Control structures

If statement

The if statement is a decision making statement. It is used to control the flow of execution of the statements and also used to test logically whether the condition is true or false. It is always used in conjunction with condition. This statement is used when a question requires answer.

Example 1The following program illustrates the use of if statement.

/* Program to check whether the entered number is less than 25*/

#include<stdio.h>
main()                                                                                            
{                                                                                                    
 int i; /* Local definition */
 /* Statement */
 printf("\nEnter the number < 10....");
scanf("%d",&i);
 if(i<10)
printf("\nThe entered number %d is < 10", i);
} /* main */

Out put:
             Enter the number < 10....5
             The entered number 5 is <10

If-Else Statements

It is basically two way decision making statement and always used in conjunction with condition. It is used to control the flow of execution and also used to carry out the logical test and then pickup one of the two possible actions depending on the logical test.

Example : 
Program to find floating point input using if-else statement.

/* Program to check floating point input using if-else statement */

#include<stdio.h>
#include<conio.h>
void main()
{
      /* Local definitions */
        float years, seconds;
      /* Statements */
        clrscr();
      printf("Input you age in years : ");
      life=scanf("%f",&years);
      if(life==0)       /* if statement checks the conditions */
                   printf("The input is not a floating-point number.\n");
                   else
                       {
                          seconds=years*365*24*60*60;
                          printf("You have lived for %f seconds\n",seconds);
                        }          /* if */
getch();
} /* main */
Output:
Input you age in years : 24
You have lived for 756864000.000000 seconds

Nested If-Else Statements

When a series of if..else statements are occurred in a program, we can write an entire if..else statement in another if..else statement called nesting, and the statement is called nested if.

Example : Program to display the types of character using if-else statement.

/* Program to display the types of character */
#include<stdio.h>
#include<conio.h>
void main()
{
char chr;   /* Local definitions */
/* Statements */
clrscr();
printf("Enter a single character : ");
scanf(",&chr);
/* nested if statement is used to checks the conditios */
if((chr >= 'a' && chr <= 'z') || (chr >= 'A' && chr <= 'Z'))
printf("Entered character is an alphabetic.\n");
else
if(chr >= '0' && chr <= '9')
printf("Entered character is an digit.\n");
else
printf("Entered character is an special character.\n");
getch();
}  /* main */

Output 1:
         Enter a single character : M
         Entered character is an alphabetic.
Output 2:
         Enter a single character : 5
         Entered character is an digit.
Output 3:
Enter a single character : %26
Entered character is an special character.


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