Lab # 5: Relational Operators, Logical Operators and Decisions
Lab # 5: Relational Operators, Logical
Operators and Decisions
EC-102 – Computer Systems and Programming
Usama Wajhi
School of Mechanical and Manufacturing Engineering (SMME),
National University of Sciences and Technology (NUST)
October 19, 2016
Lab # 5: Relational Operators, Logical Operators and Decisions
Outline
1 Relational Operators
Importance
Definition
Examples
Relational Operators in C++
2 Logical Operators
Importance
Definition
Logical Operators in C++
AND Operator
OR Operator
NOT Operator
Examples
3 Decision Making in C++
Introduction
Decision Making in C++
The if Statement
The if...else Statement
Lab # 5: Relational Operators, Logical Operators and Decisions
Relational Operators – Why do we need them?
Most programs decide what to do in
response to changing circumstances
The flow of control jumps from one
part of the program to another in
response to such events
Program statements that cause such
jumps are called control statements
e.g. decisions and loops
How many times a loop is executed or
whether a decision results in the
execution of a section of code depends
on whether certain expressions turn
out to be true or false
Lab # 5: Relational Operators, Logical Operators and Decisions
Relational Operators – What are they?
A relational operator compares two values
The comparison involves such relationships as equal-to,
lesser-than, and greater-than
The result of the comparison is true or false
Lab # 5: Relational Operators, Logical Operators and Decisions
Relational Operators – Examples
Example # 1
1 // this program demonstrates relational operators in a
comparison of int , float and char constants
2 # include < iostream >
3 using namespace std ;
4 int main () {
5 cout << (10 > 20) << endl ; // false
6 cout << (10 < 20) << endl ; // true
7 cout << (20 == 20) << endl ; // true
8
9 cout << (20.5 > 20.0) << endl ; // true
10 cout << (20.5 == 2.5) << endl ; // false
11
12 cout << ( ’a ’ == ’a ’) << endl ; // true
13 cout << ( ’a ’ > ’b ’) << endl ; // false
14 return 0;
15 }
Lab # 5: Relational Operators, Logical Operators and Decisions
Relational Operators – Examples
Example # 2
1 // relational operators in a comparison of int
variables
2 # include < iostream >
3 using namespace std ;
4 int main () {
5 int jane = 44; // assignment statement
6 int harry = 12;
7
8 cout << ( jane == harry ) << endl ;
9 cout << ( harry <= 12) << endl ;
10 cout << ( jane > harry ) << endl ;
11 cout << ( jane >= 44) << endl ;
12 cout << ( harry != 12) << endl ;
13 cout << (7 < harry ) << endl ;
14 return 0;
15 }
Lab # 5: Relational Operators, Logical Operators and Decisions
Relational Operators in C++
Here’s a complete list of C++ relational operators,
Operator Meaning
> Greater than
< Lesser than
== Equal to
!= Not equal to
>= Greater than or equal to
<= Lesser than or equal to
Lab # 5: Relational Operators, Logical Operators and Decisions
Logical Operators – Why do we need them?
While relational operators can be
used to test whether a particular
condition is true or false, they can
only test one condition at a time
Often we need to know whether
multiple conditions are true at once
Other times, we need to know
whether any one of the multiple
conditions is true
Lab # 5: Relational Operators, Logical Operators and Decisions
Logical Operators – What are they?
A relational operator is used to combine two Boolean
expressions
For example, to check whether a number x entered by the
user satisfies the expression 20 < x < 30, we would need to
logically connect both the expressions (x > 20) and (x <
30) and see if there combination yields true or false
The logical connection in this case is the word AND
The result of logical operation is either true or false
Lab # 5: Relational Operators, Logical Operators and Decisions
Logical Operators in C++
Here’s a complete list of C++ logical operators,
Operator Meaning
&& AND
|| OR
! NOT
Lab # 5: Relational Operators, Logical Operators and Decisions
Logical AND Operator (&&)
Expression 1 Expression 2 Expression 1 && Expression 2
false false false
false true false
true false false
true true true
Lab # 5: Relational Operators, Logical Operators and Decisions
Logical OR Operator (||)
Expression 1 Expression 2 Expression 1 || Expression 2
false false false
false true true
true false true
true true true
Lab # 5: Relational Operators, Logical Operators and Decisions
Logical NOT Operator (!)
Logical NOT operator is a unary operator
It can be used to reverse the meaning of a Boolean expression
Expression !Expression
false true
true false
Lab # 5: Relational Operators, Logical Operators and Decisions
Logical Operators – Example
1 // this program demonstrates logical operators
2 # include < iostream >
3 using namespace std ;
4 int main ()
5 {
6 int jane = 44;
7 int harry = 12;
8
9 cout << ( jane == harry && harry <= 12) << endl ;
10 cout << ( jane == harry || harry <= 12) << endl ;
11 cout << !( jane == harry ) << endl ;
12
13 cout << ( jane > harry && jane >= 44) << endl ;
14 cout << ( jane > harry || jane >= 44) << endl ;
15 cout << !( jane > harry || jane >= 44) << endl ;
16 return 0;
17 }
Lab # 5: Relational Operators, Logical Operators and Decisions
Decision Making
Decision making is about deciding the order of execution of
statements based on certain conditions
These statements require the programmer to specify:
One or more expressions to be evaluated or tested by the
program along with
One or more statements to be executed if the condition turns
out to be true, and optionally
One or more statements to be executed if the expression turns
out to be false
Lab # 5: Relational Operators, Logical Operators and Decisions
Decision Making in C++
Decisions can be made in C++ in many ways. The most
important is with the if...else statement. This statement
can also be used without the else, as a simple if statement.
Another decision statement, switch, creates branches for the
multiple alternative sections of code, depending on the value
of a single variable
Lab # 5: Relational Operators, Logical Operators and Decisions
The if Statement
Flowchart
Algorithm
1 Start
2 Declare variable x
3 Read value of x
4 If x is greater than 100 then display
“The number is greater than 100”
5 Stop
Lab # 5: Relational Operators, Logical Operators and Decisions
The if Statement
Code
1 // this program demonstrates IF statement
2 # include < iostream >
3 using namespace std ;
4 int main ()
5 {
6 int x ;
7
8 cout << " Enter a number : " ;
9 cin >> x ;
10
11 if ( x > 100)
12 {
13 cout << " That number is greater than 100\ n " ;
14 }
15
16 return 0;
17 }
Lab # 5: Relational Operators, Logical Operators and Decisions
The if...else Statement
Flowchart
Algorithm
1 Start
2 Declare variable x
3 Read value of x
4 If x is greater than 100 then
display “The number is
greater than 100”
5 Else, display “The number is
not greater than 100”
6 Stop
Lab # 5: Relational Operators, Logical Operators and Decisions
The if...else Statement
Code
1 // this program demonstrates IF statement
2 # include < iostream >
3 using namespace std ;
4 int main ()
5 {
6 int x ;
7 cout << " Enter a number : " ;
8 cin >> x ;
9
10 if ( x > 100)
11 {
12 cout << " That number is greater than 100\ n " ;
13 }
14 else
15 {
16 cout << " The number is not greater than 100\ n "
;
