Set Lecture #3:
Conditional and Iterative
Structures
Control Structures
uninitialized variables
if branching
if / else branching
logical operators
nesting of control structures
proper indenting and spacing conventions
java identifier naming conventions
named constants
while loop
do-while loop
for loop
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
Control Flow and Conditionals
Control flow: the order in which statements
are executed
General rule: top to bottom
Several Control Structures that change that
Conditional statements: permit control flow to
be dependent on (true/false) conditions
if
if-else
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
if
and
1
if-else
The if and if-else statements should have the following form:
if (condition) {
statements;
}
tests the condition
if true statement is done; otherwise it is skipped
if (condition) {
statements1;
} else {
statements2;
}
tests the condition
if true, statements1 is done; otherwise statements2 is done
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
2
1
Java and White Space
You can add: carriage returns, spaces, tabs
wherever you want in Java
Properly used, this makes your program easier to read and understand
http://java.sun.com/docs/codeconv/html/CodeConv
TOC.doc.html
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
3
Logical Operators
Used for forming more complex conditions.
and
&&
if ( temp >= 97 && temp <= 99 ) {
System.out.println( Patient is healthy );
}
or
||
if ( months >= 3 || miles >= 3000 ) {
System.out.println( Change your oil );
}
not:
!
if ( ! phone.equals( 301-555-1212 ) ) {
System.out.println( Sorry, wrong number );
}
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
4
Blocks
W hat happens?
if (i > 10)
i = 10;
saturate = true;
Desired: both i, saturate are set only when i > 10
Actual: only the i=10 statement is dependant
Only one statement can be associated with if
The saturate assignment statement is not part of
the if
Blocks solve this problem
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
5
2
Blocks
W hat happens?
if (i > 10)
i = 10;
saturate = true;
else
k = 100;
Desired: both i, saturate are set only when i > 10
Actual: syntax error
Only one statement can be associated with if
The saturate assignment statement is not part of the if
The else cant find the if it belongs to
Blocks solve this problem also
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
6
What Blocks Are
Blocks are sequences of statements glued together into one
Form:
{
<statement 1>;
<statement 2>;
}
Example:
if (i > 10) {
i = 10;
saturate = true;
block
} else {
i = i+1;
}
block
if, if-else, {} are statement constructors
They take statement(s) and convert them into a new statement
Implications: if statements, etc. can also appear inside (be nested within) one another
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
7
Issues with if-else
Nested If/Elses can be Ugly and Confusing!
indent and block carefully
The Dangling Else Problem
Java rule: else is associated with innermost
possible if
Cascading Elses
W E WILL USE { } FOR ALL IF, IF-ELSE,
IF-ELSE-IF, STATEMENTS
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
8
3
In Projects
You must use meaningful variable names
it must tell the purpose of that variable what it is meant to hold
it can not have so much abbreviation that only you can read it
You must use Java convention indenting and brace placement
the indenting show the purpose in nesting
with braces in the Java determined places with respect to the
lines of code
Java convention of capitalization of identifiers
variables and methods start with lower case
classes and interfaces start with upper case
variables, methods, classes and interface use camelCase
constants are all uppercase with underscores between words
You must have Fully Blocked if statements and looping
structures
You must have all lines less than or equal to 80 columns of text
You must use "named constants" for any literal values that will
not change during program execution.
CMSC 131 Fall 2009
9
Jan Plane (adapted from Bonnie Dorr)
Named Constants
If same value should be used in several places, how to ensure consistency?
i.e. Check on temperature may be performed more than once
i.e. Same prompt might be printed in several places
final int MAX_OK_TEMP = 99;
Just like a regular variable declaration/initialization, except
Special term final
Necessity of initial value
Any valid variable name will work, convention but is to use all capitals
Difference from non-final variables: assignment attempt leads to error!
literals (= named values)
e.g.
if (temp >= 212 || temp <= 32)
if (temp >= BOILING || temp <= FREEZING)
e.g.
System.out.print (Enter integer: );
System.out.print (PROMPT);
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
10
Naming Rules and Conventions
W hat is legal for variable names?
Letters, digits, $, _
Cant start variable name with digit
Avoid reserved words
Avoid names starting or ending with $ or _
Use camelCase:
Variables & Methods use lower-case for first letter
Classes/Interfaces use upper-case for first letter
Naming Conventions: Standards developed over time.
Variables and methods: Start with lowercase, and use uppercase for each new word:
dataList2 myFavoriteM artian showM eT heM oney
Class names: Start with uppercase and uppercase for each new word:
String JOptionPane
M yFavoriteClass
Named constants (variables whose value never changes): All uppercase with underscores between words:
MAX_LENGTH
DAYS_PER_WEEK
BOILING_POINT
Make variable names not too long, not too short
Bad:
crtItm
Bad :
theCurrentItemBeingProcessed
Good :
currentItem
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
11
4
Meaningful Variable Names
Choose names for your variables to reflect their purpose
Bad
String string = ;
System.out.println (Enter name: );
string = sc.next();
if (string.equals (John Doe))
Good
String name = ;
System.out.println (Enter name: );
name = sc.next();
if (name.equals (John Doe))
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
12
Loops in Java
So far our programs execute every program
statement at most once
Often, we want to perform operations more than
once:
Sum all numbers from 1 to 10
Repeatedly prompt user for input
Loops allow statements to be executed multiple
times. Loop types in Java:
while
do-while
for
Call iteration
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
13
while and do-while Loops
while and do-while loops contain:
A statement, called the body
A boolean condition
Idea: the body is executed one more time as long as the condition is true
while-loop: The condition is tested before each body execution
while( condition ){
body
}
do-while-loop: The condition is tested after each body execution
do{
body
} while ( condition );
Main difference: do-while loop bodies always executed at least once
because it is bottom tested rather than top tested
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
14
5
Types of loops
indefinite iteration
usually tests something that is coming from
outside the loop structure (e.g. input)
needs to eventually change from true to false
counted iteration
something that is controlled inside the loop
to start at some value and count up or down until
some set ending point
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
15
for loop
for-loop: The counter is set, the condition is
tested before each body execution, the
update is performed at the end of each
iteration
for( initialization; condition; update){
body
}
Usually used for counted loops, but any of
the parts can be left empty.
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
16
Infinite Loops
Loops can run forever if condition never becomes
false
Be careful when programming loops!
Add statements for termination into loop body first
Often these statements are at end of body
e.g.
while (i <= 10) {
System.out.println(i);
i = i + 1;
}
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
17
6
Variables, Blocks and Scoping
Variables can be declared anywhere in a
Java program
W hen are the declarations active?
After they are executed
Only inside the block in which they are declared
Scope rules formalize which variable
declaration are active when
Global variables: scope is entire program
Local variables: scope is a block
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
18
Nested Loops
while, do-while are statement constructors
(like if and if-else: they use blocks)
Loops can thus be used inside other loops!
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
19
Nesting Example
public class NestedLoops {
public static void main(String[] args) {
int rowNumber = 1;
while (rowNumber < 10) {
int colNumber = 1;
while (colNumber < 10) {
System.out.print((rowNumber + colNumber) % 2);
colNumber = colNumber + 1;
}
System.out.println();
rowNumber = rowNumber + 1;
}
}
}
Inner loop
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
Outer loop
20
7