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Chapter 5: Conditionals
and loops
Conditionals and Loops


Now we will examine programming statements that allow us to:

make decisions

repeat processing steps in a loop
Chapter 5 focuses on:

boolean expressions

conditional statements

comparing data

repetition statements
2
Outline
Altering flow of control
Boolean expressions
Conditional Statements
The while Statement
Other Repetition Statements
Normal flow of control


Flow of control

The order in which statements are executed

Execution normally proceeds in a linear fashion

JAVA application begins with the first line of the main method

And proceeds step by step to the end of the main method
Some programming statements allow us to

decide or not to execute a particular statement

execute a statement over and over, repetitively
Conditional statements


A conditional statement

lets us choose which statement will be executed next

is called sometimes selection statement
The Java conditional statement are the

if statement

if-else statement

switch statement
The if statement: syntax

The if statement has the following syntax
if is a Java
reserved word
The condition must be a
boolean expression. It must
evaluate to either true or false.
if ( condition )
statement;
If the condition is true, the statement is executed.
If it is false, the statement is skipped.

Example

if (count > 20)
System.out.println(“count exceeded”);
Altering the flow of control:
loop

Loops

allow to execute programs over and over again

based on a boolean expression


That determines how many times the statement is executed
include

while, do, and for statements

Each type has unique characteristics
Conditional expressions

All conditionals and loops

are based on conditional expressions

called Boolean expressions

Use
 Equality operators

Relational operators

Logical operators
Outline
Altering flow of control
Boolean expressions
Conditional Statements
The while Statement
Other Repetition Statements
Equality operators

The == and != are called equality operators

== tests whether two values are equal

!= tests whether two values are not equal
If (total != sum)
System.out.println(“total does not equal sum”);
Relational operators


They let us decide relative ordering between values

Less than (<)

Greater than (>)

Less than or equal (<=)

Greater than or equal (>=)
Arithmetic operations have higher precedence
Logical operators

Java has three logical operators
! Logical NOT
&& Logical AND
|| Logical OR

They all take Boolean operands

And produce Boolean results

Logical NOT is unary operator

Logical AND and OR are binary operators
Logical operators (cont’d)
logical
operator
Description
Example
Result
!
Logical Not
!a
True if a is false
and False if a is true
&&
Logical AND
a && b
True if a and b are both
true and false
otherwise
||
Logical OR
a || b
True if a or b are true
and false otherwise
Logical operators: truth table

A truth table

Shows all possible true-false combinations of terms

Since && and || each have 2 operands

There are four possible combinations of conditions a and b
a
b
a && b
a || b
true
true
true
true
true
false
false
true
false
true
false
true
false
false
false
false
Boolean expressions

Consider the example

if (total < MAX && !found)
System.out.println(“Completed!.”);

Under what condition would the println executed?
total <
MAX
found
!found
total < MAX &&
!found
false
false
true
false
false
true
false
false
true
false
true
true
true
true
false
false
Outline
Altering flow of control
Boolean expressions
Conditional Statements
The while Statement
Other Repetition Statements
The if Statement

If statement consists of

The reserved word if followed by

a Boolean expression enclosed in parentheses

followed by a statement

If (total > amount)
total = total + amount;
Condition
evaluated
true

See Age.java
statement
false
The if-else statement

Sometimes,

we want to do one thing if a condition is true,

and another thing if not

We can add an else to an if to handle this situation


if (height <= MAX)
adjustment = 0;
else
adjustment = MAX – height;
See Wages.java
The if Statement

If statement consists of

The reserved word if followed by

a Boolean expression enclosed in parentheses

followed by a statement

If (total > amount)
total = total + amount;
Condition
evaluated
true

See Age.java
statement
false
The if-else statement

Sometimes,

we want to do one thing if a condition is true,

and another thing if not

We can add an else to an if to handle this situation


if (height <= MAX)
adjustment = 0;
else
adjustment = MAX – height;
See Wages.java
Logic of an if-else statement
condition
evaluated
true
false
statement1
statement2
Using block statements

To do more than one thing as a result

Of boolean condition evaluation


Replace any single statement with a block of statement
A block of statement

is a collection of statements enclosed in braces

If (guess == answer)
System.out.println (“you got it! Good guessing!”);
else {
System.out.println (“that is not correct, sorry.”);
System.out.println (“the number was ” + answer);
}
Block statements

In an if-else statement

The if portion, or the else or both

Could be block statements
if (total > MAX)
{
System.out.println ("Error!!");
errorCount++;
}
else
{
System.out.println ("Total: " + total);
current = total*2;
}
Conditional operator

Its syntax is
condition ? expression1 : expression2


If the condition is true,

expression1 is evaluated

If it is false => expression2 is evaluated
The value of the entire conditional operator

Is the value of the selected expression
Conditional operator (cont’d)

Conditional operator

is similar to an if-else statement

is a ternary operator requiring three operands

uses the symbol ? :, which are always separated

((total >MAX) ? total+1 : total * 2

total = ((total >MAX) ? total+1 : total * 2
Conditional operator (cont’d)

total = (total > MAX) ? total+1 : total * 2

is equivalent to

if (total > Max)
total = total + 1
else
total = total * 2
The Conditional Operator:
Example

Another example:
System.out.println ("Your change is " +
count + ((count == 1) ? "Dime" :
"Dimes"));

If count equals 1,


then "Dime" is printed
If count is anything other than 1,

then "Dimes" is printed
27
Block Statements

Several statements can

be grouped together


into block statement delimited by braces
A block statement can

be used wherever a statement is called for
if (total > MAX)
{
System.out.println ("Error!!");
errorCount++;
}
28
Nested if statements

The statement executed as a result of if

Could be another if statement

This is called a nested if


if (code == ‘R’)
if (height <= 20)
System.out.println(“Situation Normal”);
else
System.out.println (“Bravo”);
is the else matched

to the inner if statement or the outer if statement?
Nested if statements (cont’d)

else clause
 is matched to the closest unmatched if that preceded it


in the previous example, else is matched to
if(height<=20)
To avoid confusion, braces
 can be used to specify if statement to which an else belongs

if (code == ‘R’) {
if (height <= 20)
System.out.println (“Situation Normal”);
}
else
System.out.println (“Bravo”);
MinOfThree.Java

import java.util.Scanner;
public class MinOfThree {
public static void main(String [] args) {
int num1, num2, num3, min = 0;
Scanner scan = new Scanner (System.in);
System.out.println(“Enter three integers: ”);
num1 = scan.nextInt();
num2 = scan.nextInt();
num3 = scan.nextInt();
if (num1 < num2)
if (num1 < num3)
min = num1;
else
min = num3;
else
if (num2<num3)
min = num2;
else
min = num3;
System.out.println(“Minimum value: “+ min);
}
}
Comparing data
Comparing Data

When comparing data using boolean expressions


understand the nuances of certain data types
Let's examine some key situations:

Comparing floating point values for equality

Comparing characters

Comparing strings (alphabetical order)

Comparing object vs. comparing object references
Comparing Float Values

You should rarely use the equality operator (==)


Two floating point values are equal


if the underlying binary representations match exactly
Computations often result


when comparing two floating point values
in slight differences that may be irrelevant
In many situations, you might consider

two floating point numbers to be "close enough"

even if they aren't exactly equal
Comparing Float Values

To determine the equality of two floats,

you may want to use the following technique:
if (Math.abs(f1 - f2) < TOLERANCE)
System.out.println ("Essentially equal");

If the difference between two floating point values

is less than the tolerance,


they are considered to be equal
The tolerance could be set to

any appropriate level, such as 0.000001
Comparing Characters

Java character data is based on


Unicode establishes a particular numeric value


for each character, and therefore an ordering
We can use relational operators


the Unicode character set
on character data based on this ordering
For example,

the character '+' is less than the character 'J'

because it comes before it in the Unicode character set
Comparing Characters

In Unicode, the digit characters (0-9)


are contiguous and in order
the uppercase letters (A-Z) & lowercase letters (a-z)

are contiguous and in order
Characters
Unicode Values
0–9
48 through 57
A–Z
65 through 90
a–z
97 through 122
Comparing Strings

Remember that in


Java a character string is an object
The equals method can be called

with strings to determine if


two strings contain the same characters in the same order
The equals method returns a boolean result
if (name1.equals(name2))
System.out.println ("Same name");
Comparing Strings

We cannot use the relational operators


to compare strings
The String class contains

a method called compareTo

to determine if one string comes before another
Comparing Strings

A call to name1.compareTo(name2)

returns zero if name1 and name2

are equal (contain the same characters)

returns a negative value if name1 is less than name2

returns a positive value if name1 is greater than name2
Comparing Strings
if (name1.compareTo(name2) < 0)
System.out.println (name1 + "comes first");
else
if (name1.compareTo(name2) == 0)
System.out.println ("Same name");
else
System.out.println (name2 + "comes first");

Because comparing characters and

strings is based on a character set,

it is called a lexicographic ordering
Lexicographic Ordering

Lexicographic ordering is not strictly alphabetical


when uppercase and lowercase characters are mixed
For example,

the string "Great" comes before the string "fantastic"

because all of the uppercase letters


Also, short strings come before longer strings


come before all of the lowercase letters in Unicode
with the same prefix (lexicographically)
Therefore "book" comes before "bookcase"
Comparing Objects

The == operator can be applied to objects

it returns true



if the two references are aliases of each other
The equals method is defined for all objects,

but unless we redefine it when we write a class,

it has the same semantics as the == operator
It has been redefined in the String class

to compare the characters in the two strings
The switch statement

The switch statement

Provides another way to decide which statement


To execute next
Evaluates an expression that attempts to match

The result of one of several possible cases

Each case contains a value and a list of statements

The flow of control transfer to the statement
 Associated with the first case value that matches
Switch statement: syntax

The general syntax of switch statement is
switch
and
case
are
reserved
words
switch ( expression )
{
case value1 :
statement-list1
case value2 :
statement-list2
case value3 :
statement-list3
case ...
}
If expression
matches value2,
control jumps
to here
The switch Statement

Often a break statement


A break statement


Transfers control to end of the switch statement
If a break statement


is used as last statement in each case's statement list
is not used, flow of control will continue to the next case
Sometimes this may be appropriate, but often

we want to execute only statements associated with one case
The switch Statement

An example of a switch statement:
switch (option)
{
case 'A':
aCount++;
break;
case 'B':
bCount++;
break;
case 'C':
cCount++;
break;
}
The switch Statement

A switch statement


can have an optional default case
The default case has no

associated value and simply uses


the reserved word default
If the default case is present,

control will transfer to it if no other case value matches
Switch example


switch(idchar) {
case ‘A’:
acount = acount+1;
break;
case ‘B’:
bcount=bcount+1;
break;
default:
System.out.println(“Error identifying
Character”);
}
Type of expression evaluated by switch

Char, byte, short, or int
Use of a switch statement

Sample program

A comment is printed according to a user’s grade



Grade = 100 => a perfect score; Grade = 90s
(Excellent)..etc
Algorithm

Ask user to enter a grade

Based on grade value, print the right comment
See GradeReport.java
Outline
Altering flow of control
Boolean expressions
Conditional Statements
The while Statement
Other Repetition Statements