Home > Articles > Programming > C#

This chapter is from the book

This chapter is from the book

Control Flow Statements, Continued

Now that we’ve described Boolean expressions in more detail we can more clearly describe the control flow statements supported by C#. Many of these statements will be familiar to experienced programmers, so you can skim this section looking for details specific to C#. Note in particular the foreach loop, as this may be new to many programmers.

The while and do/while Loops

Thus far you have learned how to write programs that do something only once. However, computers can easily perform similar operations multiple times. In order to do this, you need to create an instruction loop. The first instruction loop we will discuss is the while loop, because it is the simplest conditional loop. The general form of the while statement is as follows:

while (condition)

The computer will repeatedly execute the statement that is the “body” of the loop as long as the condition (which must be a Boolean expression) evaluates to true. If the condition evaluates to false, code execution skips the body and executes the code following the loop statement. Note that statement will continue to execute even if it causes the condition to become false. It isn’t until the condition is reevaluated “at the top of the loop” that the loop exits. The Fibonacci calculator shown in Listing 3.41 demonstrates the while loop.

Listing 3.41. while Loop Example

class FibonacciCalculator
  static void Main()
      decimal current;
      decimal previous;
      decimal temp;
      decimal input;

      System.Console.Write("Enter a positive integer:");

      // decimal.Parse convert the ReadLine to a decimal.
      input = decimal.Parse(System.Console.ReadLine());

      // Initialize current and previous to 1, the first
      // two numbers in the Fibonacci series.
      current = previous = 1;

      // While the current Fibonacci number in the series is
      // less than the value input by the user.
      while(current <= input)                                                                
          temp = current;                                                                    
          current = previous + current;                                                      
          previous = temp; // Executes even if previous                                      
          // statement caused current to exceed input                                        

        "The Fibonacci number following this is {0}",

A Fibonacci number is a member of the Fibonacci series, which includes all numbers that are the sum of the previous two numbers in the series, beginning with 1 and 1. In Listing 3.41, you prompt the user for an integer. Then you use a while loop to find the first Fibonacci number that is greater than the number the user entered.

The do/while loop is very similar to the while loop except that a do/while loop is preferred when the number of repetitions is from 1 to n and n is not known when iterating begins. This pattern frequently occurs when prompting a user for input. Listing 3.42 is taken from the tic-tac-toe program.

Listing 3.42. do/while Loop Example

// Repeatedly request player to move until he
// enter a valid position on the board.
bool valid;
  valid = false;

  // Request a move from the current player.
    "\nPlayer {0}: Enter move:", currentPlayer);
  input = System.Console.ReadLine();

  // Check the current player's input.
  // ...

} while (!valid);

In Listing 3.42, you always initialize valid to false at the beginning of each iteration, or loop repetition. Next, you prompt and retrieve the number the user input. Although not shown here, you then check whether the input was correct, and if it was, you assign valid equal to true. Since the code uses a do/while statement rather than a while statement, the user will be prompted for input at least once.

The general form of the do/while loop is as follows:

while (condition);

As with all the control flow statements, a code block is generally used as the single statement in order to allow multiple statements to be executed as the loop body. However, any single statement except for a labeled statement or a local variable declaration can be used.

The for Loop

The for loop iterates a code block until a specified condition is reached. In that way, it is very similar to the while loop. The difference is that the for loop has built-in syntax for initializing, incrementing, and testing the value of a counter, known as the loop variable. Because there is a specific location in the loop syntax for an increment operation, the increment and decrement operators are frequently used as part of a for loop.

Listing 3.43 shows the for loop used to display an integer in binary form. The results of this listing appear in Output 3.21.

Listing 3.43. Using the for Loop

public class BinaryConverter
  public static void Main()
      const int size = 64;
      ulong value;
      char bit;

      System.Console.Write ("Enter an integer: ");
      // Use long.Parse() so as to support negative numbers
      // Assumes unchecked assignment to ulong.
      value = (ulong)long.Parse(System.Console.ReadLine());

      // Set initial mask to 100....
      ulong mask = 1UL << size - 1;
      for (int count = 0; count < size; count++)
          bit = ((mask & value) > 0) ? '1': '0';
          // Shift mask one location over to the right
          mask >>= 1;

Output 3.21.

Enter an integer: -42

Listing 3.43 performs a bit mask 64 times, once for each bit in the number. The three parts of the for loop header first declare and initialize the variable count, then describe the condition that must be met for the loop body to be executed, and finally describe the operation that updates the loop variable. The general form of the for loop is as follows:

for (initial ; condition ; loop)

Here is a breakdown of the for loop.

  • The initial section performs operations that precede the first iteration. In Listing 3.43, it declares and initializes the variable count. The initial expression does not have to be a declaration of a new variable (though it frequently is). It is possible, for example, to declare the variable beforehand and simply initialize it in the for loop, or to skip the initialization section entirely by leaving it blank. Variables declared here are in scope throughout the header and body of the for statement.
  • The condition portion of the for loop specifies an end condition. The loop exits when this condition is false exactly like the while loop does. The for loop will execute the body only as long as the condition evaluates to true. In the preceding example, the loop exits when count is greater than or equal to 64.
  • The loop expression executes after each iteration. In the preceding example, count++ executes after the right shift of the mask (mask >>= 1), but before the condition is evaluated. During the sixty-fourth iteration, count is incremented to 64, causing the condition to become false, and therefore terminating the loop.
  • The statement portion of the for loop is the “loop body” code that executes while the conditional expression remains true.

If you wrote out each for loop execution step in pseudocode without using a for loop expression, it would look like this.

  1. Declare and initialize count to 0.
  2. If count is less than 64, continue to step 3; otherwise, go to step 7.
  3. Calculate bit and display it.
  4. Shift the mask.
  5. Increment count by one.
  6. Jump back to line 2.
  7. Continue the execution of the program after the loop.

The for statement doesn’t require any of the elements in its header. for(;;){ ... } is perfectly valid; although there still needs to be a means to escape from the loop to avoid executing infinitely. (If the condition is missing, it is assumed to be the constant true.)

The initial and loop expressions have an unusual syntax to support loops that require multiple loop variables, as shown in Listing 3.44.

Listing 3.44. for Loop Using Multiple Expressions

for(int x=0, y=5; ((x<=5) && (y>=0)); y--, x++)
      x, (x>y? '>' : '<'), y);

The results of Listing 3.44 appear in Output 3.22.

Output 3.22.

0<5     1<4     2<3     3>2     4>1     5>0

Here the initialization clause contains a complex declaration that declares and initializes two loop variables, but this is at least similar to a declaration statement that declares multiple local variables. The loop clause is quite unusual, as it can consist of a comma-separated list of expressions, not just a single expression.

The for loop is little more than a more convenient way to write a while loop; you can always rewrite a for loop like this:


The foreach Loop

The last loop statement in the C# language is foreach. The foreach loop iterates through a collection of items, setting a loop variable to represent each item in turn. In the body of the loop, operations may be performed on the item. A nice property of the foreach loop is that every item is iterated over exactly once; it is not possible to accidentally miscount and iterate past the end of the collection as can happen with other loops.

The general form of the foreach statement is as follows:

foreach(type variable in collection)

Here is a breakdown of the foreach statement.

  • type is used to declare the data type of the variable for each item within the collection. It may be var, in which case the compiler infers the type of the item from the type of the collection.
  • variable is a read-only variable into which the foreach loop will automatically assign the next item within the collection. The scope of the variable is limited to the body of the loop.
  • collection is an expression, such as an array, representing any number of items.
  • statement is the loop body that executes for each iteration of the loop.

Consider the foreach loop in the context of the simple example shown in Listing 3.45.

Listing 3.45. Determining Remaining Moves Using the foreach Loop

class TicTacToe      // Declares the TicTacToe class.
  static void Main() // Declares the entry point of the program.
      // Hardcode initial board as follows
      // ---+---+---
      //  1 | 2 | 3
      // ---+---+---
      //  4 | 5 | 6
      // ---+---+---
      //  7 | 8 | 9
      // ---+---+---
      char[] cells = {
        '1', '2', '3', '4', '5', '6', '7', '8', '9'

          "The available moves are as follows: ");

      // Write out the initial available moves
      foreach (char cell in cells)                                                                 
        if (cell != 'O' && cell != 'X')                                                            
            System.Console.Write("{0} ", cell);                                                    

Output 3.23 shows the results of Listing 3.45.

Output 3.23.

The available moves are as follows: 1 2 3 4 5 6 7 8 9

When the execution engine reaches the foreach statement, it assigns to the variable cell the first item in the cells array—in this case, the value '1'. It then executes the code within the block that makes up the foreach loop body. The if statement determines whether the value of cell is 'O' or 'X'. If it is neither, the value of cell is written out to the console. The next iteration then assigns the next array value to cell, and so on.

It is important to note that the compiler prevents modification of the variable (cell) during the execution of a foreach loop. Also, the loop variable has a subtly different behavior in C# 5 than it did in previous versions; the difference is only apparent when the loop body contains a lambda expression or anonymous method that uses the loop variable. See Chapter 12 for details.

The switch Statement

A switch statement is simpler to understand than a complex if statement when you have a value that must be compared against may different constant values. The switch statement looks like this:

   case constant:

Here is a breakdown of the switch statement.

  • expression is the value that is being compared against the different constants. The type of this expression determines the “governing type” of the switch. Allowable governing data types are bool, sbyte, byte, short, ushort, int, uint, long, ulong, char, any enum type (covered in Chapter 8), the corresponding nullable types of each of those value types, and string.
  • constant is any constant expression compatible with the governing type.
  • A group of one or more case labels (or the default label) followed by a group of one or more statements is called a switch section. The pattern above has two switch sections; Listing 3.47 shows a switch statement with three switch sections.
  • statements is one or more statements to be executed when the expression equals one of the constant values mentioned in a label in the switch section. The end point of the group of statements must not be reachable. Typically the last statement is a jump statement such as a break, return, or goto statement.

A switch statement should have at least one switch section; switch(x){} is legal but will generate a warning. Also, earlier the guideline was to avoid omitting braces in general. One exception is to omit braces for case and break statements because they serve to indicate the beginning and end of a block.

Listing 3.47, with a switch statement, is semantically equivalent to the series of if statements in Listing 3.46.

Listing 3.47. Replacing the if Statement with a switch Statement

static bool ValidateAndMove(
  int[] playerPositions, int currentPlayer, string input)
  bool valid = false;

  // Check the current player's input.
  switch (input)
    case "1" :
    case "2" :
    case "3" :
    case "4" :
    case "5" :
    case "6" :
    case "7" :
    case "8" :
    case "9" :
      // Save/move as the player directed.
      valid = true;

    case "" :
    case "quit" :
      valid = true;
    default :
      // If none of the other case statements
      // is encountered then the text is invalid.
        "\nERROR:  Enter a value from 1-9. "
        + "Push ENTER to quit");

  return valid;

In Listing 3.47, input is the test expression. Since input is a string, the governing type is string . If the value of input is one of the strings 1, 2, 3, 4, 5, 6, 7, 8, or 9, the move is valid and you change the appropriate cell to match that of the current user’s token (X or O). Once execution encounters a break statement, control leaves the switch statement.

The next switch section describes how to handle the empty string or the string quit; it sets valid to true if input equals either value. The default switch section is executed if no other switch section had a case label that matched the test expression.

There are several things to note about the switch statement.

  • A switch statement with no switch sections will generate a compiler warning, but the statement will still compile.
  • Switch sections can appear in any order; the default section does not have to appear last. In fact, the default switch section does not have to appear at all; it is optional.
  • The C# language requires that the end point of every switch section, including the last section, be unreachable. This means that switch sections usually end with a break, return, or goto.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020