Decision Making in C

Decision making is the process of deciding whether a block of code can be executed or not based on certain condition.

If the condition is satisfied, then the block of statement will be executed, else the block of statement will not be executed.

In C, there are 3 types of decision making statements. They are

• if statement
• if-else statement
• else-if statement
• switch statement

if statement

The if statement is used to check for any condition(s) and if the condition is true then it executes the block of statement after it.

Syntax

if(condition)
{
	// block of statements.
}

Example

#include <stdio.h>

void main( )
{
    int x, y;
    x = 15;
    y = 10;
    if(x>y)
    {
        printf("%d is greater than %d",x,y);
    }
	getch();
}

Output

15 is greater than 10

if-else statement

The if-else statement is used to check for a condition, if the condition is true then the if block will be executed. If the condition is false then, the else statement will be executed.

Syntax

if(condition)
{
	// block of statements
}
else
{
	// block of statements
}

Example

#include <stdio.h>

void main( )
{
    int x, y;
    x = 15;
    y = 20;
    if(x>y)
    {
        printf("%d is greater than %d",x,y);
    }
	else
	{
		printf("%d is greater than %d",y,x);
	}
	getch();
}

Output

20 is greater than 15

else-if statement

The else-if statement is used to check more than two conditions.

Syntax

if(condition-1)
{
	// block of statement.
}
else if(condition-2)
{
	// block of statement.
}
else if(condition-3)
{
	// block of statement.
}	
else 
{
	// block of statement.
}

Here the conditions are executed from the top order. If any condition is satisfied down the order, then that statement will be executed, if none of the condition is satisfied, then the else block will be executed.

Example

#include <stdio.h>

void main( )
{
    int x, y,z;
	printf("Enter 3 numbers to find the biggest amoung them\n");
	scanf("%d%d%d",&x,&y,&z);
    if(x>y && x>z)
    {
        printf("%d is greater than %d and %d",x,y,z);
    }
	else if(y>z)
	{
		printf("%d is greater than %d and %d",y,x,z);
	}
	else{
		printf("%d is greater than %d and %d",z,x,y);
	}
	getch();
}

Nested if-else statement

There may be some scenario in which we need to check some condition inside the if statement. We can have multiple if statement inside an if or else statement.

Syntax

if(condition)
{
	// block of statement.
	 if(condition)
	{
		// block of statement.
	}
}
else
{
	// block of statement.
	if(condition)
	{
		// block of statement.
	}
}

Example

#include <stdio.h>

void main( )
{
    int x, y,z;
	printf("Enter 3 numbers to find the biggest amoung them\n");
	scanf("%d%d%d",&x,&y,&z);
    if(x>y)
    {
		if(x>z)
		{
		  printf("%d is greater than %d and %d",x,y,z);
		}
    }
	else if(y>z)
	{
		printf("%d is greater than %d and %d",y,x,z);
	}
	else{
		printf("%d is greater than %d and %d",z,x,y);
	}
	getch();
}

Switch Statement

If we need to check a list of conditions and execute the set of statements based on condition we can go for else-if statement as well as switch statement.

The expression is given in the switch and check is done in the case, hence its called switch-case.

Syntax

switch(expression/variable)
{
	case  value:
		//statements
	break;  //optional
	default: //optional
	//statements
	break;
}

Example

#include<stdio.h>

#include<conio.h>

void main() {
  int day;
  clrscr();
  printf("Enter any number from 1 to 7\n");
  scanf("%d", & day);
  switch (day) {
  case 1:
    printf("Sunday");
    break;
  case 2:
    printf("Monday");
    break;
  case 3:
    printf("Tuesday");
    break;
  case 4:
    printf("Wednesday");
    break;
  case 5:
    printf("Thursday");
    break;
  case 6:
    printf("Friday");
    break;
  case 7:
    printf("Saturday");
    break;
  default:
    printf("Invalid option provided.");
  }
  getch();
}

Output 1

Enter any number from 1 to 7
1
Sunday

Output 2

Enter any number from 1 to 7
8
Invalid option provided.

In the above example, we ask for an integer value from 1 to 7 and store it in variable day.

Now we pass the day variable to switch statement. The case statement is used to check whether the value of day variable(defined in switch statement) is from 1 to 7. If the condition is true, then it will execute the block inside that case statement.

When a break statement is reached, the switch is terminated and the flow of the program moves to the next line following the switch statement.

If, no case statements are satisfied, then statement inside the default will be executed.

Rules

• With switch statement can use only byte, short, int, char data type.
• The value for a case must be same as the variable in switch.
• If no break appears, then the flow of the program will fall through to subsequent cases until a break is reached.