Lesson 6 :Loops


There may be a situation, when you need to execute a block of code several number of times. In general, the 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 a group of statements multiple times and following is the general from of a loop statement in most of the programming languages:

loop_architecture

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

Loop TypeDescription

while loop

It repeats a statement or a group of statements while a given condition is true. It tests the condition before executing the loop body.

for loop

It executes a sequence of statements multiple times and abbreviates the code that manages the loop variable.

do...while loop

It is similar to a while statement, except that it tests the condition at the end of the loop body

nested loops

You can use one or more loop inside any another while, for or do..while loop.

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

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 is skipped and the first statement after the while loop is 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

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 theincrement 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 testing for a condition). After the condition becomes false, the for loop terminates.

Flow Diagram

for_loop

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

Do...While Loop

Unlike for and while loops, which test the loop condition at the start of the loop, thedo...while loop checks its condition at the end 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

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 statement

Terminates the loop or switch statement and transfers execution to the statement immediately following the loop or switch.

continue statement

Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating.

Break Statement

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

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 Statement

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

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

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.

Example

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.


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