CS 1713/1711 Introduction to Computer Science Lecture 21

CS 1713 Lecture 21: Loops
October 18, 1999

Program control:
- Sequence
- Selection
- Repetition
The while loop:
Form: while ( repetitionCondition )
loopStatement
Example: Printing n asterisks on a line
```
   countStar = 0;
   while (countStar < n) {
      System.out.print("*");
      countStar++;
   } 
   System.out.println();
```
Interpretation: If repetitionCondition is true, the loopStatement is executed and the repetitionCondition is reevaluated and tested. When the repetitionCondition is false. As soon as repetitionCondition is false, the statement after the loop is executed.

Example: Have kermit the Frog hop 10 times and output its position.

    Frog kermit = new Frog();
    int i = 0;
    while (i < 10) {
        kermit.Hop();
        i++;
    }
    System.out.println("Kermit is now at " + kermit.GetPosition());

Example: Write a helper function to return the sum of the squares of the integers from 1 to n.
Example: Write a helper function to return the maximum of n calls to Math.random.
Example: Write a helper function called findFirstVowel that has a String parameter s. The helper returns the position of the first vowel in s or -1 if s contains no vowels.

Example:Write a howManyVowels helper function. This function has a String parameter s and returns the number of vowels in the string.

public int howManyVowels(String s) {
   int count = 0;
   int pos = 0;
   String vowels = new String("AEIOUaeiou");
   char thisChar;

   while (pos < s.Length()) {
      thisChar = s.charAt(pos);
      if (vowels.indexOf(thisChar) != -1)
          count++;
      pos++;
   }
   return count;
}

Example: Write a code segment to call the WaitForButtonPress method of the VendingButton object vend 10 times and count the number of times it was pushed.
Example: Write a helper function called generateMonth
```
public void generateMonth(String monthName, int startDay, int numberDays) {

}
```
that outputs the calendar for the specified month.

Objective: Learn to use a counting while loop.

Last revision: October 17, 1999 at 8:30 am