Department of Computer Science
1
Many times we encounter situations where we
may change the order of execution of statements
based on certain conditions, until certain
conditions are met and involves a kind of decision
making to see whether a particular condition has
been met or not , in this context C language
provides decision making capabilities with the help
of following statements:
1.If statement
2. Switch statement
3. Conditional operator statement
4. Goto statement
Since these statements control the flow / order of execution they are
also referred to as control statements/ and popularly known as decision
making statements
2
Entry
False
Test
Expression
True
3
 Determines
whether a statement or block is
executed
 Implements the selection instructions within
an algorithm
 Decides what to do by evaluating a Boolean
expression
 If the expression is true (non-zero), the
statement or block is executed
if ( expression )
statement
4
 Statements
are lines of instructions in the
programs ending with a semicolon (;)
 A compound statement or block is a
series of statements surrounded by bracts
{
number = number + 1;
printf("%d\n", number);
}
An empty statement is a single
semicolon
5
Example: oddnum.c
Read in a number, and
print it if it is odd
output “Enter an integer”
input number
if (number is odd)
then
{
output the number
}
6
Example: oddnum.c
Read in a number, and
print it if it is odd
#include <stdio.h>
/* Read in a number, and echo it
if it is odd */
int main()
{
output “Enter an integer”
input number
if (number is odd)
then
{
output the number
}
return 0;
}
7
Example: oddnum.c
Read in a number, and
print it if it is odd
#include <stdio.h>
/* Read in a number, and echo it
if it is odd */
int main()
{
int number;
output “Enter an integer”
input number
printf("Enter an integer: ");
scanf("%d", &number);
if (number is odd)
then
{
output the number
}
return 0;
}
8
Example: oddnum.c
Read in a number, and
print it if it is odd
#include <stdio.h>
/* Read in a number, and echo it
if it is odd. */
int main()
{
int number;
output “Enter an integer”
input number
printf("Enter an integer: ");
scanf("%d", &number);
if (number is odd)
then
{
output the number
}
if (number % 2 != 0)
{
printf("%d\n", number);
}
return 0;
}
9
Example: oddnum.c
Read in a number, and
print it if it is odd
#include <stdio.h>
/* Read in a number, and echo it
if it is odd. */
int main()
{
int number;
output “Enter an integer”
input number Do not put
printf("Enter an integer: ");
scanf("%d", &number);
“then” here!
if (number is odd)
then
{
output the number
}
if (number % 2 != 0)
{
printf("%d\n", number);
}
return 0;
}
10
Example: oddnum.c
Read in a number, and
print it if it is odd
#include <stdio.h>
/* Read in a number, and echo it
if it is odd. */
int main()
{
int number;
output “Enter an integer”
input number
printf("Enter an integer: ");
scanf("%d", &number);
if (number is odd)
then
{
output the number
}
if (number % 2 != 0)
{
printf("%d\n", number);
}
Do not put
semicolonreturn
here!
0;
}
11
Example: oddnum.c
Read in a number, and
print it if it is odd
#include <stdio.h>
/* Read in a number, and echo it
if it is odd. */
int main()
{
int number;
output “Enter an integer”
input number
printf("Enter an integer: ");
scanf("%d", &number);
if (number is odd)
then
{
output the number
}
if (number % 2 != 0)
{
printf("%d\n", number);
}
return 0;
}
12
Common mistake
if (number % 2 != 0);
{
printf ("%d is an odd ", number);
}
printf ("number\n");
13
Common mistake
if (number % 2 != 0);
{
printf("%d is an odd ", number);
}
printf("number\n");
No semicolon here!
The semicolon is an empty statement
14
Common mistake
if (number = 0)
{
printf ("%d\n", number);
}
printf ("%d\n", number);
15
Common mistake
if (number = 0)
{
printf("%d\n", number);
}
printf("%d\n", number);
Should be ==
16
Syntax:
If (test expression)
{
Statement – block;
}
Test
Expression
?
True
Statement Block
False
Statement X
Next Statement
Statement –x;
17
Syntax:
If (test expression)
{
True block Statement(s);
}
else
{
False block Statement(s);
}
Statement -x
True
Test
Expression
?
True- block
Statement
False
False- Block
Statement
Statement- X
18
Example: oddeven.c
Read in a number, and
determine if it’s odd or
even
#include <stdio.h>
/* Determine whether an input number
is odd or even. */
main()
{
int number;
output “Enter an integer”
input number
printf("Enter an integer: ");
scanf("%d", &number);
if (number is odd)
then
{
output: number “ is an
odd number”
}
else
{
output: number “ is an
even number”
}
if (number % 2 != 0)
{
printf("%d is an odd number\n",
number);
}
}
19
Example: oddeven.c
Read in a number, and
determine if it’s odd or
even
#include <stdio.h>
/* Determine whether an input number
is odd or even. */
main()
{
int number;
output “Enter an integer”
input number
printf("Enter an integer: ");
scanf("%d", &number);
if (number is odd)
then
{
output: number “ is an
odd number”
}
else
{
output: number “ is an
even number”
}
if (number % 2 != 0)
{
printf("%d is an odd number\n",
number);
}
else
{
printf("%d is an even number\n",
number);
}
}
20
Example: oddeven.c
Read in a number, and
determine if it’s odd or
even
#include <stdio.h>
/* Determine whether an input number
is odd or even. */
main()
{
int number;
printf("Enter an integer: ");
scanf("%d", &number);
output “Enter an
Nointeger”
input number
semicolons
if (number ishere!
odd)
then
if (number % 2 != 0)
{
printf("%d is an odd number\n",
number);
}
else
{
printf("%d is an even number\n",
number);
}
{
output: number “ is an odd
number”
}
else
{
output: number “ is an even
number”
}
}
21
Example: oddeven.c
Read in a number, and
determine if it’s odd or
even
#include <stdio.h>
/* Determine whether an input number
is odd or even. */
main()
{
int number;
printf("Enter an integer: ");
scanf("%d", &number);
output “Enter an integer”
input number
if (number % 2 != 0)
{
printf("%d is an odd number\n",
number);
}
else
{
printf("%d is an even number\n",
number);
}
if (number is odd)
then
{
output: number “ is an odd
number”
}
else
{
output: number “ is an even
number”
}
}
22
Entry
False
Test
Expression
?
True
False
Statement -3
Statement -2
Test
Expression
?
True
Statement -1
Statement- X
Next Statement
23
Test
Expression
?
Test
Expression
?
Statement -1
Statement -2
Test
Expression
?
Statement -3
Test
Expression
?
Statement -n
Statement -x
Next
Statement
Default
statement
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Syntax:
switch (expression)
{
case value-1: block-1
break;
case value-2: block-2
break;
case value-3: block-3
break;
…………………….
……………………..
Default: default block
break;
}
Statement- x;
switch
Expression
Expression = value 1
block 1
Expression = value 2
block 2
(no match) default
default
block
Statement X
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