Basics of Java Programming variables assignment and input

Basics of Java Programming variables, assignment, and input CSC 1051 – Algorithms and Data Structures I Dr. Mary-Angela Papalaskari Department of Computing Sciences Villanova University Course website: www. csc. villanova. edu/~map/1051/f 13/ Some slides in this presentation are adapted from the slides accompanying Java Software Solutions by Lewis &Loftus CSC 1051 M. A. Papalaskari, Villanova University

Variables • A variable is a name for a location in memory • A variable must be declared by specifying the variable's name and the type of information that it will hold data type variable name int sum; double miles. Per. Gallon; String name, pet. Name; CSC 1051 M. A. Papalaskari, Villanova University

Java Primitive Data Types • integers: – byte, short, int, long • floating point numbers: www. toonpool. com/cartoons/PRIMITIVE%20 CAVEMAN%20 WORDS_25547# – float, double • characters: – char • Logical values (true/false): – boolean CSC 1051 M. A. Papalaskari, Villanova University

Assignment Statement • Changes the value of a variable • The assignment operator is the = sign total = 55 - discount; • The expression on the right is evaluated and the result is stored in the variable on the left CSC 1051 M. A. Papalaskari, Villanova University

Combined declaration and assignment A variable can be given an initial value in the declaration int age = 18; double x = 3. 2, y = -0. 80; String name = scan. next. Line(); CSC 1051 M. A. Papalaskari, Villanova University

Combined declaration and assignment A variable can be given an initial value in the declaration - a new value can be assigned later: int age = 18; double x = 3. 2, y = -0. 80; String name = scan. next. Line(); age = 19; x = x + 0. 5; name = scan. next. Line(); CSC 1051 M. A. Papalaskari, Villanova University

Combined declaration and assignment – CANNOT declare twice A variable can be given an initial value in the declaration - a new value can be assigned later: int age = 18; double x = 3. 2, y = -0. 80; String name = scan. next. Line(); int age = 19; Error: declaring variable again CSC 1051 M. A. Papalaskari, Villanova University

CONSTANTS: like variables, but value cannot change – declare using final modifier: final int INCHES_PER_FOOT = 12; final double LBS_PER_KG = 2. 2; Convention: Use UPPER_CASE identifiers CSC 1051 M. A. Papalaskari, Villanova University

Arithmetic Operators Addition Subtraction Multiplication Division Remainder + * / % • If either or both operands used by an arithmetic operator are floating point, then the result is a floating point CSC 1051 M. A. Papalaskari, Villanova University

Division and Remainder • If both operands are integers, the division result is an integer (the fractional part is discarded): 14 / 3 143 / 60 8 / 12 20 / 16 • % gives the remainder of the division: 14 % 3 8 % 12 143 % 60 20 % 16 CSC 1051 M. A. Papalaskari, Villanova University

Operator Precedence result = total + count / max - offset; Order of evaluation: 1. Multiplication, division, remainder 2. addition, subtraction, string concatenation – Operators with the same precedence: left right – Use parentheses to override default order CSC 1051 M. A. Papalaskari, Villanova University

Examples int x = 5; System. out. println ("Ans= " + (x + 1)); System. out. println ("Ans= " + x + 1); System. out. println ("Ans= " + (x + 1 * 3)); System. out. println ("Ans= " + ((x + 1) * 3)); System. out. println ("Ans= " + (x / 3 / 2)); System. out. println ("Ans= " + (x / (3 / 2))); CSC 1051 M. A. Papalaskari, Villanova University

More examples a + b + c + d + e a – b / c + d * e a / (b + c) - d % e a / (b * (c + (d - e))) CSC 1051 M. A. Papalaskari, Villanova University

Assignment Revisited • lower precedence than the arithmetic operators • evaluated right left First the expression on the right hand side of the = operator is evaluated Then the result is stored in the variable on the left hand side answer = 4 sum / 4 + MAX * lowest; 1 3 2 CSC 1051 M. A. Papalaskari, Villanova University

Assignment Revisited • The right and left hand sides of an assignment statement can contain the same variable First, one is added to the original value of count = count + 1; Then the result is stored back into count (overwriting the original value) CSC 1051 M. A. Papalaskari, Villanova University

Increment and Decrement • The increment operator (++) adds one to its operand • The decrement operator (--) subtracts one from its operand • The statement count++; is functionally equivalent to count = count + 1; CSC 1051 M. A. Papalaskari, Villanova University

Assignment operator • Assignment ( = ) copies the value of the right side into the memory location associated with the left side • It does not set up an ongoing equivalence int daves. Age = 21; int sues. Age = daves. Age; daves. Age = 22; System. out. println (daves. Age); // prints 22 System. out. println (sues. Age); // prints 21 CSC 1051 M. A. Papalaskari, Villanova University

Interactive Programs • The Scanner class has methods for reading input • We declare a Scanner object to read input from the keyboard: Scanner scan = new Scanner (System. in); Scanner object keyboard input Copyright © 2012 Pearson Education, Inc.

Reading Input • Once created, the Scanner object can be used to invoke various input methods, such as: answer = scan. next. Line(); Scanner object • The next. Line method reads all of the input until the end of the line is found Copyright © 2012 Pearson Education, Inc.

Reading Input • The Scanner class is part of the java. util class library, and must be imported into a program to be used • The import statement goes at beginning of your program (above class definition) import java. util. Scanner; (See Echo. java ) • The details of object creation and class libraries are discussed further in Chapter 3 Copyright © 2012 Pearson Education, Inc.

//********************************** // Echo. java Author: Lewis/Loftus // // Demonstrates the use of the next. Line method of the Scanner class // to read a string from the user. //********************************** import java. util. Scanner; public class Echo { //--------------------------------// Reads a character string from the user and prints it. //--------------------------------public static void main (String[] args) { String message; Scanner scan = new Scanner (System. in); System. out. println ("Enter a line of text: "); message = scan. next. Line(); System. out. println ("You entered: "" + message + """); } } Copyright © 2012 Pearson Education, Inc.

Sample Run //********************************** // Echo. java Author: Lewis/Loftus Enter a line of text: // // Demonstrates thefries use of the next. Line method of the Scanner class You want with that? // to read a string from the user. You entered: "You want fries with that? " //********************************** import java. util. Scanner; public class Echo { //--------------------------------// Reads a character string from the user and prints it. //--------------------------------public static void main (String[] args) { String message; Scanner scan = new Scanner (System. in); System. out. println ("Enter a line of text: "); message = scan. next. Line(); System. out. println ("You entered: "" + message + """); } } Copyright © 2012 Pearson Education, Inc.

Reading in numbers • next. Int reads in an integer: • Example: age = scan. next. Int(); • next. Double similar method for type double • White space (space, tab, new line) can be used to separate input tokens • next reads the next input token and returns it as a string • See Gas. Mileage. java Copyright © 2012 Pearson Education, Inc.

//********************************** // Gas. Mileage. java Author: Lewis/Loftus // // Demonstrates the use of the Scanner class to read numeric data. //********************************** import java. util. Scanner; public class Gas. Mileage { //--------------------------------// Calculates fuel efficiency based on values entered by the // user. //--------------------------------public static void main (String[] args) { int miles; double gallons, mpg; Scanner scan = new Scanner (System. in); continue Copyright © 2012 Pearson Education, Inc.

continue System. out. print ("Enter the number of miles: "); miles = scan. next. Int(); System. out. print ("Enter the gallons of fuel used: "); gallons = scan. next. Double(); mpg = miles / gallons; System. out. println ("Miles Per Gallon: " + mpg); } } Copyright © 2012 Pearson Education, Inc.

continue System. out. print ("Enter the number of miles: "); miles = scan. next. Int(); System. out. print ("Enter the gallons of fuel used: "); gallons = scan. next. Double(); mpg = miles / gallons; System. out. println ("Miles Per Gallon: " + mpg); } } Sample Run Enter the number of miles: 328 Enter the gallons of fuel used: 11. 2 Miles Per Gallon: 29. 28571429 Copyright © 2012 Pearson Education, Inc.

Homework • Read Sections 2. 1 – 2. 4 and 2. 6 – Always do all self-review exercises when you review material • Do Exercises EX 2. 2 – 2. 7 and 2. 11 CSC 1051 M. A. Papalaskari, Villanova University