Chapter 4
Control Structures I
Objectives
► Examine
relational and logical operators
► Explore how to form and evaluate logical
(Boolean) expressions
► Learn how to use the selection control structures
if, if…else, and switch in a program
Control Structures
► Three methods of
 In sequence
 Branching
 Looping
► Branch: Altering
processing a program
the flow of program execution
by making a selection or choice
► Loop: Altering the flow of program execution by
repetition of statement(s)
Flowcharts
► Flowcharts
use some basic symbols
►
To contain calculations
►
To make decisions
►
To connect different parts of an
algorithm
Selection
► One-Way
Selection
► Two-Way Selection
► Compound (Block of) Statements
► Multiple Selections (Nested if)
► Conditional Operator
► switch Structures
Decisions and Sequence Structures
►
When an action is to be taken only if a particular
condition holds a decision statement is used.
► The condition which must hold may be logical or
relational. The value of the condition must be boolean
► Each possible path through a condition statement will
contain a sequence of steps to be executed
► The condition and the sequences of steps that are
executed for each outcome of the condition form a
selection structure.
► A selection structure is a type of control structure
One way selection
► Expression
referred to as decision maker
► Statement referred to as action statement
if ( condition )
{
// Series of actions to be taken
// when the condition is TRUE
action 1;
action 2;
}
Flowchart for a one way selection
► A simple
decision uses a decision box to hold the
condition
► The sequence of statements is held in a sequence
box of boxes
condition
T
Statement 1:
Statement n:
F
Two way selection
if (condition)
{
// Series of actions to be taken when the condition is TRUE
action 1;


action n;
}
else
{
// Series of actions to be taken when the condition is FALSE
action 1;

action n;
}
Flowchart for a two way selection
► A selection
structure uses a decision box, and
sequence boxes. There may be multiple
sequence boxes along each path
condition
F
Statement 1:
Statement n:
T
Statement 1:
Statement n:
Multiple Selections (Nested if)
if (condition)
{ // Series of actions to be taken when the condition is TRUE
action 1;
action n;
}
else
{
if (condition 2)
{ // Series of actions to be taken when condition is FALSE and condition2 is TRUE
action 1;
action n;
}
else
{ // Series of actions to be taken when condition and condition 2 are FALSE
action 1;
action n;
}
}
Multiple Selections (else if)
if (condition)
{
// Series of actions to be taken when the condition is TRUE
action 1;
action n;
}
else if (condition 2)
{
// Series of actions to be taken when condition is FALSE and condition2 is TRUE
action 1;
action n;
}
else
{
// Series of actions to be taken when condition and condition 2 are FALSE
action 1;
action n;
}
Flowchart for a decision
Statement 1:
T
condition
F
condition2
F
Statement 1:
Statement n:
T
Statement n:
Statement 1:
Statement n:
The switch statement
► An
alternative method to if statements with
multiple else clauses
► The controlling expression (selector) must
have an integral type
► Case labels are particular values of the
controlling expression
► Break statements exit from the switch structure
switch Structures
switch(controling expression)
{
case value1: statements1
break;
case value2: statements2
break;
...
case valuen: statementsn
break;
default: statements
}
switch Statement
Defining the condition
► Each
decision statement is based upon a
condition
► The condition is a statement with a logical value
(boolean true or false), and is commonly referred
to as a logical expression
► A logical expression may consist of boolean
operands and a logical operation
Binary Logical Operators
►
►
►
►
►
&
|
^
&&
||
AND
OR
Exclusive OR
Short Circuit AND
Short Circuit OR
Unary Logical Operators
►
!
Not
AND and OR Operators
The & , && (And) operator
The | , || (Or) operator
Logical (Boolean) Operators
The ^ (XOR) operator
false
true
true
false
The ! (not) Operator
^
Short-Circuit Evaluation
► Definition: a
process in which the computer
evaluates a logical expression from left to right
and stops as soon as the value of the expression
is known
Relational Operators
► Relational
Operator
 Allows you to make comparisons in a program
 Binary operator
► Condition is
represented by a logical expression
► A Logical expression has a value of either true or
false
Relational Operators and Primitive
Data Types
► Can
be used with integral and floating-point data
types
► Can be used with the char data type
► A relational expression consists of a relational
operator and two integral, floating point, or
character variables
► A relational expression is a type of logical
expression with a logical (boolean) value of true
or false
Binary Relational Operators in Java
►
►
►
►
<
<=
>
>=
less than
less than or equal to
greater than
greater than or equal to
Binary Equality Operators in Java
►
►
==
!=
equal to
not equal to
Relational Operators and the
Unicode Collating Sequence
Comparing Strings
► class
String
 Method compareTo
 Method equals
► Given
string str1 and str2
integer < 0 if str1 < str2
Str1.compareTo(str2) = { 0 if str1 = str2
integer > 0 if str1 > str2
Assignment operators
►A =
B
assign value of expression B to
variable A, store result in A
► A &= B add the value of expression B to
variable A, store result in A
► A |= B
subtract the value of expression B
from variable A, store result in A
► A ^= B
multiply the value of expression B
by variable A, store result in A