1. Variables and Assignment Statements

In all but the smallest programs, an executing program is constantly working with values. These values are kept in little sections of main memory called variables. 

Chapter Topics:

• Variables
  
• Assignment Statements
  
• Expressions
  
• Arithmetic Operators
  

Question 1:
Do you imagine that a variable can change its value?

Source: Bradley Kjell, http://programmedlessons.org/Java9/chap09/ch09_01.html
This work is licensed under a Creative Commons Attribution-NonCommercial 3.0 License.

2. Variables

Answer:
Yes — that is why is is called a variable.

Variables

Question 2:
Must a variable always have a data type?

3. Declaration of a Variable

Answer:
Yes. Otherwise it would not be clear what its bits represent.
Remember that the meaning of a bit pattern is determined by its context. It is the data type that 
 gives the bits in a variable a context.

Declaration of a Variable

long payAmount = 123;

Question 3:
What do you think the program prints on the monitor? (You should be able to figure this out.)

4. Simulated Java Program

Answer:
The variable contains: 123

Simulated Java Program

  
  public class Example
  {
    public static void main ( String[] args )
    {
      long payAmount = ;    // declaration of a variable
  
      System.out.println("The variable contains: " + payAmount );
    }
  }
  
  

Question 4:
Try entering something like "rats" in the declaration. Does the program compile successfully?

5. Syntax of Variable Declaration

Answer:
No

Syntax of Variable Deceleration

dataType   variableName;

dataType   variableName  =  initialValue ;

dataType   variableNameOne, variableNameTwo ;

dataType   variableNameOne  =  initialValueOne, 

           variableNameTwo  =  initialValueTwo ;

Question 4:
Is the following correct?
int answer;
double rate = 0.05;

6. Names for Variables

Answer:
Yes — as long as the names  answer  and  rate  have not already been used.

Variables

Question 6:
Which of the following variable declarations are correct? (click on a declaration to verify your answer)
int myPay, yourPay; 

long good-by ; 

short shrift = 0; 

double bubble = 0, toil= 9, trouble = 8 

byte the bullet ; 

int  double; 

char thisMustBeTooLong ; 

int 8ball; 

float a=12.3; b=67.5; c= -45.44;

7. Calculation

Answer:
The first value was stored in a variable of data type long an integer type. Integers do not have fractional parts.
 The second forty was the result of a calculation involving a variable of data type double, a floating point type, 
 which does have a fractional part. (Here, the fractional part was zero.)

Calculation

public class Example
{
  public static void main ( String[] args )
  {
    long   hoursWorked = 40;    
    double payRate = 10.0, taxRate = 0.10;    

    System.out.println("Hours Worked: " + hoursWorked );
    System.out.println("pay Amount  : " + (hoursWorked * payRate) );
    System.out.println("tax Amount  : " + (hoursWorked * payRate * taxRate) );
  }
}

(hoursWorked * payRate)

Question 8:
Would it be better to write some of those statements on two lines instead one one?

8. Several Lines per Statement

Answer:
Maybe.

Several Lines per Statment

You can use several lines for a statement. Anywhere a space character is OK you can split a statement. This means you can't split a statement in the middle of a name, nor between the quote marks of a string literal, nor in the middle of a numeric literal. Recall that a "literal" is an explicit value in the program. Here is the program with some statements correctly put on two lines:

public class Example
{
  public static void main ( String[] args )
  {
    long   hoursWorked = 40;    
    double payRate = 10.0, 
           taxRate = 0.10;    
    System.out.println("Hours Worked: " + 
hoursWorked );
    System.out.println("pay Amount  : " 
+ (hoursWorked * payRate) );
    System.out.println("tax Amount  : " + (hoursWorked 
* payRate * taxRate) );
  }
}

public class Example
{
  public static void main ( String[] args )
  {
    long   hoursWorked = 40;    
    double payRate = 10.0, 
           taxRate = 0.10;    
    System.out.println("Hours Worked: " + 
        hoursWorked );
    System.out.println("pay Amount  : " +
       (hoursWorked * payRate) );
    System.out.println("tax Amount  : " + 
       (hoursWorked * payRate * taxRate) );
  }
}             

It is a good idea to indent the second half of a split statement further than the start of the statement.

Question 9:
Is the following correct?
cla
   ss Example
{

  public static void main ( String[] args )
  {
    long   hoursWorked = 40;    
    double payRate = 10.0, taxRate = 0.10;    

    System.out.println("Hours 
        Worked: " + hoursWorked );

    System.out.println("pay Amount  : " + (hours
        Worked * payRate) );

    System.out.println("tax Amount  : " + (
        hoursWorked * payRate * taxRate) );
  }
}

9. Assignment Statements

Answer:
No. The incorrect splittings are highlighted in red:
cla
    ss Example
{

  public static void main ( String[] args )
  {
    long   hoursWorked = 40;    
    double payRate = 10.0, taxRate = 0.10;    

    System.out.println("Hours 
        Worked: " + hoursWorked );

    System.out.println("pay Amount  : " + (hours
        Worked * payRate) );

    System.out.println("tax Amount  : " + (
        hoursWorked * payRate * taxRate) );
}
The last statement is correct, although not done in a good style for easy human comprehension. The extra blank lines are OK.

Assignment Statement

public class AssignmentExample
{
  public static void main ( String[] args )
  {
    long payAmount ;  // payAmount is declared without an initial value
    payAmount = 123;  // an assignment statement
    System.out.println("The variable contains: " + payAmount );
  }
}

So far, the example programs have been using the value initially put into a variable. Programs can change the value in a variable. An assignment statement changes the value that is held in a variable. The program uses an assignment statement.

The assignment statement puts the value 123 into the variable. In other words, while the program is executing there will be a 64 bit section of memory that holds the value 123.

Remember that the word "execute" is often used to mean "run". You speak of "executing a program" or "executing" a line of the program.

Question 10:
What does the program print to the monitor?

10. Assignment Statement Syntax

Answer:
The variable contains: 123
The program prints out the same thing as the first example program. However, this program did not 
 initialize the variable and so had to put a value into it later.

Assignment Statement Syntax

Assignment statements look like this:

variableName  =  expression ;

• The equal sign = is the assignment operator.
• variableName is the name of a variable that has been declared previously in the program.
• expression is a collection of characters that calls for a value to be calculated.

Here are some example assignment statements (assume that the variables have already been declared):

total = 3 + 5;

price = 34.56;

tax = total*0.05;

In the source file, the variable must be declared before any assignment statement that uses that variable.

Question 11:
Is the following correct?
int sum;

sum = 42 - 12 ;

11. Assignment Statement Semantics

Answer:
Yes, the statements are syntactically correct.

Assignment Statement Semantics

total = 3 + 5;

points = 23;

Question 12:
What happens FIRST when the following statement executes?
value = 2*3 ;

12. Two Steps

Answer:
FIRST, do the multiplication 2*3 to get the value 6.

Two Steps

Question 13:
What will this program fragment write?
value = 2*3;
System.out.println( "value holds: " + value );

13. More Practice

Answer:
value holds: 6

More Practice

int extra;
extra = 5;

variableName = expression;

Question 14:
What will this program fragment write?
int quantity = 7;

quantity = 13;
System.out.println( "quantity holds: " + quantity );

14. Adding a Number to a Variable

Answer:
quantity holds:  13
The assignment statement replaced the value that was originally in quantity with the value 13.

Adding a Number to a Variable

value = extra + 2;

Question 15:
What will this program print out:
// Example assignment statements
int extra, value;

extra = 5;
value = extra + 2;
System.out.println( "value now holds: " + value );

15. Same Variable Twice in an Assignment Statement

Answer:
The program will print out:
value now holds: 7

Adding a Number to a Variable

value = 5;
value = 12 + value;

Question 16:
What does the following program fragment write?
value = 5;
System.out.println("value is: " + value );

value = value + 10;
System.out.println("value is: " + value );

16. A Sequence that Counts

Answer:
The program will print out:
value is: 5
value is: 15

A Sequence that Counts

Look at this program fragment. Click inside the program to see how it works.

int count = 0;                  // statement 1
System.out.println( count ) ;   // statement 2
count = count + 1;              // statement 3
System.out.println( count ) ;   // statement 4

0
1

Question 17:
Think of a way to write 0, 1, and 2.

17. Counting Higher

Answer:
Put a copy of statements 3 and 4 at the end.

Counting Higher

The following fragment:

int count = 0;                  // statement 1
System.out.println( count ) ;   // statement 2

count = count + 1;              // statement 3
System.out.println( count ) ;   // statement 4

count = count + 1;               
System.out.println( count ) ;

prints out

0
1
2

The statement

count = count + 1;

increments the number in count. Sometimes programmers call this "incrementing a variable" although (of course) it is really the number in the variable that is incremented.

sum = 2*sum ;

Question 18:
What does the following assignment statement do:
sum = 2*sum ;
(What are the two steps, in order?)

18. Expressions

Answer:
Evaluate the expression: get the value in sum and multiply it by two.
Then, put that value into sum.

Expressions

(32 - y) / ( x + 5 )

32 - y) / ( x  5 + )

Question 19:
Which of the following expressions are correct? (Assume that the variables 
 have been properly declared elsewhere.)
Expression
53
12 - 3)
x + 34
*z   99
sum + value
Correct or Not?

19. More Practice

Answer:
Expression
  53
  12 - 3)
  x + 34
  *z   99
  sum + value
Correct or Not?
  Correct
  NOT Correct
  Correct
  NOT Correct
  Correct

More Practice

Let's try some more. Again, assume that all the variables have been correctly declared.

Question 20:
Why is the last expression not correct?

20. Spaces Don't Much Matter

Answer:
Multiplication must be shown with a * operator. (Another answer is that 3.14y 
 is not a correct variable name.)

Spaces Don't Much Matter

(hoursWorked*payRate)-deduction

(hoursWorked * payRate) - deduction

( hours Worked * pay Rate) -deduction

12-4   /   2+2

12 - 4/2 + 2

Question 21:
Based on what you know about algebra, what is the value of this expression:
12 - 4/2 + 2

21. Arithmetic Operators

Answer:
12, since the expression means: 12 - 2 + 2

Arithmetic Operators

An arithmetic operator is a symbol that asks for two numbers to be combined by arithmetic. As the previous question illustrates, if several operators are used in an expression, the operations are done in a specific order.

Operators of higher precedence are done first. The table shows the precedence of some Java operators.

Some operators have equal precedence. For example, addition + and subtraction - have the same precedence.

The unary minus and unary plus operators are used as part of a negative or a positive number. For example, -23 means negative twenty-three and +23 means positive twenty-three. More on this later.

When both operands (the numbers) are integers, these operators do integer arithmetic. If one operand or both operands are floating point, then these operators do floating point arithmetic. This is especially important to remember with division, because the result of integer division is an integer.

For example:

However,

Question 22:
What is the value of the following expressions? In each expression, do the 
 highest precedence operator first.
Expression   
16 - 12 / 4    
 2 + 6 / 2  
  8 + 4*2  
  8+4 * 2  
  12/2 - 3  
  6/8 + 2  
Value
 
 
 
 
 
 

22. Evaluation by Rewriting

Answer:
Expression
16 - 12 / 4
2 + 6 / 2
8 + 4*2
8+4 * 2
12/2 - 3
6/8 + 2
Value
13
5
16
16
3
2
The last result is correct. First 6/8 is done using integer division, 
 resulting in 0. Then that 0 is added to 2.

Evaluation by Rewriting

When evaluating an expression, it can be helpful to do it one step at a time and to rewrite the expression after each step. Look at:

16 - 12 / 4

Do the division first, since it has highest precedence. Next, rewrite the expression replacing the division with its value:

16 - 3

Now evaluate the resulting expression:

13

You can write the process like this:

16 - 12 / 4
     ------
16 -    3
---------
   13

The dashed lines show what was done at each step.

Question 23:
What is the value of the following expression?
24 / 2  - 8

23. Evaluate Equal Precedence from Left to Right

Answer:
24 / 2  - 8
------
  12    - 8
  ---------
       4

Evaluate Equal Precedence from Left to Right

When there are two (or more) operators of equal precedence, evaluate the expression from left to right.

2 * 7 * 3 
-----
  14  * 3
  -------
     42

Since both operators are *, each has equal precedence, so calculate 2 * 7 first, then use that result with the second operator.

Here is a second example:

4 - 2 + 5
-----
  2   + 5
  -------
      7

Now the operators are different, but they both have the same precedence, so they are evaluated left to right.

Usually it doesn't matter if evaluation is done left to right or in any other order. In algebra it makes no difference. But with floating point math it sometimes makes an important difference. Also, when an expression uses a method it can make a very big difference. (Methods are discussed in part 6 of these notes.)

Question 24:
What is the value of the following expression?
2 + 4/2 + 1

24. Unary Minus

Answer:
2 + 4/2 + 1
    ---
2 +  2  + 1
------
  4     + 1
  ---------
       5

Unary Minus

-97.34

95 - 12

-12 + 3

Question 25:
What is the value of the following expression?
+24 + 3 * -4

25. Unary Minus

Answer:
2 + 4/2 + 1
    ---
2 +  2  + 1
------
  4     + 1
  ---------
       5

Unary Minus

-97.34

95 - 12

-12 + 3

Question 25:
What is the value of the following expression?
+24 + 3 * -4

26. Arrange what you want with Parentheses

Answer:
+24 + 3 * -4
      ------
+24 +   -12
-----------
     12
The first + is a unary plus, so it has high precedence and applies only to the 24.
The - is a unary minus, so it applies only to the 4.
Next in order of precedence is the *, so three times negative four is calculated 
 yielding negative twelve.
Finally the low precedence + combines the positive twenty four with the negative twelve.

Arrange what you want with Parentheses

To show exactly what numbers go with each operator, use parentheses. For example

-1 * ( 9 - 2 ) * 3  

means do 9 - 2 first. The ( ) groups together what you want done first. After doing the subtraction, the ( 9 - 2 ) becomes a 7:

-1 * 7 * 3  

Now follow the left-to-right rule for operators of equal precedence:

-1 * 7 * 3  
------
  -7   * 3
  --------
     -21

Question 26:
What is the value of each of the following expressions?
Expression
(8 - 2) / 2
(2 + 6) / 2 - 9
(8 + 4) * 2
8 + 4 * 2
8 + (4 * 2)
Value

27. Extra Parentheses

Answer:
Expression
(8 - 2) / 2
(2 + 6) / 2 - 9
(8 + 4) * 2
8 + 4 * 2
8 + (4 * 2)
Value
3
-5
24
16
16

Extra Parentheses

Question 27:
What is the value of each of the following expressions?
Expression
8 + 2 / 2 + 3
(8 + 2) / (2 + 3)
(8 + 2) / 2 + 3
8 + 2 / (2 + 3)
Value

28. Nested Parentheses

Answer:
Expression
(8 + 2) / (2 + 3)
(8 + 2) / 2 + 3
8 + 2 / (2 + 3)
Value
12
2
8
8
The last answer is correct. It is done like this:
8 + 2 / (2 + 3 )
        --------
8 + 2 / 5
    -----
8 + 0
-----
8
2/5 is 0 with integer division.

Nested Parentheses

( ( ( 32 - 16 ) / ( 2 * 2 ) ) -  ( 4 - 8 ) ) + 7
    -----------

( (     16      / ( 2 * 2 ) ) -  ( 4 - 8 ) ) + 7
                  ---------

( (     16      /     4     ) -  ( 4 - 8 )  ) + 7
  ---------------------------

(               4             -  ( 4 - 8 )  ) + 7
                                  -------

(               4             -      -4     ) + 7
---------------------------------------------
                    
                     8                        + 7
                     ----------------------------

                                  15        

Question 28:
What is the value of this expression:
(12 / 4 ) + ( 12 / 3)

29. End of Chapter

Answer:
7

End of the Chapter