19.Need of conditional execution and Control structures

 

Need of conditional execution:

A C program executes procedurally statement after statement in a pre-expected order.

In order to develop real-time applications, we need to conditionally execute the program.

 

Explanation through example:

If we want to accept any number and want to print whether the number is a zero or a non-zero, then a procedural program alone can’t solve.

 

Example :

#include<stdio.h>
void main()
{
int n;
printf("Enter any number:");
scanf("%d",&n);
printf("Zero");
printf("\n Non-zero");
}

Execution 1:

Enter any number: 12

Zero

Non-zero

 

Execution 2:

Enter any number: 0

Zero

Non-zero

 

Example explained:

The above example gives the same output for any input because program is not conditionally executing.

When the input is 12 then printf("Zero"); must be skipped and printf("Non-zero"); must be executed.

When the input is 0 then printf("Zero"); must be executed and printf("Non-zero"); must be skipped.

It requires conditional execution.

C language provides number of control structures to promote conditional execution.

 

 

Control structures/ Control Statement:

Control structure or control statement is a statement that controls the program execution.

Control structures in C language are classified into.

 

 

 

Decision making control structures are used to skip or execute a statement or a block of statements.

Looping or iterative control structures are used to execute a statement or a block of statements repeatedly until a condition is false.

 

Need of condition:

Both the decision and looping control structures controls the execution of statements according to the condition.

A condition is a relational or logical expression, the result of which is always either true (1) or false (0).

 

Relational operators:

 

Operator	Meaning
<	Less than
>	Greater than
< =	Less than or equal to
> =	Greater than or equal to
= =	Equal to
! =	Not Equal to

  

Note:

In C language ‘=’ (assigning operator) is used to assign a value or result of an expression to the variable.

‘= =" is used to compare any two values, the result of which is either true (1) or false (0).

 

 

Relational expression:

Combination of operands, relational operators and constants is called a relational expression.

Any relational expression returns (results) either true or false.

In C language true is represented with 1 and false is represented with 0.

 

Example:

#include<stdio.h>
void main()
{
int a,b,c,d,e,f;
a=10<20 b="40">80;
c=20<=30;
d=48>=80;
e=20==30;
f=20!=30;
printf("a=%d \n b=%d \n c=%d \n d=%d \n e=%d \n f=%d",a,b,c,d,e,f);
}

Output:

a=1

b=0

c=1

d=0

e=0

f=1

 

Example explained:

In C language any relational expression returns either true (1) or false (0).

 

if conditional control structure:

It is a basic and fundamental control structure used to execute or skip a statement or block of statements.

According to the syntax

The condition (relational expression) is placed within ( ).

If the condition is true, then the statement or block of statements under if conditional statement is executed, otherwise skipped from the execution.

Multiple statements can be controlled by placing within a block { }

 

 

 

 

 

 

 

 

 

 

 

 

 

Example :

#include<stdio.h>
void main()
{
printf("Zero");

if(10<<20 if="" none="" printf="">40)
   printf("\n one");
if(10>40)
   printf("\n Two");
if(1)
   printf("\n three");
if(0)
   printf("\n Four");

printf("five");

<20 if="" none="" printf="">}

Output:

zero

One

 three

 five

Example explained:

printf("Zero"); is executed as a normal statement.

printf("\nOne"); is executed because 10<40 returns true (1)

printf("\nTwo"); is skipped from execution because 10>40 returns false (0)

printf("\nThree"); is executed because 1 is considered as true

printf("\nFour"); is executed not executed because 0 is considered as false

printf("\nFive"); is executed as a normal statement.

 

Let us practice some examples:

Specification 1:

Accept a number and print whether the number is a positive, negative or zero.

 

Program :

#include<stdio.h>
void main()
{
int n;
printf("Enter any number:");
scanf("%d",&n);
if(n==0)
   printf("Zero");
if(n>0)
   printf("Positive number");
if(n<0)
  printf("negitive number");

Execution 1 :

Enter any number: 25

Positive number

Execution 2 :

Enter any number: -40

Negative number

Execution 3 :

Enter any number: 0

Zero

Example explained:

At any given case only a single condition will be true, left all the conditions are false.

 

 

Specification 2 :

Accept any number and print whether the given number is an even or odd.

 

Program :

#include<stdio.h>
void main()
{
int n;
printf("Enter any number:");
scanf("%d",&n);
if(n%2==0)
  printf("Even number");
if(n%2==1)
  printf("Odd number");
}

Execution 1 :

Enter any number: 25

Odd number

Execution 2 :

Enter any number: 24

Even number

 

 

Specification 3 :

Accept two numbers and print whether equal, if not then print the biggest number.

 

Program :

#include<stdio.h>
void main()
{
int a,b;
printf("Enter two numbers:");
scanf("%d%d",&a,&b);
if(a==b)
   printf("Equals");
if(a>b)
   printf("Biggest number %d",a);
if(b>a)
   printf("Biggest number %d",b);
}

Execution 1 :

Enter two numbers: 10  10

Equals

 

Execution 2 :

Enter two numbers: 40  80

Biggest number 80

 

Execution 3:

Enter two numbers: 80 20

Biggest number 80

 

Specification 4:

Accept the price, quantity of a product and print the bill, discount and the net bill. Allow 5% discount if bill exceeds to 500 else allow 3% discount.

 

Program :

#include<stdio.h>
void main()
{
int qty;
float price,bill,dis,net_bill;
printf("Price:");
scanf("%f",&price);
printf("Quantity:");
scanf("%d",&qty);
bill=price*qty;
if(bill>500)
    dis=bill*5/100;
if(bill<=500)
    dis=bill*3/100;
net_bill=bill-dis;
printf("Bill %f",bill);
printf("\n Discount %f",dis);
printf("\n Net bill %f",net_bill);
}

Execution 1:

Price: 20

Quantity: 5

Bill 100

Discount 3

Net bill 97

 

Execution 2:

Price: 100

Quantity 10

Bill 1000

Discount 50

Net bill 950

 

Specification 5:

Accept any digit from 0 to 6 and print appropriate day. For example Sunday for 0, Monday for 1… Saturday for 6

 

Program:

#include<stdio.h>
void main()
{
int n;
printf("Enter any digit from 0 to 6:");
scanf("%d",&n);
if(n==0)
   printf("Sunday");
if(n==1)
   printf("Monday");
if(n==2)
   printf("Tuesday");
if(n==3)
   printf("Wednesday");
if(n==4)
   printf("Thursday");
if(n==5)
   printf("Friday");
if(n==6)
   printf("Saturday");
}

Execution 1:

Enter any digit from 0 to 6 : 3

Wednesday

 

Execution 2:

Enter any digit from 0 to 6: 1

Monday

 

 

Summery :-

A problem may have any number of possibilities; we need to write a condition for each possibility and solution under every condition to be executed if the condition is true.