Chapter 4: C language Decision Statements

In this chapter we are going to study:

If condition
If-else condition
If-else-if condition
Nested-if
Switch Statement

These conditional statements help us to navigate the program flow based on certain conditions.

Above mentioned are the control statements that C language supports.

If condition:

If statement is used, when we need to execute a set of statements if the condition is true. If it becomes false, we shall start to execute the next statements.

The condition expression should be a Boolean expression and should result in only true or false.

The syntax of simple if statement:

if(condition_expression)
{
  //execute statements if condition is true
}

Programming example to check if pin is valid or not:

#include<stdio.h>
int main()
{
  int pin;
  printf("Enter pin number \n");
  scanf("%d", &pin);


  if (pin == 8525)
  {
    printf("Pin is valid \n");
  }
printf("Thank You \n");
return 0;
}

if else statement

if else statement is used when there is a need to execute set of statements if the condition is true or execute another set of statements if the condition is false.

For example, if you entered the correct password, the system should allow to log in, else the system should throw an error message.

Syntax:

if(condition_expression)
{
  //execute statements if condition is true
}
else
{
  //execute statements if condition is false
}

Programming example to check if login password is valid or not:

#include<stdio.h>
int main()
{
  int password;
  printf("Enter Login password \n");
  scanf("%d", &password);


  if (password == 8525)
  {
    printf("Password is valid \n");
  }
  else
  {
    printf("Password is in-valid \n");
  }

    printf("Thank You \n");
return 0;
}

If-else-if statement

Consider the following example:

If today is Saturday I need to go to movie

else if today is Sunday I need to buy grocery

else I have to go to office as this is weekday.

In these type of situations, we need to use if..else..if statements.

Below is the syntax

if (condition1)
{
  //Statements 1
}

else if (condition2)
{
  //statements 2
}
else
{
  //statements 3
}

if the condition1 is true statements1 will be executed and shall be out of the if..else ladder. If condition1 is false, it will go and check for condition2 if true statements2 will be executed and come out of if..else ladder. If condition2 is also false, then default statements3 will be executed.

Programming example to give grades to a student.

#include<stdio.h>
int main()
{
  int marks;
  printf("Enter marks of a student \n");
  scanf("%d", &marks);


  if (marks >= 80)
  {
    printf("Student has got A grade \n");
  }
  else if (marks >= 80)
  {
    printf("Student has got B grade \n");
  }
  else
  {
    printf("Student has got C grade \n");
  }

    printf("Thank You \n");
return 0;
}

Nested if statement.

There can be an if statement inside an if block. Then it is called as nested if.

Consider the below example: While searching a store, we look if the product is available or not. If the product is available, then we shall check the price of the product and keep it the shopping cart.

Syntax:

if (condition1)
{
  //statements1 

  if(condition2)
  {
    //statements2
  }

}
else
{
  //statements3

}

From the above we can see that if condition 1 is true, all the statements under condition 1 will be executed. Then condition2 will be checked, if true all the statements under condition 2 will be executed.

Program example:

#include<stdio.h>

int main()
{
  int num_1 = 1;
  int num_2 = 2;
  
  if (num_1 == 1)
  {
    printf("Inside if condition \"num_1 ==1\" \n");
      if (num_2 == 2) // Nested if statement
      {
        printf("Inside if condition \"num_2 == 2\" \n");
      }
  }
  else
  {
    printf(" In else part");
  }
return 0;
}

Go ahead, change the values of “num_1” and “num_2” to get different output.

Switch statement

Instead of using if…else..if ladder, we can use the switch statement. The switch statement can be useful in situations where a particular code needs to be executed based on multiple choices like a menu.

Syntax:

switch(value)
{
  case value1:
    //statements1
  break;

  case value2:
    //statements2
  break;

  case value3:
    //statements3
  break;

  default:
    //statements4
}

Note:

In switch statement, break is necessary. If you miss break statement, then the next case will be executed.
Default statement will be executed when none of the cases matches.
The case in the switch statement should always be a number or an alphabet. You cannot include Boolean expression there.

Programming Example:

#include<stdio.h>

int main()
{
	int num = 4;


	switch(num)
	{
		case 1:
			printf("The number is one\n");
			break;

		case 2:
			printf("The number is two\n");
			break;

		case 3:
			printf("The number is three\n");
			break;

		case 4:
			printf("The number is four\n");
			break;

		case 5: // we have missed the break statement. Check what happens when the number is 5
			printf("The number is five\n");

		case 6:
			printf("The number is six\n");
			break;

		default:
			printf("This is default case\n");
	}	


	return 0;
}

Output:

When the number is 4.

The number is four



When the number is 5, the output is:



The number is five

The number is six