1. CSC 103
Lecture 7 & 8
Introduction to Computers and Programming

2. The Decision Control
2
 We need to alter our actions in the face of changing
circumstances
 If the weather is fine then I will go for a walk
 If the highway is busy I would take a diversion
 If the pitch takes spin, we would win the match
 If I am feeling well then I will watch a movie
 You can notice that all these decisions depend on
some condition being met
 C language too must be able to perform different sets
of actions depending on the circumstances

3. 3
 Decision control instruction can be implemented in C
using:
(a) The if statement
(b)The if-else statement
(c)The conditional operators

4. The if Statement
4
 C uses the keyword if to implement the decision
control instruction
 Syntax is :
if ( expression )
execute this statement ;
 The keyword if tells the compiler that what follows is a
decision control instruction
 The expression following the keyword if is always
enclosed within a pair of parentheses
 if the expression is true then the statement is
executed otherwise not

5. 5
 In general, the condition is expressed using C’s ‘Relational
Operator’
 Following are different relational operators:
 != is inequality or not equals to operator
 == is used for comparison of two quantities
 = is an assignment operator

6. Example
6
/* Demonstration of if statement */
main( )
{
int num ;
printf ( "Enter a number less than 10 " ) ;
scanf ( "%d", &num ) ;
if ( num <= 10 )
printf ( “The number is less than 10!" ) ;
}

7. 7
 The general form of if statement is:
if (expression)
statement;
 Here the expression can be any valid expression including a
relational expression
 We can use arithmetic expressions in the if statement
e.g.
if (3+5-2)
printf(“ This works”);
if (a=10)
printf(“ Even this works”);
if(-5)
printf (“And even this works”);

8. if-else Statement
8
 Statements are executed when if condition is true
 We also want to execute statements when condition is
false
if ( condition )
do this ;
else
do this ;
if block
else block

9. Some if-else rules in C
9
 else block should come immediately after if block
 When if and else blocks contain only one statement
 then we can drop pair of braces i.e. { }
 Example:
 Take two values from user and determine which one is
smaller and which one is greater.
 Solution:
 First we draw a flow chart

10. Making decisions in C
 Sometimes your programs need to make logical
choices.
 Example:
IF score is higher than 50
THEN grade is "pass"
ELSE grade is "fail"
10

11. Making decisions in C
 Example:
IF score is higher than 50
THEN grade is PASS
ELSE grade is FAIL
 In C, this corresponds to one statement with 3 parts:
if (score > 50) {
grade = PASS;
}
else {
grade = FAIL;
}
11

12. Making decisions in C
 Part 1 : the condition
An expression that evaluates to TRUE or FALSE
if (score > 50)
{
grade = PASS;
}
else
{
grade = FAIL;
}
12

13. Making decisions in C
 Part 2 : the TRUE part
A block of statements that are executed if the condition
has evaluated to TRUE
if (score > 50)
{
grade = PASS;
}
else
{
grade = FAIL;
}
True part
13

14. Making decisions in C
 Part 3 : the FALSE part
A block of statements that are executed if the condition
has evaluated to FALSE
if (score > 50)
{
grade = PASS;
}
else
{
grade = FAIL;
}
if the condition
evaluates to FALSE,
the true part is skipped.
False part
14

15. Making decisions in C
 If the true part (or false part) consists of only one
statement, then the curly braces may be omitted.
 Example: these two statements are equivalent:
if (score > 50)
{
grade = PASS;
}
else
{
grade = FAIL;
}
if (score > 50)
grade = PASS;
else
grade = FAIL;
15

16. Making decisions in C
 Sometimes there are more than two parts. In those cases you
may use cascading (a.k.a. nested) if/else statements:
if (score > 90)
lettergrade = 'A';
else if (score > 75)
lettergrade = 'B';
else if (score > 60)
lettergrade = 'C';
else if (score > 50)
lettergrade = 'D';
else
lettergrade = 'F';
16

17. Making decisions in C
 if(condition)
statement;
 if (condition) {
statements;
statements; …
}
 if (condition) {
statements;
statements;…
}
else {
statements;
statements;...
}
3 forms of if statements;
(note indenting)
Note condition is
always in parentheses,
All TRUE parts and
all FALSE parts are
a single statement
or a single block
of statements
{}17

18. Some Valid if Statements in C
18
if ( 3 + 2 % 5 )
printf ( "This works" ) ;
if ( a = 10 )
printf ( "Even this works" ) ;
if ( -5 )
printf ( "Surprisingly even this works" ) ;
“Condition is false only if expression evaluates to 0”

19. Multiple Statements within if (Flowchart)
19

20. Multiple Statements within if (C Language)
20
/* Calculation of bonus */
main( )
{
int bonus, cy, yoj, yr_of_ser ;
printf ( "Enter current year and year of joining " ) ;
scanf ( "%d %d", &cy, &yoj ) ;
yr_of_ser = cy - yoj ;
if ( yr_of_ser > 3 )
{
bonus = 2500 ;
printf ( "Bonus = Rs. %d", bonus ) ;
}
}

21. 21
#include<stdio.h>
#include<conio.h>
main()
{
int a,b;
printf("Enter 1st Number: ");
scanf("%d", &a);
printf("Enter 2nd Number: ");
scanf("%d", &b);
if( a > b )
printf("%d is greater than %d", a, b);
else
printf("%d is greater than %d", b, a);
getch();
}

22. Some tests
22
Test
No.
Output Remarks
01
Enter 1st Number: 15
Enter 2nd Number: 25
25 is greater than 15
2nd number is greater than 1st number
02
Enter 1st Number: 35
Enter 2nd Number: 20
35 is greater than 20
1st number is greater than 2nd number
03
Enter 1st Number: 300
Enter 2nd Number: 300
300 is greater than 300
Both numbers are equal…..Logical Error!

23. nested if-else
23
if ( condition )
do this ;
else
{
if ( condition )
do this ;
else
do this ;
}
“Modify our flow chart to solve logical error”

24. 24
#include<stdio.h>
#include<conio.h>
main()
{
int a,b;
printf("Enter 1st Number: ");
scanf("%d", &a);
printf("Enter 2nd Number: ");
scanf("%d", &b);
if( a > b )
printf("%d is greater than %d", a, b);
else {
if( a == b )
printf("%d is equal to %d", a, b);
else
printf("%d is greater than %d", b, a);
}
getch();
}

25. else if Statement
25
 Every else is associated with its previous if
 The last else goes to work only if all the conditions fail
 In else if the last else is optional
 The else if clause is nothing different, it’s just a way of
rearranging the else with the if that follows it
 e.g.

26. Use of Logical Operators
26
 C allows usage of three logical operators
 &&, ||, !
 These are to be read as AND, OR, NOT respectively
 && and || are composed of double symbols
 Don’t use the single symbols & , |
 These single symbols also have meaning. These are
bitwise operators
 The && and || allow two or more conditions to be
combined in an if statement

27. 27
Write a program to calculate the division obtained
by a student. The marks obtained by the student
in 5 different subjects are input through the
keyboard
 There are two ways to solve this problem

28. 28
/* Method – I */
main( )
{
int m1, m2, m3, m4, m5, per ;
printf ( "Enter marks in five subjects " ) ;
scanf ( "%d %d %d %d %d", &m1, &m2, &m3, &m4, &m5 ) ;
per = ( m1 + m2 + m3 + m4 + m5 ) / 5 ;
if ( per >= 60 )
printf ( "First division n") ;
else
{
if ( per >= 50 )
printf ( "Second divisionn" ) ;
else
{
if ( per >= 40 )
printf ( "Third divisionn" ) ;
else
printf ( "Failn" ) ;
}
}
}

29. 29
 This program uses nested if-else
 Three disadvantages of this method
 As the number of conditions go on increasing the level of
indentation also goes on increasing. As a result the
whole program creeps to the right.
 Care needs to be exercised to match the corresponding
ifs and elses.
 Care needs to be exercised to match the corresponding
pair of braces.
 All these three problems can be eliminated by usage
of ‘Logical operators’

30. 30
/* Method – II */
main( )
{
int m1, m2, m3, m4, m5, per ;
printf ( "Enter marks in five subjects " ) ;
scanf ( "%d %d %d %d %d", &m1, &m2, &m3, &m4, &m5 ) ;
per = ( m1 + m2 + m3 + m4 + m5 ) / 5 ;
if ( per >= 60 )
printf ( "First divisionn" ) ;
if ( ( per >= 50 ) && ( per < 60 ) )
printf ( "Second divisionn" ) ;
if ( ( per >= 40 ) && ( per < 50 ) )
printf ( "Third divisionn" ) ;
if ( per < 40 )
printf ( "Failn" ) ;
}

31. 31
 In the second if statement, the && operator is used to
combine two conditions
 ‘Second division’ gets printed if both conditions are
true
 Two advantages of this method
 The matching of the ifs with their corresponding elses
gets avoided
 In spite of using several conditions, the program doesn’t
expand to the right

32. Solution Using else if
32
/* Use of else if */
main( )
{
int m1, m2, m3, m4, m5, per ;
printf ( "Enter marks in five subjects " ) ;
scanf ( "%d %d %d %d %d", &m1, &m2, &m3, &m4, &m5 ) ;
per = ( m1+ m2 + m3 + m4+ m5 ) / 5 ;
if ( per >= 60 )
printf ( "First divisionn" ) ;
else if ( per >= 50 )
printf ( "Second divisionn" ) ;
else if ( per >= 40 )
printf ( "Third divisionn" ) ;
else
printf ( "failn" ) ;
}

33. Problem!
33
 Find smallest and greatest number among three input
numbers?

34. The ! (NOT) Operator
34
 The third logical operator is ! (NOT operator)
 This operator reverse the result of an expression
 If the result of an expression is TRUE, the ! operator will
make it FALSE and vice versa
 e.g.
 !(y<10) it means that “not y less than 10”
 if y is less than 10, the expression will be FALSE
 the same condition can be written as (y>=10)
 The NOT operator is often used to reverse the logical
value of a single variable
 if (!flag) is same as if(flag==0)

35. Hierarchy of Operators Revisited
35
 Since we have added the logical operators, so it’s time to
review the priorities of these operators
 The higher the position of an operator in the table, the
higher is its priority

36. Summary of Logical Operators
36
 Following figure summarizes the working of logical
operators
X 1

37. Logical Operators (Exercise)
37
 A company insures its drivers in the following cases:
 If the driver is married.
 If the driver is unmarried, male & above 30 years of age.
 If the driver is unmarried, female & above 25 years of
age.
 In all other cases the driver is not insured. If the
marital status, sex and age of the driver are the inputs,
write a program to determine whether the driver is to
be insured or not.

38. 38
// One way to solve the problem
main( )
{
char sex, ms ;
int age ;
printf ( "Enter age, sex, marital status " ) ;
scanf ( "%d %c %c", &age, &sex, &ms ) ;
if ( ms == 'M' )
printf ( "Driver is insured" ) ;
else
{
if ( sex == 'M' )
{
if ( age > 30 )
printf ( "Driver is insured" ) ;
else
printf ( "Driver is not insured" ) ;
}
else
{
if ( age > 25 )
printf ( "Driver is insured" ) ;
else
printf ( "Driver is not insured" ) ;
}
}
}

39. Solution by using Logical Operators
39
main( )
{
char sex, ms ;
int age ;
printf ( "Enter age, sex, marital status " ) ;
scanf ( "%d %c %c“, &age, &sex, &ms ) ;
if ( ( ms == 'M') || ( ms == 'U' && sex == 'M' && age > 30 ) ||
( ms == 'U' && sex == 'F' && age > 25 ) )
printf ( "Driver is insured" ) ;
else
printf ( "Driver is not insured" ) ;
}

40. The Conditional Operators
40
 The conditional operators are ? and :
 Sometimes called Ternary Operators since they take three
arguments
 The general form is:
expression 1 ? expression 2 :
expression 3
 It means that if expression 1 is TRUE then the value
returned will be expression 2 otherwise the value
returned will be expression 3
 e.g.
int x,y;
scanf (“%d”, &x);
y=(x>5 ? 3 : 4)
 This statement will store 3 in y if x is greater than 5
otherwise it will store 4 in y

41. 41
 Another example
int x,y;
scanf("%d", &x);
y = (x >= 10 && x <= 50 ? 1 : 0);
 Here if x>=10 && x<=50 evaluates to TRUE then y=1
otherwise y=0

42. The following points may be noted about conditional operators
42
 It’s not necessary that the conditional operators should be used only in
arithmetic statements
int i ;
scanf ( "%d", &i ) ;
i == 1 ? printf ("You have entered 1" ) : printf (
"Entered value is other than 1" ) ;
 The conditional operators can be nested as shown below:
int big, a, b, c ;
big = ( a > b ? ( a > c ? 3: 4 ) : ( b > c ? 6: 8 ) ) ;

43. 43
 Check out the following conditional expression:
a > b ? g = a : g = b ;
 This will give you an error ‘Lvalue Required’. To remove the error
just enclose the statement in : part within a pair of parenthesis
a > b ? g = a : ( g = b ) ;
 In absence of parentheses the compiler believes that b is
being assigned to the result of the expression to the left
of second =.
 The limitation of the conditional operators is that after
the ? or after the : only one C statement can occur

44. What is the Output?
44
#include<stdio.h>
#include<conio.h>
main()
{
int a, b, g;
a=10;b=5;
a > b ? g = a : (g = b) ;
g == 10 ? printf ( "true" ) : printf ( "false" );
getch();
}

45. Some Rules
45
 The default scope of the if statement is only the next
statement.
 So, to execute more than one statement they must be written
in a pair of braces.
 An if block need not always be associated with an else
block.
 However, an else block is always associated with an if
statement.
 && and || are binary operators, whereas, ! is a unary
operator
 In C every test expression is evaluated in terms of zero and
non-zero values.
 A zero value is considered to be false and a non-zero value is
considered to be true.
 Assignment statements used with conditional operators
must be enclosed within a pair of parenthesis.

46. Output of program!
46
main( )
{
int a = 300, b, c ;
if ( a >= 400 )
b = 300 ;
c = 200 ;
printf ( "n%d %d", b, c ) ;
}
main( )
{
int x = 10, y = 20 ;
if ( x == y ) ;
printf ( "n%d %d", x, y ) ;
}

47. 47
main( )
{
int i = 4, j = -1, k = 0, y, z ;
y = i + 5 && j + 1 || k + 2 ;
z = i + 5 || j + 1 && k + 2 ;
printf ( "ny = %d z = %d", y, z ) ;
}