Variables and Expressions

1. Variables and Expressions

2. Programming Contests • ACM International Collegiate Programming Contest by IBM: http://mcicpc.cs.atu.edu • Google Code Jam (prize $15,000): http://code.google.com/codejam/ • Facebook Hacker Cup (prize $10,000): https://www.facebook.com/hackercup • TopCoder (prize $25,000): http://community.topcoder.com/tc

3. Objectives • Describe the Java data types(e.g., “int”, “fload”, “char”, etc) used for simple data. • Write Java statements to declare variables of different types. • Write assignment statements, expressions containing variables and constants.

4. Variables • Variablesstore data such as numbers and letters. • Think of them as places to store data. • They are implemented as memory locations. • The data stored by a variable is called its value. • The value is stored in the memory location. • The value of a variable can be changed using proper Java statements.

5. Declaring Variables • When you declare a variable, you provide its type and name: intnumberOfBaskets, eggsPerBasket; • A variable's type determines what kinds of values it can hold (“int”, “double”, “char”, etc.). • Declare a variable just before it is used or at the beginning of your program: public static void main (String[] args) { /* declare variables here */ . . .}

6. Data Types • A primitive data type is used for simple and nondecomposable values such as an individual number or individual character. • int, double, and char are primitive types • In addition to the 8primitive data types, Java also provides special support for character strings via the java.lang.String class. • String s = “this is a string”

7. Same kind with different sizes

8. Examples of Primitive Values • Integer types: 0 -1365 12000 • Floating-point types: 0.99 -22.8 3.14159 5.0 • Character type: ‘a’ ‘A’ ‘#’ ‘ ’ • Boolean type: true false

9. Java Identifiers • Identifiers are the names of variables, methods, classes, packages and interfaces. They can be arbitrarily long. • The first character cannot be a digit. • Identifiers may contain only • Letters A/a~Z/z, digits 0 ~ 9, underscore _, dollar sign $ • Identifiers may not contain any • Spaces, dots . , asterisks *, or other characters: • Java is case sensitive: “stuff”, “Stuff”, “STUFF” are different.

10. Keywords and Proper Naming • Words such as if are called keywords or reserved words and have special and predefined meanings: • They cannot be used as identifiers, e.g., int, public. • Primitive types begin with a lowercase letter, e.g., int. • Classes begin with an uppercase letter, e.g., String. • Variables begin with a lowercase letters, e.g., myName. • Choose names that are helpful like count or speed, not cor s. • Multiword names are punctuatedusing uppercase letters.

11. Assignment Statements • Assignment statement is used to assign a value to a variable: answer = 42; • The equal sign is called the assignment operator. • Say, a variable, named answer, is assigned a value of 42. • Syntax: variable = expression where expression can be another variable, a literal or constant (e.g., a number), or something that combines variables and literals using operators such as + and -. firstInitial = 'W'; amount = 3.99; eggsPerBasket= eggsPerBasket – 2;

12. Initializing Variables • A variable that has been declared, but no yet given a value is said to be uninitialized. • Uninitialized primitive variables have a default value, e.g., int= 0, boolean = false, String = null. • To protect against an uninitialized variable, assign a value at the time the variable is declared: • Example: intcount = 0; char grade = 'A'; • Syntax: type variable_1 = expression_1;

13. Assignment Evaluation • The expression on the right-hand side of the assignment operator =is evaluated first. • The result is used to set the value of the variable on the left-hand side of the assignment operator. score = numberOfCards + handicap; eggsPerBasket = eggsPerBasket - 2;

15. It is a math operation Not a math operation

16. Sample Screen Output

17. e Notation & Imprecision • Floating-point constants can be written with digits after a decimal pointor using e notation. • The e notation is also called floating-point notation or scientific notation. • Examples: • 865000000.0can be written as 8.65e8 • 0.000483can be written as 4.83e-4 • Floating-point numbers often are only approximations since they are stored with a finite number of bits. • Hence, 1.0/3.0= 0.33333333333333 is slightly less than 1/3.

18. Constants & Named Constants • Literal expressions like 2,3.14, or 'y' are called constants. • Constant numbers can be preceded by a + or –sign. • Java provides mechanism to: • Define a variable • Initialize it • Fix the value so it cannot be changed public static final Type Variable = Constant; • Example: public static final double PI = 3.14159;

19. Assignment Compatibilities • Java is said to be strongly typed. • You can't, for example, assign a floating point value to a variable declared to store an integer. • A value of one type can be assigned to a variable • of any type further to the right byte --> short --> int --> long --> float --> double • but not to a variable of any type further to the left. • You can also assign a value of type char to a variable of type int.

20. Type Casting • A type cast temporarily changes the value of a variable from the declared type to some other type. • For example: double distance; distance = 9.0; intpoints; points = (int)distance; • Illegal without (int). The value of pointsor (int)distanceis 9but the value of distance, both before & after the cast, is 9.0. • Any nonzero value to the right of the decimal point is truncated rather than rounded.

21. Arithmetic Operators • Arithmetic expressions can be formed by +, –, *, and /operatorswith variables or numbers, known as operands. • When both operands are of the same type, the result is of that type. • When one of the operands is a floating-point type and the other is an integer, the result is a floating-point type. • Example: if hoursWorked is an intwith a value of 40, and payRate is a double to which 8.25 has been assigned hoursWorked* payRate is a double with a value of 330.0.

22. Arithmetic Operations • Expressions with two or more operators can be viewed as a series of steps, each involving only two operands. • The result of one step produces one of the operands to be used in the next step: e.g., balance + (balance * rate) • In general, if one of operands is a floating-point type and the rest are integers, the result will be a floating-pointtype. • The result is the rightmost type from the following list that occurs in the expression. byte --> short --> int --> long --> float --> double

23. Division & Mod Operators • The division operator /behaves as expected if one of the operands is a floating-point type, i.e., the result is a FP type. • However, when both operands are integer types, the result is truncated, not rounded: • Hence, 99/100= 0.99has a value of 0. • The mod operator %is used with operators of integer type to obtain the remainder after integer division. • Example: 14 divided by 4 is 3 with a remainder of 2, i.e., 14 % 4is 2. • The mod operator has many uses, e.g., determining if an integer is odd or even.

24. Parentheses and Precedence • Parentheses can communicate the order in which arithmetic operations are performed, for examples: (cost + tax) * discount cost + (tax * discount) • Without parentheses, an expressions is evaluated according to the rules of precedence. It is negation like -2 It is subtraction like 4-2

25. Precedence Rules • Even when ( )are not needed, they can be used to make the code clearer, though * has a higher priority than +, using: balance + (interestRate * balance) rather than balance + interestRate * balance • When unary operators have equal precedence, the operator on the right acts before the operation(s) on the left. • When binary operators have equal precedence, the operator on the left acts before the operator(s) on the right. balance +interestRate – balance

27. Specialized Assignment Opt. • Assignment operators can be combined with arithmetic operators including –,+,*,/, and %,discussed later: amount = amount + 5; Also, can be written as amount += 5; • Increment and decrement operations, i.e., adding or subtracting by one: count++; ++count; count = count + 1; count--; --count; count = count – 1;

28. Special Case of Inc. and Dec. • After executing: int m = 4; int result = 3 * (++m) The result has a value of 15 and mhas a value of 5. • After executing: int m = 4; int result = 3 * (m++) The result has a value of 12 and mhas a value of 5.

29. Case Study • The Vending Machine Change: first we define the requirements of the program: • The user enters an amount between 1 cent and 99 cents. • The program determines a combination of coins equal to that amount. • For example, • 55cents can be 2 quarters and 1 nickel or • 87cents can be 3 quarters, 1 dime, 0 nickels and 2 pennies, i.e., (3 * 25) + (1 * 10) + (0 * 5) + (2 * 1) = 87

30. Case Study (Cont.) • The initial version of the algorithm: • First read the amount. • Now findmaximum number of quarters in the amount. • Then, subtractthe value of quarters from the amount. • Repeat steps 2 and 3for dimes, nickels, and pennies. • Finally, printthe amount and the quantities of each coin.

31. Case Study (Cont.) • The algorithm doesn't work properly • The original amount is changed by the intermediate computation and operations. • As a result, the original value of amount is lost. • Therefore, changethe list of variables: intamount, originalAmount; int quarters, dimes,nickles, pennies;

32. Case Study (Cont.) • The modified version of the algorithm: • First readthe amount. • Make a copy of the amount. • Now findthe maximum number of quarters in amount. • Then subtractthe value of quarters from the amount. • Repeat steps 3 and 4 for dimes, nickels, and pennies. • Finally, printthe amount and the quantities of each coin.