CS 46A - Lecture 3

Pre-class reading

• Sections 1.8, 2.1 - 2.3
• Did you take the quiz?

Algorithms

• Algorithm: A sequence of steps that is:
  • unambiguous
  • executable
  • terminating
• Algorithm for deciding which car to buy, based on total cost:

  For each car, compute the total cost as follows: 
  	    annual fuel consumed = annual miles driven / fuel efficiency 
  	    annual fuel cost = price per gallon x annual fuel consumed 
  	    operating cost = 10 x annual fuel cost 
  	    total cost = purchase price + operating cost 
  	    If total cost1 < total cost2 
  	    Choose car1 
  	    Else 
  	    Choose car2

Pseudocode

• Pseudocode: An informal description of an algorithm

• Describe how a value is set or changed:

  total cost = purchase price + operating cost

• Describe decisions and repetitions:

  For each car 
  	       operating cost = 10 x annual fuel cost 
  	       total cost = purchase price + operating cost

  Use indentation to indicate which statements should be selected or repeated

• Indicate results:

  Choose car1

Program Development Process

Types

• A type defines a set of values and the operations that can be carried out on the values
• Examples:
  • 13 has type int
  • "Hello, World" has type String
  • System.out has type PrintStream
• Java has separate types for integers and floating-point numbers
  • The double type denotes floating-point numbers
• A value such as 13 or 1.3 that occurs in a Java program is called a number literal

Number Literals

Number Types

• Number types are primitive types
  • Numbers are not objects
• Numbers can be combined by arithmetic operators such as +, -, and *

Lecture 3 Clicker Question 1

What is the type of the values 0, 0.0, and "0"?

1. int, double, string
2. int, float, string
3. primitive, primitive, object
4. None of the above

Variables

• Use a variable to store a value that you want to use at a later time
• A variable has a type, a name, and a value:

  String greeting = "Hello, World!";
  PrintStream printer = System.out;
  int width = 13;

• Variables can be used in place of the values that they store:

  printer.println(greeting); // Same as System.out.println("Hello, World!") 
  printer.println(width); // Same as System.out.println(20)

• It is an error to store a value whose type does not match the type of the variable:

  String greeting = 20; // ERROR: Types don’t match

Variable Declarations

Identifiers

• Identifier: name of a variable, method, or class
• Rules for identifiers in Java:
  • Can be made up of letters, digits, and the underscore (_) and dollar sign ($) characters
  • Cannot start with a digit
  • Cannot use other symbols such as ? or %
  • Spaces are not permitted inside identifiers
  • You cannot use reserved words such as public
  • They are case sensitive
• By convention, variable names start with a lowercase letter
  • Camel case: Capitalize the first letter of a word in a compound word such as farewellMessage
• By convention, class names start with an uppercase letter
• Do not use the $ symbol in names — it is intended for names that are automatically generated by tools

Syntax 2.1 Variable Declaration

Variable Names

Lecture 3 Clicker Question 2

How many of the following are legal identifiers?

Greeting1
g
void
101dalmatians
Hello, World
<greeting>

1. one of them
2. two of them
3. three of them
4. four of them

The Assignment Operator

• Assignment operator =
• Used to change the value of a variable

int width = 10;
      

The Assignment Operator

int width = 10;
width = 20;

Uninitialized Variables

• It is an error to use a variable that has never had a value assigned to it:

  int height; 
  	 width = height;   // ERROR—uninitialized variable height

  

• Remedy: assign a value to the variable before you use it:

  int height = 30; 
  	 width = height; // OK

• Even better, initialize the variable when you declare it:

  int height = 30; 
  	 int width = height; // OK

Syntax 2.2 Assignment

Assignment

• The right-hand side of the = symbol can be a mathematical expression:

  width = height + 10;

• Means compute the value of width + 10 and store that value in the variable width

Lecture 3 Clicker Question 3

What is the value of x and y after the following statements?

double x = 10;
double y = 2 * x;
x = 2 * y;
y = 2 * y;

1. x = 10, y = 20
2. x = 20, y = 40
3. x = 40, y = 40
4. x = 40,  y = 80

Lecture 3 Clicker Question 4

Suppose you have a variable

String greeting = "Hello, World!";

How do you change the value of the greeting variable to "Hello, Nina!"?

1. greeting = "Hello, Nina!";
2. String greeting = "Hello, World!";
3. greeting.changeTo("Hello, Nina!");
4. greeting.substring(6, 11) = "Nina";