Decision Making

Decision making structures requires the programmer to specify one or more conditions to be evaluated or tested by the program, along with a statement or statements to be executed if the condition is determined to be true, and optionally, other statements to be executed if the condition is determined to be false.

C# provides following types of decision making statements :

  • IF Statement
  • IF...ELSE Statement
  • Nested IF Statement
  • SWITCH Statement
  • Nested SWITCH Statement

IF Statement

An if statement consists of a boolean expression followed by one or more statements.

Syntax : 

if(boolean_expression) {
   /* statement(s) will execute if the boolean expression is true */
}

If the boolean expression evaluates to true, then the block of code inside the if statement is executed. If boolean expression evaluates to false, then the first set of code after the end of the if statement(after the closing curly brace) is executed.

Example 

using System;

namespace DecisionMaking {
   class Program {
      static void Main(string[] args) {
         /* local variable definition */
         int a = 10;
        
         /* check the boolean condition using if statement */
         if (a < 20) {
            /* if condition is true then print the following */
            Console.WriteLine("a is less than 20");
         }
         Console.WriteLine("value of a is : {0}", a);
         Console.ReadLine();
      }
   }
}

IF...ELSE Statement

An if statement can be followed by an optional else statement, which executes when the boolean expression is false.

Syntax :

if(boolean_expression) {
   /* statement(s) will execute if the boolean expression is true */
} else {
   /* statement(s) will execute if the boolean expression is false */
}

If the boolean expression evaluates to true, then the if block of code is executed, otherwise else block of code is executed.

Example

using System;

namespace DecisionMaking {
   class Program {
      static void Main(string[] args) {
         /* local variable definition */
         int a = 100;
         
         /* check the boolean condition */
         if (a < 20) {
            /* if condition is true then print the following */
            Console.WriteLine("a is less than 20");
         } else {
            /* if condition is false then print the following */
            Console.WriteLine("a is not less than 20");
         }
         Console.WriteLine("value of a is : {0}", a);
         Console.ReadLine();
      }
   }
}

Nested IF Statement

It is always legal in C# to nest if-else statements, which means you can use one if or else if statement inside another if or else if statement(s).

Syntax : 

if( boolean_expression 1) {
   /* Executes when the boolean expression 1 is true */
   if(boolean_expression 2) {
      /* Executes when the boolean expression 2 is true */
   }
}

You can nest else if...else in the similar way as you have nested if statement.

Example

using System;

namespace DecisionMaking {
   class Program {
      static void Main(string[] args) {
         //* local variable definition */
         int a = 100;
         int b = 200;
         
         /* check the boolean condition */
         if (a == 100) {
            
            /* if condition is true then check the following */
            if (b == 200) {
               /* if condition is true then print the following */
               Console.WriteLine("Value of a is 100 and b is 200");
            }
         }
         Console.WriteLine("Exact value of a is : {0}", a);
         Console.WriteLine("Exact value of b is : {0}", b);
         Console.ReadLine();
      }
   }
}

SWITCH Statement

A switch statement allows a variable to be tested for equality against a list of values. Each value is called a case, and the variable being switched on is checked for each switch case.

Syntax : 

switch(expression) {
   case constant-expression  :
      statement(s);
      break; /* optional */
   case constant-expression  :
      statement(s);
      break; /* optional */
  
   /* you can have any number of case statements */
   default : /* Optional */
   statement(s);
}

Rules :

  • The expression used in a switch statement must have an integral or enumerated type, or be of a class type in which the class has a single conversion function to an integral or enumerated type.
  • You can have any number of case statements within a switch. Each case is followed by the value to be compared to and a colon.
  • The constant-expression for a case must be the same data type as the variable in the switch, and it must be a constant or a literal.
  • When the variable being switched on is equal to a case, the statements following that case will execute until a break statement is reached.
  • When a break statement is reached, the switch terminates, and the flow of control jumps to the next line following the switch statement.
  • Not every case needs to contain a break. If no break appears, the flow of control will fall through to subsequent cases until a break is reached.
  • A switch statement can have an optional default case, which must appear at the end of the switch. The default case can be used for performing a task when none of the cases is true. No break is needed in the default case.

Example

using System;

namespace DecisionMaking {
   class Program {
      static void Main(string[] args) {
         /* local variable definition */
         char grade = 'B';
         
         switch (grade) {
            case 'A':
               Console.WriteLine("Excellent!");
               break;
            case 'B':
            case 'C':
               Console.WriteLine("Well done");
               break;
            case 'D':
               Console.WriteLine("You passed");
               break;
            case 'F':
               Console.WriteLine("Better try again");
               break;
               default:
            Console.WriteLine("Invalid grade");
               break;
         }
         Console.WriteLine("Your grade is  {0}", grade);
         Console.ReadLine();
      }
   }
}

Nested SWITCH Statement

It is possible to have a switch as part of the statement sequence of an outer switch. Even if the case constants of the inner and outer switch contain common values, no conflicts will arise.

Syntax : 

switch(ch1) {
   case 'A':
   Console.WriteLine("This A is part of outer switch" );
   
   switch(ch2) {
      case 'A':
         Console.WriteLine("This A is part of inner switch" );
         break;
      case 'B': /* inner B case code */
   }
   break;
   case 'B': /* outer B case code */
}

Example

using System;

namespace DecisionMaking {
   class Program {
      static void Main(string[] args) {
         int a = 100;
         int b = 200;
         
         switch (a) {
            case 100: 
            Console.WriteLine("This is part of outer switch ");
            
            switch (b) {
               case 200:
               Console.WriteLine("This is part of inner switch ");
               break;
            }
            break;
         }
         Console.WriteLine("Exact value of a is : {0}", a);
         Console.WriteLine("Exact value of b is : {0}", b);
         Console.ReadLine();
      }
   }
}