CMPT 128: Introduction to
Computing Science for
Engineering Students
Introduction to branching
© Janice Regan, CMPT 128, Jan 2007
0
Control Structures
 Control structures are used to manage the
order in which statements in computer
programs will be executed
 Three different approaches
 In sequence
 Branching
 Looping
© Janice Regan, CMPT 128, 2007-2013
1
Control Structures
 Branch: Altering the flow of program
execution by making a selection or choice
 Loop: Altering the flow of program
execution by repetition of a particular
block of statement(s)
© Janice Regan, CMPT 128, 2007-2013
2
Branching
 One way selection
 Two way selection
 Multiple way selection
© Janice Regan, CMPT 128, 2007-2013
3
Flowcharts
 Flowcharts use some basic symbols

To start or end a function

To contain calculations

To make decisions

To connect different parts of
an algorithm
© Janice Regan, CMPT 128, 2007-2013
4
One-way selection
 Simplest form of a branch.
 Evaluate a condition that can be True or False
 If the condition is true a series of actions are
executed
 If the condition is false that series of actions are
not executed
 C and C++ implementation:
 if statement
© Janice Regan, CMPT 128, 2007-2013
5
Flowchart: one way selection

Write the condition that
needs to be satisfied in the
decision box (diamond).

Based upon the value of the
condition (boolean T or F)
choose what to do next
C and C++ implementation
if statement
condition
T
Statement 1;
⋮
Statement n;
F

The sequence of statements
to be executed if the
condition is true is placed in
the box
© Janice Regan, CMPT 128, 2007-2013
6
One-way selection
 Example if statement in C and C++:
setFlagOrderBoxes = 0;
if (numberOfBoxes < minimumBoxInventory)
setFlagOrderBoxes = 1;
setFlagOrderBags = 2; // always executed
 setFlagOrderBags is always set to 2, even if the condition
in the if statement is false
© Janice Regan, CMPT 128, 2007-2013
7
C++ Compound/Block Statement
 Only one statement following the if
statement is part of the if control structure
 What if we need more than one statement done
if the condition is true?
 Must use a block of statements (also called a
compound statement)
 C++ uses { }, to contain all the statements in
a block of statements
© Janice Regan, CMPT 128, 2007-2013
8
One way selection: sample
 Do a series of actions only if a given condition holds
 If the condition does not hold skip the actions
if (myScore > yourScore)
{
cout << “My score was lower than yours “;
difference = yourScore – myScore;
cout << “I only got “ << difference <<
“ less points than you did” ;
}
cout << “Thanks for studying with me”;
© Janice Regan, CMPT 128, 2007-2013
9
Two-way selection
 Evaluate a condition that can be True or False
 One “Set of things to do” if the condition is
true,
 A different “Set of things to do” if the condition
is false
 C and C++ implementation:
 if-else statement
© Janice Regan, CMPT 128, 2007-2013
10
Flowchart: two way selection



Based upon the value of
the condition (boolean T or
F) choose what to do next
The sequence of
statements to be executed
if the condition is true is
placed in the box at the
right
The sequence of
statements to be executed
if the condition is false is
placed in the box below the
condition
© Janice Regan, CMPT 128, 2007-2013
condition
F
T
Statement 1;
Statement
n;
⋮
Statement 1;
Statement
n;
⋮
Implemented in C and C++
as an if-else statement
11
Example of two-way selection
 Example if-else statement in C and C++:
if (examScore > 50)
myCourseGrade = “PASS”;
else
myCourseGrade = “FAIL”;
 NOTE: Only one statement follows the if, and
one statement follows the else
© Janice Regan, CMPT 128, 2007-2013
12
Two way selection in C++
 Complete one of two possible series of actions


First series of actions is complete if condition is true
Second series of actions is completed if condition is false

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;
}
© Janice Regan, CMPT 128, 2007-2013
13
Two way selection in C++
 CODING STANDARD: always use {} to make blocks
if (X<Y && Y<Z) //Never X<Y<Z
{

//Series of actions to be taken when the condition
is true
cout < < “Y is between X and Z” ;
}
else
{
// Series of actions to be taken when the condition is false
cout<< “ Y is not between X and Z”;
}
cout << endl << “Y is “ << Y;
cout << “ X is “ << X << “ Z Is “ << Z;
© Janice Regan, CMPT 128, 2007-2013
14
Truth Table &&
The && (And) operator
EXPRESSION1
EXPRESSION2
T
T
F
F
© Janice Regan, CMPT 128, 2007-2013
T
F
T
F
EXPRESSION1 &&
EXPRESSION2
T
F
F
F
15
Multiple-way selection
 If condition1 is true the 1st series of actions is completed
 If condition1 is false and condition2 is true the 2nd series
of actions is completed
 If condition1 and condition2 are false and condition3 is
true the 3rd series of actions is completed
 …
 If all conditions are false the final series of actions is
completed
 Implemented in C++ as an if-elseif-else statement
© Janice Regan, CMPT 128, 2007-2013
16
Flowchart for multiple selection
Statement 1;
T
condition
F
condition2
T
⋮
Statement n;
Statement 1;
⋮
Statement n;
F
Statement 1;
⋮
Statement n;
© Janice Regan, CMPT 128, 2007-2013
17
Example of Multiple selection
 Example:
if (examScore > 80)
myCourseGrade = “A”;
else if (examScore > 70)
myCourseGrade = “B ”;
else
myCourseGrade = “FAIL”;
© Janice Regan, CMPT 128, 2007-2013
18
Example of Multiple selection
 Example: always use { } to create blocks
if ( X > 80 || Y < 100)
{
cout << “ (X >80 , Y any value) OR (X any value, Y<100)” ;
}
else if (X < 80)
{
cout << “X < 80 and Y >= 100” ;
}
else
{
cout <“X =80 Y >= 100”;
}
© Janice Regan, CMPT 128, 2007-2013
19
Truth Tables ||
The || (Inclusive Or) operator
EXPRESSION1
EXPRESSION2
T
T
F
F
© Janice Regan, CMPT 128, 2007-2013
T
F
T
F
EXPRESSION1 ||
EXPRESSION2
T
T
T
F
20
Selection: decision statements
Each decision statement contains a condition
 The condition is an expression with a logical
value (true or false)
 The condition is a Boolean expression
 A relational expression (a type of logical
expression)
 Another type of logical expression
 A Boolean variable or constant
© Janice Regan, CMPT 128, 2007-2013
21
Binary Relational Operators C, C++




<
<=
>
>=
less than
less than or equal to
greater than
greater than or equal to
Binary Equality Operators in C, C++


==
!=
equal to
not equal to
 Evaluated left to right
© Janice Regan, CMPT 128, 2007-2013
22
Relational Expressions
 A type of logical expression
 Combines two numbers, strings, characters
to give a value of true or false
 A simple relational expression is
 Two numerical values combined using a
binary relational operator
 A more complex relational expression is
 A combination of simple relational
expressions
© Janice Regan, CMPT 128, 2007-2013
23
Other logical expressions
Other types of logical expression include
 Two logical variables / constants combined
with a binary logical operator
 One logical variable or constant
 One logical variable or constant operated on
by a unary logical operator
 Two relational expressions combined with a
binary logical operator
 …
 Any more complex logical expression
© Janice Regan, CMPT 128, 2007-2013
24
Binary Logical Operators C, C++


&&
||
Logical AND
Logical OR
Unary Logical Operators C, C++

!
Not
 Evaluated left to right
 Arguments of logical operators have values of
true or false
© Janice Regan, CMPT 128, 2007-2013
25