CPSC 124, Fall 2005
Answers to Quiz #2

Question 1: Suppose that str is a variable of type String that has value "Fred". What is the value of str.charAt(1)? Why?

Answer: The value of str.charAt(1) is the character 'r'.   str.charAt(n) is the character at position number n in the string str. Since positions are numbered 0, 1, 2, ..., the character at position 1 in "Fred" is 'r'.

Question 2: Suppose that the statement   ok = !( x < 0 || x > 100 );   occurs in a Java program. Assume that x is a variable of type double that has value 98.6. What type of variable does ok have to be, and and what value does this statement assign to ok?

Answer: The expression "!( x < 0 || x > 100 )" is a test that computes a boolean value (either true or false). (The expression means "it is not the case that x is less than 0 or x is greater than 100".) Since the boolean value of this test is assigned to ok, ok must have type boolean. In this case, x is 98.6. So "x < 0" is false and "x > 100" is also false. This means that "x < 0 or x > 100" is false. Since the "!" operator reverses a boolean value, "!( x < 0 || x > 100 )" is true. So, the value assigned to ok is true.

Question 3: Give three different examples of literals, representing three different types of values. State the type of each literal, using a valid Java type name.

Answer: We have seen five major types of literals: int, double, String, boolean, and char. Here is an example of a literal of each of these types:

               int:      42

               double:   17.3

               String:   "Hello World!"

               boolean:  false
               char:     'A'

Question 4: What is an infinite loop, and how could such a thing happen?

Answer: An infinite loop is a loop that continues running indefinitely. This can happen when the exit condition, which is tested to determine whether or not to end the loop, never becomes true. When a program is in an infinite loop, it must be terminated externally in some way (such as pressing CONTROL-C on the command line in Linux).

Part 2: Write a code segment that asks the user to enter an integer, then reads the integer from the user, then writes out all the integers from 1 up to the number entered. The output numbers should be on one line, separated by spaces. For example, if the user types in the number 7, then the output would be:   1 2 3 4 5 6 7

Answer: Here is one possible answer:

              int userNum;    // The number read from the user.
              int outputNum;  // A number to be printed.
              System.out.print("Please enter an integer: ");
              userNum = TextIO.getlnInt();
              outputNum = 1;  // The first number to be printed.
              while (outputNum <= userNum) {
                  System.out.print(outputNum);   // Output the number.
                  System.out.print(" ");         // Output a space.
                  outputNum++;                   // Go on to the next number

David Eck, 16 September 2005