Chapter 2 and 3 Quiz

What does it mean to prime a loop?

The condition at the beginning of a while loop has to make sense even the first time it is tested, before the body of the loop is executed. To prime the loop is to set things up before the loop starts so that the test makes sense (that is, the variables that it contains have reasonable values). For example, if the test in the loop is "while the user's response is yes," then you will have to prime the loop by getting a response from the user (or making one up) before the loop.

What is a type

A "type" represents a set of possible values.

What is a block statement? How are block statements used in Java programs?

A block statement is just a sequence of Java statements enclosed between braces, { and }. The body of a subroutine is a block statement. Block statements are often used in control structures. A block statement is generally used to group together several statements so that they can be used in a situation that only calls for a single statement. For example, the syntax of a while loop calls for a single statement: "while (condition) do statement". However, the statement can be a block statement, giving the structure: "while (condition) { statement; statement; ...}".

In Java, classes have two fundamentally different purposes. What are they?

A class can be used to group together variables and subroutines that are contained in the class. These are called the static members of the class. For example, the subroutine Math.sqrt is a static member of the class called Math. Also, the main routine in any program is a static member of a class. The second possible purpose of a class is to describe and create objects. The class specifies what variables and subroutines are contained in those objects. In this role, classes are used in object-oriented programming (which we haven't studied yet in any detail.)

What is a literal?

A literal is a sequence of characters used in a program to represent a constant value. For example, 'A' is a literal that represents the value A, of type char, and 17L is a literal that represents the number 17 as a value of type long. A literal is a way of writing a value, and should not be confused with the value itself.

What does the computer do when it executes a variable declaration statement. Give an example.

A variable is a "box", or location, in the computer's memory that has a name. The box holds a value of some specified type. A variable declaration statement is a statement such as int x; which creates the variable x. When the computer executes a variable declaration, it creates the box in memory and associates a name (in this case, x) with that box. Later in the program, that variable can be referred to by name.

Integrated Development Environments such as Eclipse often use syntax coloring, which assigns various colors to the characters in a program to reflect the syntax of the language. A student notices that Eclipse colors the word String differently from int, double, and boolean. The student asks why String should be a different color, since all these words are names of types. What's the answer to the student's question?

Although String, like int, double, and boolean, is a type name, there is a fundamental difference between String and the other types. String is the name of a class, which int, double, and boolean are primitive types. Eclipse colors all class names in the same way that it does String, and it uses a different color for the primitive types. (Although the difference between classes and primitive types might not seem very important to you now, it really is an important distinction and it's reasonable for Eclipse to use different colors for the two concepts.)

What is an algorithm?

An algorithm is an unambiguous, step-by-step procedure for performing a certain task, which is guaranteed to finish after a finite number of steps. (An algorithm is not the same thing as a program, but it can be the idea behind a program.)

Explain what is meant by an animation and how a computer displays an animation.

An animation consists of a series of "frames." Each frame is a still image, but there are slight differences from one frame to the next. When the images are displayed rapidly one frame after another, the eye perceives motion. A computer displays an animation by showing one image on the screen, then replacing it with the next image, then the next, and so on.

What is the main difference between a while loop and a do..while loop?

Both types of loop repeat a block of statements until some condition becomes false. The main difference is that in a while loop, the condition is tested at the beginning of the loop, and in a do..while loop, the condition is tested at the end of the loop. It is possible that the body of a while loop might not be executed at all. However, the body of a do..while loop is executed at least once since the condition for ending the loop is not tested until the body of the loop has been executed.

What is the difference between the statement "x = TextIO.getDouble();" and the statement "x = TextIO.getlnDouble();"

Either statements will read a real number input by the user, and store that number in the variable, x. They would both read and return exactly the same value. The difference is that in the second statement, using getlnDouble, after reading the value, the computer will continue reading characters from input up to and including the next carriage return. These extra characters are discarded.

Write a for loop that will print out all the multiples of 3 from 3 to 36, that is: 3 6 9 12 15 18 21 24 27 30 33 36.

Here are two possible answers. Assume that N has been declared to be a variable of type int: for ( N = 3; N <= 36; N = N + 3 ) { System.out.println( N ); } or for ( N = 3; N <= 36; N++ ) { if ( N % 3 == 0 ) System.out.println( N ); }

What is meant by precedence of operators?

If two or more operators are used in an expression, and if there are no parentheses to indicate the order in which the operators are to be evaluated, then the computer needs some way of deciding which operator to evaluate first. The order is determined by the precedence of the operators. For example, * has higher precedence than +, so the expression 3+5*7 is evaluated as if it were written 3+(5*7).

Explain briefly what is meant by "pseudocode" and how is it useful in the development of algorithms.

Pseudocode refers to informal descriptions of algorithms, written in a language that imitates the structure of a programming language, but without the strict syntax. Pseudocode can be used in the process of developing an algorithm with stepwise refinement. You can start with a brief pseudocode description of the algorithm and then add detail to the description through a series of refinements until you have something that can be translated easily into a program written in an actual programming language.

Show the exact output that would be produced by the following main() routine: public static void main(String[] args) { int N; N = 1; while (N <= 32) { N = 2 * N; System.out.println(N); } }

The exact output printed by this program is: 2 4 8 16 32 64 (The hard part to get right is the 64 at the end. The value of N doubles each time through the loop. For the final execution of the loop, N starts out with the value 32, but N is doubled to 64 before it is printed.)

Suppose that s1 and s2 are variables of type String, whose values are expected to be string representations of values of type int. Write a code segment that will compute and print the integer sum of those values, or will print an error message if the values cannot successfully be converted into integers. (Use a try..catch statement.)

The function Integer.parseInt can be used to convert the strings into integers. This function will throw an exception of type NumberFormatException if the conversion fails. A try..catch exception can catch this exception and print an error message. So, the code segment can be written: try { int n1, n2; // The values of s1 and s2 as integers. int sum; // The sum of n1 and n2. n1 = Integer.parseInt(s1); n2 = Integer.parseInt(s2); sum = n1 + n2; // (If an exception occurs, we don't get to this point.) System.out.println("The sum is " + sum); } catch ( NumberFormatException e ) { System.out.println("Error! Unable to convert strings to integers.); }

What is the boolean type? Where are boolean values used?

The only values of type boolean are true and false. Expressions of type boolean are used in places where true/false values are expected, such as the conditions in while loops and if statements.

Explain what is meant by an assignment statement, and give an example. What are assignment statements used for?

The operator ++ is used to add 1 to the value of a variable. For example, "count++" has the same effect as "count = count + 1". The operator && represents the word and. It can be used to combine two boolean values, as in "(x > 0 && y > 0)", which means, "x is greater than 0 and y is greater than 0." The operation != means "is not equal to", as in "if (x != 0)", meaning "if x is not equal to zero.".

Give the meaning of each of the following Java operators: a) ++ b) && c) !=

The operator ++ is used to add 1 to the value of a variable. For example, "count++" has the same effect as "count = count + 1". The operator && represents the word and. It can be used to combine two boolean values, as in "(x > 0 && y > 0)", which means, "x is greater than 0 and y is greater than 0." The operation != means "is not equal to", as in "if (x != 0)", meaning "if x is not equal to zero.".

ill in the following main() routine so that it will ask the user to enter an integer, read the user's response, and tell the user whether the number entered is even or odd. (You can use TextIO.getInt() to read the integer. Recall that an integer n is even if n % 2 == 0.) public static void main(String[] args) { // Fill in the body of this subroutine! }

The problem already gives an outline of the program. The last step, telling the user whether the number is even or odd, requires an if statement to decide between the two possibilities. public static void main (String[] args) { int n; // the number read from the user TextIO.put("Type an integer: "); // ask the use to enter an integer n = TextIO.getInt(); // read the user's response if (n % 2 == 0) // tell the user whether the number is even or odd System.out.println("That's an even number."); else System.out.println("That's an odd number."); }

Explain why the value of the expression 2 + 3 + "test" is the string "5test" while the value of the expression "test" + 2 + 3 is the string "test23". What is the value of "test" + 2 * 3 ?

The reason is somewhat technical. The difference is due to the order of evaluation. When several + operators are used in a row, with no parentheses, they are evaluated from left to right. 2 + 3 + "test" is interpreted as (2 + 3) + "test", so 2 and 3 are added together, giving 5, and then the 5 is concatenated onto the string "test". On the other hand, "test" + 2 + 3 is interpreted as ("test" + 2) + 3, so the 2 is first concatenated onto the "test", giving "test2", and then the 3 is concatenated onto that. In the case of "test" + 2 * 3, the precedence rules for + and * come into play. Since * has higher precedence, this expression is interpreted as "test" + (2 * 3), which evaluates to "test6".

Briefly explain what is meant by the syntax and the semantics of a programming language. Give an example to illustrate the difference between a syntax error and a semantics error.

The syntax of a language is its grammar, and the semantics is its meaning. A program with a syntax error cannot be compiled. A program with a semantic error can be compiled and run, but gives an incorrect result. A missing semicolon in a program is an example of a syntax error, because the compiler will find the error and report it. If N is an integer variable, then the statement "frac = 1/N;" is probably an error of semantics. The value of 1/N will be 0 for any N greater than 1. It's likely that the programmer meant to say 1.0/N.

Show the exact output produced by the following main() routine: public static void main(String[] args) { int x,y; x = 5; y = 1; while (x > 0) { x = x - 1; y = y * x; System.out.println(y); } }

The way to answer this question is to trace exactly what the program does, step-by-step. The output is shown below on the right. On the left is a table that shows the values of the variables x and y as the program is being executed. value of x | value of y OUTPUT --------------|-------------- ------------- 5 | 1 [ before loop] 4 | 4 [ = 1*4 ] 4 3 | 12 [ = 4*3 ] 12 2 | 24 [ = 12*2 ] 24 1 | 24 [ = 24*1 ] 24 0 | 0 [ = 24*0 ] 0

What output is produced by the following program segment? Why? (Recall that name.charAt(i) is the i-th character in the string, name.) String name; int i; boolean startWord; name = "Richard M. Nixon"; startWord = true; for (i = 0; i < name.length(); i++) { if (startWord) System.out.println(name.charAt(i)); if (name.charAt(i) == ' ') startWord = true; else startWord = false; }

This is a tough one! The output from this program consists of the three lines: R M N As the for loop in this code segment is executed, name.charAt(i) represents each of the characters in the string "Richard M. Nixon" in succession. The statement System.out.println(name.charAt(i)) outputs the single character name.charAt(i) on a line by itself. However, this output statement occurs inside an if statement, so only some of the characters are output. The character is output if startWord is true. This variable is initialized to true, so when i is 0, startWord is true, and the first character in the string, 'R', is output. Then, since 'R' does not equal ' ', startWorld becomes false, so no more characters are output until startWord becomes true again. This happens when name.charAt(i) is a space, that is, just before the 'M' is processed and again just before the 'N' is processed. In fact whatever the value of name, this for statement would print the first character in name and every character in name that follows a space. (It is almost true that this for statement prints the first character of each word in the string, but something goes wrong when there are two spaces in a row. What happens in this case?)