Topic 4 Variables

What we will do today

•Explain and look at examples of
–primitive data types
–expressions
–variables
–assignment statements

Programs that examine data

•We have already seen that we can print text on the screen
using println and String literals:
 System.out.println("Hello, world!");
•Now we will learn how to print and manipulate other kinds of
data, such as numbers:
 System.out.println(42);
 System.out.println(3 + 5 * 7);
 System.out.println(12.5 / 8.0);
•data: Numbers, characters, or other values that are
processed by a human or computer.
– Useful computer programs manipulate data.

Data types

•Most programming languages (like Java) have a
notion of data types and ask the programmer to
specify what type of data is being manipulated.
•type: A category or set of data values.
– Example: integer, real number, string
•Internally, the computer stores all data as 0s and
1s.
– example: 42 --> 101010
– example: "hi" --> 0110100001101001
•Counting with dots exercise

Java's primitive types

•The expressions in today's slides so far have
been integers.
– Integers are one of Java's data types.
•primitive types: Java's built-in simple data
types for numbers, text characters, and logic.
– Java has eight primitive types total.
– Types that are not primitive are called object types.
– We'll use these four primitive types in this class:

Expressions

•expression: A data value, or a set of operations
that compute a data value.
 – Example: 1 + 4 * 3
 – The simplest expression is a literal value.
 – A more complex expression can have operators and/or parentheses.
 • The values that an operator applies to are called operands.
•5 common arithmetic operators we will use:
 + (addition)
 - (subtraction or negation)
 * (multiplication)
 / (division)
 % (modulus, a.k.a. remainder)

Evaluating expressions

•When your Java program executes and encounters a line
with an expression, the expression is evaluated (its value is
computed).
– The expression 3 * 4 is evaluated to obtain 12.
– System.out.println(3 * 4) prints 12, not 3 * 4.
(How could we print 3 * 4 on the screen?)
•When an expression contains more than one operator of the
same kind, it is evaluated
left-to-right.
– Example: 1 + 2 + 3 is (1 + 2) + 3 which is 6
– Example: 1 - 2 - 3 is (1 - 2) - 3 which is -4
(not the same as 1 - (2 - 3) which is 2)
•Show the Dr. Java interaction pane code pad

Integer division with /

•14 / 4 evaluates to 3, not 3.5.
– Back to division in 4^th grade
– In Java, when we divide integers, the result is also an integer: the
integer quotient.
– The integer quotient of dividing 14 by 4 is 3.
The integer remainder of dividing 14 by 4 is 2.
– Imagine that you were doing long division:

– Examples:
 • 35 / 5 evaluates to 7
 • 84 / 10 evaluates to 8
 • 156 / 100 evaluates to 1
– Dividing by 0 causes your program to crash.
– Try it!

Integer remainder with %

•The % operator computes the remainder from a
division of integers.
– Example: 14 % 4 is 2
– Example: 218 % 5 is 3

•What do the following expressions evaluate to?

– 45 % 6
– 2 % 2
– 8 % 20
– 11 % 0

Applications of % operator

•What expression obtains the last digit (units place)
of a number?
– Example: From 230857, obtain the 7.
•How could we obtain the last 4 digits of a Social
Security Number?
– Example: From 658236489, obtain 6489.
•What expression obtains the second-to-last digit
(tens place) of a number?
– Example: From 7342, obtain the 4.
•Can the % operator help us determine whether a
number is odd? Can it help us determine whether
a number is divisible by, say, 27?

Operator precedence

•How does Java evaluate 1 + 3 * 4?
Is it (1 + 3) * 4, or is it 1 + (3 * 4)?
– In a complex expression with several operators, Java uses internal
rules of precedence to decide the order in which to apply the
operators.
•precedence: Order in which operations are computed in an
expression.
– Multiplicative operators have a higher level of precedence than
additive operators, so they are evaluated first.
 • * / % before + -
– In our example, * has higher precedence than +, just like on a
scientific calculator, so 1 + 3 * 4 evaluates to 13.
– Parentheses can be used to override a precedence.
(1 + 3) * 4 evaluates to 16.

Precedence examples

Precedence examples

•What do the following expressions evaluate to?
9 / 5
695 % 20
7 + 6 * 5
7 * 6 + 5
248 % 100 / 5
6 * 3 - 9 / 4
(5 - 7) * 4
6 + (18 % (17 - 12))
•Which parentheses above are unnecessary
(which do not change the order of
evaluation?)

Real numbers

•The expressions we have seen so far used integers, but
Java also can manipulate real numbers (numbers with a
decimal point).
– Examples: 6.022, -15.9997, 42.0, 2.143e17
•The operators we saw, + - * / % , as well as
parentheses ( ) , all work for real numbers as well.
– The / operator produces a more precise answer when used on real
numbers, rather than an integer quotient.
 • Example: 15.0 / 2.0 evaluates to 7.5
– The % operator is not often used on real numbers.
•The same rules of precedence that apply to integers also
apply to real numbers.
– ()before * / % before + -

Real number example

Real number precision

•Strange things are afoot with real numbers:
System.out.println( 11.0 – 10.91 );
– The mathematically correct answer is 0.09
– Instead, we get this:

•Unfortunately, the computer represents real numbers in an
imprecise way internally, so some calculations with them are
off by a very slight amount.
– We cannot do anything to change this.
– We will generally ignore this problem for this course and tolerate the
precision errors, but later on we will learn some ways to produce a
better output for examples like above.
– Example. Write 1/3 base 10 as a decimal in base 10 and then in base 3

Mixing integers and reals

•When a Java operator is used on an integer and a real
number, the result is a real number.
– Example: 3 * 4.2 evaluates to 12.6
– Example: 1 + 1.0 evaluates to 2.0
The kind of number that results from a given operator
depends only on its operands, not any other operands.

The computer's memory

•Think of the computer like a calculator for a
moment.
– We have already seen how to calculate values.
•A flexible calculator has "memory" keys to store
and retrieve a computed value.
– In what situation(s) is this useful?
•How can we save and restore
a value that our Java program
previously calculated, like the
memory keys (MC / MR, STO / RCL)
on the calculator?

Variables

•variable: A piece of your computer's memory that is given a
name and type, and can store a value.
– We use variables to store the results of a computation and use those
results later in our program.
– Unlike a cheap calculator, which may only have enough to store a
few values, we can declare as many variables as we want, limited
only by the memory are program is allowed to use.
•Variables are a bit like the 6 preset stations on your car
stereo, except we can, essentially, have as many of them as
we want, and we give them names, not numbers.

Declaring variables

•variable declaration statement: A Java statement that
creates a new variable of a given type.
– A variable is declared by writing a statement that says its type, and
then its name. (The name is an identifier.)

•Declaration statement syntax:
<type> <name> ;
– Example: int x;
– Example: double myGPA;

•It is also legal to declare multiple variables of the same type
on one line:
<type> <name>, <name>, ..., <name> ;
– Example: int a, b, c;

More on declaring variables

•Declaring a variable sets aside a chunk of memory in which
you can store a value.
int x;
int y;
– A (crude) diagram of part of the computer's memory:

(The memory has no value in it yet.)

•The compiler will fail if you try to declare a variable twice, or
declare two variables with the same name.
– Illegal:
int x;
int x; // variable x already exists! ERROR

•When tracing code, draw boxes for variables!!

Assignment statements

•assignment statement: A Java statement that stores a
value into a variable's memory location.
– Variables must be declared before they can be assigned a value.

•Assignment statement syntax:
<name> = <value> ;
– Example: x = 3;
– Example: myGPA = 3.95;

– Another (crude) diagram of part of the computer's memory:

– Technically, = is an operator like + or *, called the assignment
operator, with very low precedence (it is carried out last).

More about assignment

•The <value> assigned to a variable can be a complex
expression. The expression will be evaluated, and the
variable will store the result.
– Example:
x = (2 + 8) / 3 * 5;
(The variable x now stores the value 15)

•A variable can be assigned a value more than once in the
program.
– Example (Draw the boxes!!):
int x;
x = 3;
System.out.println(x); // 3
x = 4 + 7;
System.out.println(x); // 11

Using variables' values

•Once a variable has been assigned a value, it can be used
in an expression, just like a literal value.
int x;
x = 3;
System.out.println(x * 5 - 1);
– The above has output equivalent to:
System.out.println(3 * 5 - 1);

•A variable that has not been assigned a value cannot be
used in an expression or println statement.
– Illegal:
int x;
System.out.println(x); // ERROR -- x has no value

Assignment and algebra

•Though the assignment statement uses the =
character, it is not like an algebraic equation.
= means, "store the value on the right into the memory of
the variable on the left“
in Java = is a verb, not a statement of fact
– Illegal:
3 = 1 + 2;
(because 3 is not a piece of the computer's memory)
1 + 2 = x; // syntax error

•What do you suppose happens when a variable is
used on both sides of an assignment statement?
int x;
x = 3;
x = x + 2; // what happens?

Assignment and types

•A variable can only store a value of its own type.
– Illegal: x = 2.5; // ERROR: x can only store an int
– (Technically, the value does not need to be the same type as the
variable--it can be any type that Java knows how to convert into the
variable's type... see below.)

•An int value can be stored in a variable of type double.
The value is converted into the equivalent real number.
– Legal: double myGPA = 4;

Assignment examples

•What is the output of the following Java code?
int number;
number = 2 + 3 * 4;
System.out.println(number - 1);
number = 16 % 6;
System.out.println(2 * number);

•What is the output of the following Java code?
double average;
average = (9 + 8) / 2;
System.out.println(average);
average = (average * 2 + 10 + 8) / 4;
System.out.println(average);

Declaration and initialization

•A variable can be declared and assigned an initial value in
the same statement, to save lines in your program.

•Declaration and initialization statement syntax:
<type> <name> = <value> ;
– Example: double myGPA = 3.95;
– Example: int x = (11 % 3) + 12;

•It is also legal to declare/initialize several at once:
<type> <name> = <value> , <name> = <value> ;
– Example: int a = 2, b = 3, c = -4;
– Example: double grade = 3.5, delta = 0.1;

Multiple declaration error

•The compiler will fail if you try to declare-and-initialize a
variable twice.
– Illegal:
int x = 3;
System.out.println(x);
int x = 5; // variable x already exists! ERROR
System.out.println(x);
– This is the same as trying to declare x twice.

•What should the code have been if the programmer wanted
to change the value of x to 5 ?

Integer or real number?

•Categorize each of the following quantities by whether an
int or double variable would best to store it:

1. Temperature in degrees Celsius
2. The population of lemmings
3. Your grade point average
4. A person's age in years
5. A person's weight in pounds
6. A person's height in meters
7. Number of miles traveled today
8. Number of dry days in the past month
9. The number of games the volleyball team wins this season
10. Number of seconds left in a game
11. The sum of a group of integers
12. The average of a group of integers

Strings in expressions

•A String can be used in an expression.
– But the only operator Strings understand is + , and its meaning is
different.
– A + operator on a String and another value causes the other value to
be attached to the String, creating a longer String. This is called
concatenation.
– Remember, the precedence of the + operator is below * / % .

Example: "hello" + 42 evaluates to "hello42"
Example: 1 + "abc" + 2 evaluates to "1abc2"
Example: "abc" + 1 + 2 evaluates to "abc12"
Example: 1 + 2 + "abc" evaluates to "3abc"
Example: "abc" + 9 * 3 evaluates to "abc27"
Example: "1" + 1 evaluates to "11"

Printing String expressions

•String expressions with + are useful so that we can print
more complicated messages that involve computed values.
double grade = (95.1 + 71.9 + 82.6) / 3.0;
System.out.println("Your grade was " + grade);
int students;
students = 11 + 17 + 4 + 19 + 14;
System.out.println("There are " + students +
" students in the course.");

Example variable exercise

•Write a Java program that stores the following data:
– Section 58615 has 17 students.
– Section 58617 has 8 students.
– Section 58620 has 11 students.
– Section 58625 has 23 students.
– Section 58627 has 24 students.
– Section 58630 has 7 students.
– The average number of students per section.

and prints the following:
There are 24 students in Section 58627.
There are an average of 15 students per section.

Modify-and-assign operators

•Java has several shortcut operators that
allow you to quickly modify a variable's value:

•Examples:
x += 3; // x = x + 3;
myGPA -= 0.5; // myGPA = myGPA - 0.5;
number *= 2; // number = number * 2;

Increment and decrement

•Since it is a very common task to increase or decrease a
variable's value by 1, there are two special operators for
this.

– These are called the increment and decrement operators.
– If <variable>++ or <variable>-- is used in an expression, the
variable's old value is used during the computation, and then
afterward the variable is incremented or decremented.
• Guideline: Don't use ++ or -- in an expression! It’s confusing!
– Example:
int x = 3;
System.out.println(x); // 3
x++;
System.out.println(x); // 4
System.out.println(x++); // 4
System.out.println(x); // 5