop body might not be executed at all, if initial test is false
• for
  • particularly good for counting or going through a sequence
  • test occurs before loop body is executed
  • initialization, test and increment explicitly listed as part of the for
• do-while
  • particularly good for menu-driven loops
  • test occurs after the body of the loop
  • loop body is always executed at least once

The while loop is the most commonly used kind of loop --- when in doubt, you can always use a while loop.

Form: while ( repetitionCondition )
                        loopBody

Example 1: Generate random integers between 0 and 5 (inclusive) until you get a 3. Output the number of values you drew.

   Random r = new Random();
   int numberValues = 0;
   while (r.nextInt(6) != 3){
      numberValues ++;
   }
   System.out.println("We had to draw " + (numberValues+1) + " values");

The repetitionCondition in the while can be anything, but when the while loop is used for counting, the form will be:

                  //<== initialize counter
   while ( /* test whether the counter exceeded the limit? */ )   {
                     // <== stuff to repeat                    
                     // <== update the loop counter
   }

Example 2: Output the numbers between 1 and 10 using a while loop.

   int num = 1;
   while (num <= 10) {
      System.out.print( num + " ");
      num ++;
   }
   System.out.println();

   1 2 3 4 5 6 7 8 9 10 
 

The for loop is naturally designed for counting out a specific number.

Form: for( initialization; test; update )
                        loop-statement

Interpretation: The initialization is done first. Then the test is performed. If the test is true , execute the body of the loop. Then perform the update step and do the test again. As soon as the test is false , control passes to the statement after the for loop. (Draw a flow chart.)

Example 3: Print n asterisks on a line

   for (count = 0; count < n; count++)
      System.out.print("*");
   System.out.println();

Form: do {
                        loop-body
              } while ( repetition-condition)

Example 4: Write code to prompt the user to enter an integer greater than 0.

   int theNumber;
   do {
      System.out.println("Please enter an integer > 0");
      theNumber = Keyboard.readInt();
   } while (theNumber <= 0);
   System.out.println("The number you have entered is " + theNumber);

Fence post problems arise when the endpoints are treated differently than the other values.

Example 5: A straightforward approach to outputting the integers between 1 and 10 separated by commas has an extra comma on the end:

   int num = 1; 
   while ( num <= 10 ) { 
      System.out.print( num + ","); 
      num ++; 
   } 
   System.out.println(); 

The problem here is that the number of numbers is one more than the number of commas

   |~~~|~~~|~~~|~~~| 
   |~~~|~~~|~~~|~~~| 

Alternative 1: Print the first fence post (number) before the loop:

   int num = 1;
   System.out.print(num);      
   num++;                       
   while ( num <= 10 ) {                           
      System.out.print("," + num); 
      num++;                      
   }
   System.out.println();

   int num = 1;
   while ( num < 10 ) {
      System.out.print(num + ",");
      num ++;
   }  
   System.out.println(num);

   int num = 1; 
   while( num <= 10) {
      System.out.print(num); 
      if ( num < 10 ) 
        System.out.print(",");
      num++; 
   }
   System.out.println(); 

• What is it? (Ans: The order in which statements are executed in a running program)
• What is the usual flow of control? (Ans: for ordinary statements it's sequential)
• How do object instantiation and method calls change it?
• How does selection change it?
• How does looping change it?

Draw a schematic of the flow of control through the program.

Example 6: The inner loop is executed completely on each iteration of the outer loop.

   for (int j = 1; j <= 3; j++) {
      for (int k = 1; k <= 2; k++) {
         System.out.println(j + "," + k);
      }
    } 

   1,1
   1,2
   2,1
   2,2
   3,1
   3,2 

Example 7: The test of the inner loop can depend on the outer loop variable.

   for (int j = 1; j <= 5; j++) { 
      for (int k = 1; k <= j; k++) { 
         System.out.print( (j *k) + " "   ); 
      } 
      System.out.println(); 
   } 

   1 
   2  4 
   3  6  9 
   4  8 12 16 
   5 10 15 20 25 

   int line;  
   int i; 
   int numberStars = 1;   // number of stars to print on line
   for (line = 1; line <= numberLines; line++) {
           // Print the leading blanks
      for (i = 1; i <= numberLines - line; i++)         
         System.out.print(' ');
               // Print the stars
      for (i = 1; i <= numberStars; i++)
         System.out.print('*');
      System.out.println();
      numberStars += 2;  // add 2 to numberStars
   }

   public static void printChars(int numBlanks, int numChars) {
      int i;
      for (i = 1; i <= numBlanks; i++) 
         System.out.print(' ');
           // Print the stars
      for (i = 1; i <= numChars; i++)
         System.out.print('*'); 
      System.out.println();
   }

   int line;  
   int numberStars = 1;   // number of stars to print on line
   for (line = 1; line <= numberLines; line++) {    
       printChars(numberLines-line, numberStars);
       numberStars += 2;  // add 2 to numberStars 
   }  

Question 1: How would you modify printChars to print any character rather than specifically asterisks?

Question 2: How could you use printChars to print other kinds of patterns? Ans: Instead of putting printChars in a loop, put explicit calls to printChars needed to form the pattern.

Priming a loop: Reading an initial value or performing other kinds of initialization so that the loop test can be evaluated the first time is called priming the loop.

   int sum = 0;
   System.out.println("Enter a number ");
   int number = Keyboard.readInt();
   while ( number != 0 ) {
      sum += number;
      System.out.println("Enter a number ");
      number = Keyboard.readInt();
   }
   System.out.println("total = " + sum);

Review of loop problems and how to fix them:

• Infinite loops - make sure that the test condition is changed in the body of the loop.

• Off-by-one - watch out for loops that are executed one too many times or one too few times. This is often fixed by changing the initialization slightly or changing whether equality is included in the test.

• Incorrect ordering in a short circuit - this common problem for selection can also be a problem for loops --- the order in compound boolean expressions often matters.

• Scoping of variables - a variable declared within the initialization part of a for loop is in existence only within that loop and cannot be referred to outside the loop.

• Fence post problems - occur when the endpoints need to be treated differently from the middle items in a sequence. (For example, the following code to print the numbers from 1 to 10 separated by commas.) Fence posts can be fixed, but treating the first item as a special case, treating the last item as a special case, or putting a crosspiece test inside the loop.

The scope of an identifier refers to the area of the program in which the item named by the identifier can be used. Variables and methods have scope. If you use an identifier outside of its scope, you will get an "undefined identifier" message.

   public static void main(String args[]) {
      int k = 1;
      while( k < 5 ){
         int ct = 0;
         System.out.println(k);
         k++;
      }
      System.out.println(k);
      //System.out.println(ct);  //ct NOT accessible here
      for (k = 1; k < 5; k++) {
         for (int j = 1; j < 5; j++) {
             System.out.println(k + "  " + j);
         }
         System.out.println(k);
         //System.out.println(j); // j NOT accessible here 
      }
      System.out.println(k); // k IS accessible here
   }