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CS100
Lecture 2
1
Today’s Topics
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Brief review of yesterday
Variables and constants
Declaration of a variable
Assignment to a variable
Declaration of a constant
The conditional (if) statement
Input
CS100
Lecture 2
2
Review
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Algorithms
Methods
Method call/method invocation
Importance of comments -- document your
code
• See the course website for information on
Java programming style
• Any questions on homework yet?
CS100
Lecture 2
3
Introduction to Variables
• Think of a variable as a box into which a
value can be placed
32
• Declarations:
int x;
int timeOfDay
CS100
x
-532
timeOfDay
1115
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What Happens in a Declaration?
• A variable is a name for a location in
memory
• When a variable is declared you instruct the
compiler to reserve enough space for the
type of variable you’re declaring
• A variable can only store one value of its
declared type
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Lecture 2
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Variable Names -- Identifiers
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Begin with a letter, _, or $
Contain letters, digits, _, and $
Generally, we don’t use _ or $
One convention: first letter small, capitalize
words within the identifier
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–
CS100
timeOfDay
f12
f0
restOfInput
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Assignment
• int timeOfDay = 3;
// timeOfDay
3
• timeOfDay = 430;
// timeOfDay
430
• timeOfDay = 30 + timeOfDay + 5;
// timeOfDay = 465
465
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Lecture 2
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Assignment Syntax
• <variable> = <expression> ;
• Note well: “=” does not mean equality in
Java
• Think of x = y as “Assign y to x” or “x
becomes y” or “x gets the value of y”
• Think of x == y as “x equals y”
• The expression “x==y” yields true if x and
y contain the same value, false otherwise
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Lecture 2
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Booleans
• Boolean is a data type like int
• It has two values: true and false
boolean answer;
int x;
int y;
x = 3;
y = 5;
answer = (x == y);
CS100
What value ends up in the
variable “answer” ?
Lecture 2
9
Some history
= was introduced as a sign for equality in the 1500s. It was
chosen because nothing could be more equal than 2 parallel
lines.
The use of = for assignment was started in Fortran in the
early 1950s. C and C++ continued this and introduced ==
for equality. This has caused great confusion and much
wasted time on the part of programmers the world over.
Algol 60 used := for assignment and = for equality.
Remember: concepts, not syntax, but be careful of the
syntax!
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Lecture 2
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Constants
• If data doesn’t change throughout program,
use constant
• Helpful to name this value
final int CS100_STUDENTS = 78;
final double PI = 3.14159;
• Use uppercase to distinguish from variables,
whose values do change throughout the
program
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Lecture 2
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Naming Conventions
• Short variable names make programs
shorter, and more manageable
• Long names can convey more meaning
• Name can almost never give full meaning,
so comment when the variable is declared.
• Avoid names such as:
“thisIsTheVariableThatStoresTheNumberOfBooksInMyLibrary”
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Conditional Statement -- If
• Conditional statements allows a choice of a
command based on some condition
// Store the maximum of x and y in z
z = x;
if (y > x)
z = y;
• Syntax:
if (boolean expression)
statement
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Lecture 2
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Block statements in if statements
• Suppose you would like to execute more
than one thing based on the condition?
// if x != y, store 0 in x, store y in z
if (y != x)
{ x = 0; z = y; }
OR
// if x != y, store 0 in x, store y in z
if (y != x) {
x = 0;
z = y;
}
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Lecture 2
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More on block statements
• Consider:
{ <statement 1>
<statement 2>
<statement 3>
…. }
• { and } are used to delimit a sequence of
statements that are to act together, just as (
and ) are used in arithmetic expressions
• Many options for positioning { and }, just
be consistent
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Lecture 2
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Second Form of if statement
• Provide multiple options:
// Store maximum of x and y in z
if (x >= y)
{ z = x; }
else
{ z = y; }
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Lecture 2
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Example of Nested Ifs
if (coin == HEADS)
if (choice == RECEIVE)
System.out.println(“You won, will receive”);
else
System.out.println(“You won, will kickoff”);
else
System.out.println(“You lost.”);
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Lecture 2
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Operator Syntax
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= = equal to
! = not equal to
< less than
> greater than
< = less than or equal to
> = greater than or equal to
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Lecture 2
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Operator Precedence
• * / % then + - + then =
• 14 + 8 / 2 is therefore 18 (not 11)
• Use parentheses to make things clear:
– (14 + 8) / 2 = 11
– Must be an equal number of left and right
parens
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Lecture 2
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Input (briefly)
• Classes that provide facilities for input and
export are available in package java.io
• Place the phrase
import java.io.*
at the top of your Java source file
• Ok if you don’t understand all of this yet
• Read Section 3.3 in text for details
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Lecture 2
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Characters and Strings (briefly)
• char date type
– ASCII plus stuff = UNICODE
– A character literal uses single quotes: ‘b’
– char firstChar = ‘b’
• String is not a primitive type, it’s a Class
• Strings represented as objects of the String
class
• String manipulation can get complicated -see text for more details
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Lecture 2
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Getting Input from user
Import java.io.*;
// Read a string from the user
String message;
message = stdin.readLine();
// Read numeric input from the user
String string1; int num1;
string1 = stdin.readLine();
num1 = Integer.parseInt(string1);
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Example
system.out.println(“What’s your name?”);
name = stdin.readLine();
system.out.println(“Your name is: ” + name);
What is your name?
Milly Lunor
Your name is: Milly Lunor
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Lecture 2
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Small Formatting Tricks
• Java uses the \ in output statements to indicate
formatting
– \t tab, \n newline, \” double quote, \’ single quote, \\
backslash
– “Name\tDOB” results in
Name DOB
– “foo\n\nbar” results in
foo
bar
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Lecture 2
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Discussion Issues
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Java is strongly-typed. What good is that?
Why use constants?
Why would you use (a > 0) vs. (a >= 1)
Is Y2K a compile-time, run-time or logical
error?
• If a system fails, who is responsible?
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Lecture 2
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