Assignment W1--Solutions

CS 100 – Summer 1999

Due Date: July 1, 1999

TA: Alan Renaud

 

  1. Lines (4) & (8) are compile-time errors—errors that are caught by the compiler before the program even runs; try to compile the program as it is written and see for yourself.

    2. The compile-time error in (8) is the undeclared per2 variable; but line (8) is also guilty of a logic error: remainder division uses the % symbol—not the / symbol.

    Line (3) is technically a syntax error, as the code should be public static void main (String args[]) not …. (String [] args)

  2. Machine code is machine-dependent; Java byte code is machine-independent.
  3. 3base and bits&bytes are not valid identifiers: 3base, because it begins with a number; and bits&bytes because it contains the ampersand symbol.

    4. The valid but still not good identifiers are index (too vague) and

    the_next_name_in_the_list_that_starts_with_b (way, way too long).

    Whether PDG_name is good or not will depend upon the context.

    In general, try to choose short and meaningful variable names; they help make your programs "self-documenting".

  4. Java is a case-sensitive language: it distinguishes between upper- and lowercase letters. Hence, b1 and B1 are different identifiers; as are the words if and If—the first is correct, while the last is not a recognized keyword.

5. Here's how the different variables match up in a table:

type

size in bits

values

byte

8

-128 to 127

short

16

-32,768 to 32767

int

32

-2147483648 to + 2147483647

float

32

-3.40292347E+38 to +3.492923447E+38

double

64

-1.79769313486231570E+308 to +1.7969313486231570E + 308

Good programmers try to choose appropriate variable types for each problem. Hence, it would not be sensible to write a factorial method with variables that are all int, as your program would very quickly need numbers bigger than int can provide. Conversely, you would be ill-advised to use a variable of type double to count the number of students in a classroom, too much byte space for too little data.
  1. The Java compiler always associates an else with the previous if unless told to do otherwise by the placement of braces {}. In the case of the present code fragment, the compiler sees it this way:

if (total == MAX)

if (total < sum)

System.out.println ("total equals MAX and total is"

+ " less than sum");

else

System.out.println("total is not equal to MAX");

The body of the first if structure is another if/else structure.

The first structure tests to see if total is equal to MAX. If true, the execution proceeds by testing if total is also less than sum; if that is also true, the proper string—"total equals MAX and total is less than sum"—is displayed. However, if the second condition is false the string "total is not equal to MAX" is displayed even though we know total is equal to MAX.

To force the code fragment to execute as it was originally intended, rewrite the structure as follows:

if (total == MAX) {

if (total < sum)

System.out.println ("total equals MAX and total is"

+ " less than sum");

}

else

System.out.println("total is not equal to MAX");