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C# Control Statements Part 2 (For , While, and Do Loops)

There are situations that require you to execute a block of code several number of times. In general statements are executed sequentially: The first statement in a function is executed first, followed by the second, and so on.

Programming languages provide various control structures that allow for more complicated execution paths.

A loop statement allows us to execute a statement or group of statements multiple times and following is the general from of a loop statement in most of the programming languages:

Loop Architecture
Loop Architecture



C# provides following types of loop to handle looping requirements. Click the following links to check their detail.

Loop TypeDescription
for loopExecutes a sequence of statements multiple times and abbreviates the code that manages the loop variable.
while loopRepeats a statement or group of statements while a given condition is true. It tests the condition before executing the loop body.
do...while loopLike a while statement, except that it tests the condition at the end of the loop body
nested loopsYou can use one or more loop inside any another while, for or do..while loop.

For Loop

A for loop is a repetition control structure that allows you to efficiently write a loop that needs to execute a specific number of times.

Syntax:
The syntax of a for loop in C# is:

for ( init; condition; increment )
{
   statement(s);
}
Here is the flow of control in a for loop:

  • The init step is executed first, and only once. This step allows you to declare and initialize any loop control variables. You are not required to put a statement here, as long as a semicolon appears.
  • Next, the condition is evaluated. If it is true, the body of the loop is executed. If it is false, the body of the loop does not execute and flow of control jumps to the next statement just after the for loop.
  • After the body of the for loop executes, the flow of control jumps back up to the increment statement. This statement allows you to update any loop control variables. This statement can be left blank, as long as a semicolon appears after the condition.
  • The condition is now evaluated again. If it is true, the loop executes and the process repeats itself (body of loop, then increment step, and then again condition). After the condition becomes false, the for loop terminates.

Flow Diagram:
for loop in C#

Example:
using System;

namespace Loops
{
    
    class Program
    {
        static void Main(string[] args)
        {
            /* for loop execution */
            for (int a = 10; a < 20; a = a + 1)
            {
                Console.WriteLine("value of a: {0}", a);
            }
            Console.ReadLine();
        }
    }
} 
When the above code is compiled and executed, it produces the following result:

value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15
value of a: 16
value of a: 17
value of a: 18
value of a: 19

While Loop

A while loop statement in C# repeatedly executes a target statement as long as a given condition is true.

Syntax:
The syntax of a while loop in C# is:

while(condition)
{
   statement(s);
}
Here, statement(s) may be a single statement or a block of statements. The condition may be any expression, and true is any non-zero value. The loop iterates while the condition is true.

When the condition becomes false, program control passes to the line immediately following the loop.

Flow Diagram:
while loop in C#

Here, key point of the while loop is that the loop might not ever run. When the condition is tested and the result is false, the loop body will be skipped and the first statement after the while loop will be executed.

Example:
using System;

namespace Loops
{
    
    class Program
    {
        static void Main(string[] args)
        {
            /* local variable definition */
            int a = 10;

            /* while loop execution */
            while (a < 20)
            {
                Console.WriteLine("value of a: {0}", a);
                a++;
            }
            Console.ReadLine();
        }
    }
} 

When the above code is compiled and executed, it produces the following result:

value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15
value of a: 16
value of a: 17
value of a: 18
value of a: 19

Do While Loop

Unlike for and while loops, which test the loop condition at the top of the loop, the do...while loop checks its condition at the bottom of the loop.

A do...while loop is similar to a while loop, except that a do...while loop is guaranteed to execute at least one time.

Syntax:
The syntax of a do...while loop in C# is:

do
{
   statement(s);

}while( condition );
Notice that the conditional expression appears at the end of the loop, so the statement(s) in the loop execute once before the condition is tested.

If the condition is true, the flow of control jumps back up to do, and the statement(s) in the loop execute again. This process repeats until the given condition becomes false.

Flow Diagram:
do...while loop in C#

Example:
using System;

namespace Loops
{
    
    class Program
    {
        static void Main(string[] args)
        {
            /* local variable definition */
            int a = 10;

            /* do loop execution */
            do
            {
               Console.WriteLine("value of a: {0}", a);
                a = a + 1;
            } while (a < 20);

            Console.ReadLine();
        }
    }
} 
When the above code is compiled and executed, it produces the following result:

value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15
value of a: 16
value of a: 17
value of a: 18
value of a: 19

Nested Loops

C# allows to use one loop inside another loop. Following section shows few examples to illustrate the concept.

Syntax:
The syntax for a nested for loop statement in C# is as follows:

for ( init; condition; increment )
{
   for ( init; condition; increment )
   {
      statement(s);
   }
   statement(s);
}
The syntax for a nested while loop statement in C# is as follows:

while(condition)
{
   while(condition)
   {
      statement(s);
   }
   statement(s);
}
The syntax for a nested do...while loop statement in C# is as follows:

do
{
   statement(s);
   do
   {
      statement(s);
   }while( condition );

}while( condition );
A final note on loop nesting is that you can put any type of loop inside of any other type of loop. For example a for loop can be inside a while loop or vice versa.

Example:
The following program uses a nested for loop to find the prime numbers from 2 to 100:

using System;

namespace Loops
{
    
   class Program
   {
      static void Main(string[] args)
      {
         /* local variable definition */
         int i, j;

         for (i = 2; i < 100; i++)
         {
            for (j = 2; j <= (i / j); j++)
               if ((i % j) == 0) break; // if factor found, not prime
            if (j > (i / j)) 
               Console.WriteLine("{0} is prime", i);
         }

         Console.ReadLine();
      }
   }
} 
When the above code is compiled and executed, it produces the following result:

2 is prime
3 is prime
5 is prime
7 is prime
11 is prime
13 is prime
17 is prime
19 is prime
23 is prime
29 is prime
31 is prime
37 is prime
41 is prime
43 is prime
47 is prime
53 is prime
59 is prime
61 is prime
67 is prime
71 is prime
73 is prime
79 is prime
83 is prime
89 is prime
97 is prime

Loop Control Statements:

Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed.

C# provides the following control statements. Click the following links to check their details.

Control StatementDescription
break statementTerminates the loop or switch statement and transfers execution to the statement immediately following the loop or switch.
continue statementCauses the loop to skip the remainder of its body and immediately retest its condition prior to reiterating.

Break

The break statement in C# has following two usage:

  • When the break statement is encountered inside a loop, the loop is immediately terminated and program control resumes at the next statement following the loop.
  • It can be used to terminate a case in the switch statement.
If you are using nested loops (i.e., one loop inside another loop), the break statement will stop the execution of the innermost loop and start executing the next line of code after the block.

Syntax:
The syntax for a break statement in C# is as follows:

break;
Flow Diagram:
c# break statement

Example:
using System;

namespace Loops
{
    
    class Program
    {
        static void Main(string[] args)
        {
            /* local variable definition */
            int a = 10;

            /* while loop execution */
            while (a < 20)
            {
                Console.WriteLine("value of a: {0}", a);
                a++;
                if (a > 15)
                {
                    /* terminate the loop using break statement */
                    break;
                }
            }
            Console.ReadLine();
        }
    }
} 
When the above code is compiled and executed, it produces the following result:

value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15

Continue

The continue statement in C# works somewhat like the break statement. Instead of forcing termination, however, continue forces the next iteration of the loop to take place, skipping any code in between.

For the for loop, continue statement causes the conditional test and increment portions of the loop to execute. For the while and do...while loops, continue statement causes the program control passes to the conditional tests.

Syntax:
The syntax for a continue statement in C# is as follows:

continue;
Flow Diagram:
C# continue statement

Example:
using System;

namespace Loops
{
    
    class Program
    {
        static void Main(string[] args)
        {
            /* local variable definition */
            int a = 10;

            /* do loop execution */
            do
            {
                if (a == 15)
                {
                    /* skip the iteration */
                    a = a + 1;
                    continue;
                }
                Console.WriteLine("value of a: {0}", a);
                a++;

            } while (a < 20);
 
            Console.ReadLine();
        }
    }
} 
When the above code is compiled and executed, it produces the following result:

value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 16
value of a: 17
value of a: 18
value of a: 19

The Infinite Loop:

A loop becomes infinite loop if a condition never becomes false. The for loop is traditionally used for this purpose. Since none of the three expressions that form the for loop are required, you can make an endless loop by leaving the conditional expression empty.

using System;

namespace Loops
{
    
    class Program
    {
        static void Main(string[] args)
        {
            for (; ; )
            {
                Console.WriteLine("Hey! I am Trapped");
            }
 
        }
    }
} 
When the conditional expression is absent, it is assumed to be true. You may have an initialization and increment expression, but programmers more commonly use the for(;;) construct to signify an infinite loop.


C# Control Statements Part 1 (If and Switch statements)

Decision making structures require that the programmer 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.

Following is the general from of a typical decision making structure found in most of the programming languages:

Decision making statements in C#


Decision making statements in C#
C# provides following types of decision making statements. Click the following links to check their detail.

StatementDescription
if statementAn if statement consists of a boolean expression followed by one or more statements.
if...else statementAn if statement can be followed by an optional else statement, which executes when the boolean expression is false.
nested if statementsYou can use one if or else if statement inside another if or else if statement(s).
switch statementswitch statement allows a variable to be tested for equality against a list of values.
nested switch statementsYou can use one switch statement inside another switchstatement(s).

If Statement

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

Syntax:
The syntax of an if statement in C# is:

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 will be 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) will be executed.

Flow Diagram:

C# if statement
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();
        }
    }
}
The above code results in the following:
a is less than 20;
value of a is : 10

If...else Statement

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

Syntax:
The syntax of an if...else statement in C# is:

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 will be executed, otherwise else block of code will be executed.

Flow Diagram:
C# if...else statement

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();
        }
    }
}
The above code results in the following:
a is not less than 20;
value of a is : 100

If...else if...else Statement

An if statement can be followed by an optional else if...else statement, which is very useful to test various conditions using single if...else if statement.

When using if , else if , else statements there are few points to keep in mind.

  • An if can have zero or one else's and it must come after any else if's.
  • An if can have zero to many else if's and they must come before the else.
  • Once an else if succeeds, none of the remaining else if's or else's will be tested.
The syntax of an if...else if...else statement in C# is:

if(boolean_expression 1)
{
   /* Executes when the boolean expression 1 is true */
}
else if( boolean_expression 2)
{
   /* Executes when the boolean expression 2 is true */
}
else if( boolean_expression 3)
{
   /* Executes when the boolean expression 3 is true */
}
else 
{
   /* executes when the none of the above condition is true */
}
Example:
using System;

namespace DecisionMaking
{
    
    class Program
    {
        static void Main(string[] args)
        {

            /* local variable definition */
            int a = 100;

            /* check the boolean condition */
            if (a == 10)
            {
                /* if condition is true then print the following */
                Console.WriteLine("Value of a is 10");
            }
            else if (a == 20)
            {
                /* if else if condition is true */
                Console.WriteLine("Value of a is 20");
            }
            else if (a == 30)
            {
                /* if else if condition is true  */
                Console.WriteLine("Value of a is 30");
            }
            else
            {
                /* if none of the conditions is true */
                Console.WriteLine("None of the values is matching");
            }
            Console.WriteLine("Exact value of a is: {0}", a);
            Console.ReadLine();
        }
    }
}
The above code results in the following:
None of the values is matching
Exact value of a is: 100

Nested If Statements

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).

The syntax for a nested if statement is as follows:

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();
        }
    }
}
When the above code is executed it will result in the following:
Value of a is 100 and b is 200
Exact value of a is : 100
Exact value of b is : 200

Switch Statements

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.

The syntax for a switch statement in C# is as follows:

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);
}
The following rules apply to a switch statement:

  • 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.
Flow Diagram:
switch statement in C#

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();
        }
    }
}
The above code results in the following:
Well done
Your grade is B

Nested Switch Statements

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.

The syntax for a nested switch statement is as follows:

switch(ch1) 
{
   case 'A': 
      printf("This A is part of outer switch" );
      switch(ch2) 
      {
         case 'A':
            printf("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();
        }
    }
} 
When the above code is compiled and executed, it produces the following result:
This is part of outer switch
This is part of inner switch
Exact value of a is : 100
Exact value of b is : 200

The ? : Operator:

The operator ? : can be used to replace if...else statements. It has the following general form:

Exp1 ? Exp2 : Exp3;
Where Exp1, Exp2, and Exp3 are expressions. Notice the use and placement of the colon.

The value of a ? expression is determined like this: Exp1 is evaluated. If it is true, then Exp2 is evaluated and becomes the value of the entire ? expression. If Exp1 is false, then Exp3 is evaluated and its value becomes the value of the expression.