More Things We Can Do With It! Overview. Circle Calculations. πr 2. π = More operators and expression types More statements

Transcription

1 More Things We Can Do With It! More operators and expression types More s 11 October 2007 Ariel Shamir 1 Overview Variables and declaration More operators and expressions String type and getting input Assignment Constants Selection 11 October 2007 Ariel Shamir 2 Circle Calculations 2πr r? π = πr 2 11 October 2007 Ariel Shamir 3 1

2 Outline of a Program class CircleCalc { public static void main(string[] args) { // my code should come here // as s 11 October 2007 Ariel Shamir 4 CircleCalc First Attempt class CircleCalc {??? public static void main(string[] args) { double r = 3.0; System.out.println(2.0 * * r); System.out.println( * * r); Can you see a problem? 11 October 2007 Ariel Shamir 5 Variables Usage One of the most important roles of programs is to deal with data. Data is most often put in variables for example: Numbers in input to be computed. A word written. A word read by the program. The date. 11 October 2007 Ariel Shamir 6 2

3 Variables A variable is an identifier that represents a storage to hold a particular type of data. Variables are actually a better name for an address in memory. count (integer) radius (real) October 2007 Ariel Shamir 7 Variable Types Variables must have a type. The type is the inner representation of the variable in the computer. The size of the memory allocated depends on the type. Type and size are very close terms in the computer world. 11 October 2007 Ariel Shamir 8 Variable Declaration Variables must be declared before they can be used (declaration ): Syntax: <Type> <variable_name> [,<name>] ; Examples: char c; int x1,x2,x3; double r; 11 October 2007 Ariel Shamir 9 3

4 Variables Memory Allocation Upon declaration memory of appropriate size is allocated. The address of the beginning of the piece of memory is associated with the variable name. short count; float radius; October 2007 Ariel Shamir 10 Variable Initialization Variables must be initialized before they can be used. Syntax: <variable_name> = <value>; Examples: x = 3; c = c ; myvalue = ; 11 October 2007 Ariel Shamir 11 Variable List Initialization A list of any size of variables of the same type can be initialized to the same value: Syntax: <variable1> = <variable2> = <variable3> = <value> ; Examples: x = y = my = account = degrees = 9 ; 11 October 2007 Ariel Shamir 12 4

5 Initialization During Declaration Both operations (declaration and initialization) can be performed in the same line: <type> Examples: int x=3; char c= c ; <variable_name> = <value>; float myvalue=1.0003f; 11 October 2007 Ariel Shamir 13 Variables Example class VariablesDeclerationExample { public static void main (String[] args) { short weeks = 14; int numberofstudents = 120; double averagefinalgrade = 78.6; System.out.println(weeks); System.out.println(numberOfStudents); System.out.println(averageFinalGrade); 11 October 2007 Ariel Shamir 14 CircleCalc Second Attempt class CircleCalc { public static void main(string[] args) { double r = 3.0; double circum, area; circum = 2 * * r; area = * r * r; System.out.println( Circumference: + circum); System.out.println( Area: + area); 11 October 2007 Ariel Shamir 15 5

6 Operators Java has 37 tokens that are operators. Java operators can be either: Unary operator - takes a single value Binary operator - takes two values Java binary operators are written in the infix notation: <operand1> <operator> <operand2> The operator is written between operands. 11 October 2007 Ariel Shamir 16 Operators Tokens + - * / % > < ==!= <= >= =? :! && ^ ~ & << >> >>> += -= *= /= %= &= = ^= <<= >>= >>>= 11 October 2007 Ariel Shamir 17 Expressions An expression can consist of a combination of operators and operands. Operands can be literal values, variables, or expressions by themselves (recursive). Types of expressions: Arithmetic expression: *4 Boolean expression: grade > 100 String expression: abc + d 11 October 2007 Ariel Shamir 18 6

7 Arithmetic Operators Binary: + addition x+y - subtraction x-y * multiplication x*y / division x/y % modulo remainder x%y Unary: - negation a = -34; 11 October 2007 Ariel Shamir 19 Operators Semantics The result of an arithmetic expression depends on the type of the operands: 3.0 / 2.0 == / 2.0 == / 2 == / 2 == 1-3 / 2 == October 2007 Ariel Shamir 20 Precedence The order in which operands are evaluated in expressions is determined by a welldefined precedence hierarchy. Operators at the same level of precedence are evaluated according to their associativity (right to left or left to right). Parentheses can be used to change precedence. 11 October 2007 Ariel Shamir 21 7

8 Precedence Example The usual rules apply: multiplication and division before addition and subtraction etc. and from left to right: / 3 * ( ) - 7 / October 2007 Ariel Shamir 22 More Examples Expression * 4 / 2 3 * (3 * 13) * (13 + 2) 4 * (11-6) * ( ) (5 * (4-1)) / 2 Result October 2007 Ariel Shamir 23 String Type String is the type of a line of text: I am a String. It is a first example of a reference data type (not a primitive type). It is widely used. 11 October 2007 Ariel Shamir 24 8

9 String Concatenation The + operator between strings has the meaning of String concatenation. When we apply + upon a String and a value of another type, that value is first converted into a String and the result is the concatenation of the two Strings. 11 October 2007 Ariel Shamir 25 Concatenation Example class AntarticaCode { public static void main(string[] args) { System.out.print( The international + code ); System.out.println( for Antarctica is + 762); Output: The international code for Antarctica is October 2007 Ariel Shamir 26 Getting Input You are already acquainted with one way of output: System.out.println(... ) How do we get input? read from keyboard? Just as System.out is an object that represents the standard output (screen), System.in is an object that represents the standard input (keyboard). We can read directly from System.in, however it is a bit complicated. An easier way: use the Scanner class. 11 October 2007 Ariel Shamir 27 9

10 The Scanner Class System.out.println( Enter number of items: ); Scanner sc = new Scanner(System.in); int numberofitems = sc.nextint(); You may use it to read any primitive data types by calling its methods: bollean b = sc.nextboolean(); float f = sc.nextfloat(); double d = sc.nextdouble(); To use the Scanner class, you must include the line: import java.util.scanner; at the head of your file. 11 October 2007 Ariel Shamir 28 CircleCalc Third Attempt class CircleCalc { public static void main(string[] args) { double r, circum, area; Scanner sc = new Scanner(System.in); System.out.println( Please enter radius: ); r = sc.nextint(); //or maybe nextdouble()? circum = 2 * * r; area = * r * r; System.out.println( Circumference: + circum); System.out.println( Area: + area); 11 October 2007 Ariel Shamir 29 Example - Adding Two Numbers // Requests two integers and prints their sum class AdditionExample { public static void main(string[] args) { int a,b; Scanner sc = new Scanner(System.in); System.out.println( Please enter first number: ); a = sc.nextint(); System.out.println( Please enter second number: ); b = sc.nextint(); int sum = a + b; System.out.println( The sum is + sum); 11 October 2007 Ariel Shamir 30 10

11 The Assignment Statement Stores a new value into a variable. The most common computer instruction. Syntax: <variable_name> = <expression>; Examples: x = 3+(2*7); c = c ; myvalue = / ; 11 October 2007 Ariel Shamir 31 Assignment Evaluation The assignment has a left side and a right side : <left_side> = <right_side>; The left side is usually a variable name and the right side some expression. The computer first evaluates the right side and only then assigns the value of the this evaluation to the left side. 11 October 2007 Ariel Shamir 32 Assignment Example class MyIncome { public static void main (String[] args) { Assignment Initialization int year = 2000; float monthincome = 10.4f; float bonus = 8.8f; float income = 12 * monthincome + bonus; System.out.println( in +year+ income was +income); year = 2001; bonus = 0; income = 12 * monthincome + bonus; System.out.println( in +year+ income was +income); 11 October 2007 Ariel Shamir 33 11

12 Blocks Block is the simplest type of composite. { // s It implements grouping of a sequence of s into a single. Block can be empty without s! 11 October 2007 Ariel Shamir 34 Block Scope { float k; // declaration k = (k+5)/32; // assignment (?) System.out.println(k); // call A variable k declared inside a block is completely inaccessible and invisible from outside that block. Such a variable is called local to the block. 11 October 2007 Ariel Shamir 35 Constants A constant is an identifier that is similar to a variable except that its value cannot be changed. You cannot assign to a constant! The final modifier is used for constant declaration: final <type> <Variable_name> = <value>; Example: final int MAX_GRADE=100; 11 October 2007 Ariel Shamir 36 12

13 CircleCalc Final Version! // Reads the radius of a circle and prints // its circumference and area class CircleCalc { static final double PI = ; public static void main(string[] args) { double r, circum, area; Scanner sc = new Scanner(System.in); system.out.println( Please enter radius: ); r = sc.nextint(); circum = 2 * PI * r; area = PI * r * r; System.out.println( Circumference: + circum); System.out.println( Area: + area); 11 October 2007 Ariel Shamir 37 Relational and Conditional Operators > greater than >= greater than or equal to < less than <= less than or equal to = = equal to! = not equal to 11 October 2007 Ariel Shamir 38 Boolean Expressions Expressions that use conditional operators are called Boolean expressions. Boolean expressions can have only two values: true False Examples: 5 >= == 1+7 5!= October 2007 Ariel Shamir 39 13

14 Boolean Operators Operator && ^! Use exp1 && exp2 exp1 exp2 exp1 ^ exp2!exp Returns true if exp1 and exp2 are both true, conditionally evaluates exp2 either exp1 or exp2 is true, conditionally evaluates exp2 if exp1 or exp2 is true but not both exp is false 11 October 2007 Ariel Shamir 40 Truth Tables or && and ^ xor T F T F T F T T T T T F T F T F T F F F F F T F 11 October 2007 Ariel Shamir 41 Bit-wise Operators Perform the operation on each bit Operator Use Evaluates ~ ~exp1 Flips all the bits of exp1 & exp1 & exp2 Always evaluates exp1 and exp2 exp1 exp2 Always evaluates exp1 and exp2 ~ = = & = October 2007 Ariel Shamir 42 14

15 Program Flow (Up Till Now) 11 October 2007 Ariel Shamir 43 Selection Statement selection? 11 October 2007 Ariel Shamir 44 If Statement Doing something under some condition: if (Boolean_condition) ; Boolean_condition is a Boolean expression that can have true or false values. If the value is true than is performed, otherwise (the value is false) is skipped. 11 October 2007 Ariel Shamir 45 15

16 If Flow Chart If? true false 11 October 2007 Ariel Shamir 46 Boolean Expression Example if ( x<1 x%2!=0 ) { System.out.println( x is not a positive even number! ); return; if ( y+2 < x &&!(y == 17) ) { System.out.println( y is not 17 and is smaller than x by more than 2! ); return; 11 October 2007 Ariel Shamir 47 If Statement Example class FailTest { public static void main(string[] args) { System.out.println( Enter your grade: ); Scanner sc = new Scanner(System.in); int grade = sc.nextint(); System.out.println( Your grade is: + grade); if (grade < 60) { System.out.println( You failed! ); 11 October 2007 Ariel Shamir 48 16

17 If.. Else Statement A choice between doing two things: if (Boolean_condition) 1; else 2; If the Boolean_condition is true, 1 is executed; if the condition is false, 2 is executed. 11 October 2007 Ariel Shamir 49 If..Else Flow Chart true If..else? false (else clause) 11 October 2007 Ariel Shamir 50 If.. Else Example class FailTest { public static void main(string[] args) { System.out.println( Enter your grade: ); Scanner sc = new Scanner(System.in); int grade = sc.nextint(); System.out.println( Your grade is: + grade); if (grade < 60){ else { System.out.println( You failed!! ); System.out.println( You passed!! ); 11 October 2007 Ariel Shamir 51 17

18 Nested If s if (a!= b) { if (a > b) { System.out.println(a + is greater ); else { System.out.println(b + is greater ); else { System.out.println( They are equal! ); 11 October 2007 Ariel Shamir 52 Always Use Blocks! if (grade >= 60) sum = sum + grade; avg = sum / NUM_STUDENTS; Problem?... So after thinking the programmer adds: if (grade >= 60) sum = sum + grade; count = count+1; avg = sum / count; 11 October 2007 Ariel Shamir 53 18