Difference between revisions of "C Sharp Looping - The for Statement"

Latest revision as of 20:00, 27 October 2016

In this chapter of C# Essentials we will continue looking at flow control in C# applications. In the preceding chapters we have looked in detail at using logical expressions to decide what C# code should be executed. Another aspect of flow control entails the definition of loops. Loops are essentially sequences of C# statements which are to be executed repeatedly until a specified condition (or conditions) are met.

Why Use Loops?

It is generally common knowledge that computers are great at performing repetitive tasks an infinite number of times, and doing so very quickly. It is also common knowledge that computers really don't do anything unless someone programs them to tell them what to do. Loop statements are the primary mechanism for telling a computer that a sequence of tasks needs to be repeated a specific number of times. Suppose, for example, that you have a requirement to add a number to itself ten times. One way to do this might be to write the following C# code:

int j = 10;

j += j;
j += j;
j += j;
j += j;
j += j;
j += j;
j += j;
j += j;
j += j;
j += j;

Whilst this is somewhat cumbersome it does work. What would happen, however, if you needed to perform this task 100 or even 10,000 times? Writing C# code to perform this as above would be prohibitive. Such a scenario is exactly what the for loop is intended to handle.

The syntax of a C# for loop is as follows:

for ( ''initializer''; ''conditional expression''; ''loop expression'' )
{
      statements to be executed
}

i = 0;

This sets the counter to be the variable i and sets it to zero. Note that if the count variable has not been previously declared it may be declared as part of the for statement:

int i = 0;

The conditional expression specifies the test to perform to verify whether the loop has been performed the required number of iterations. For example, if we want to loop 100 times:

i < 100;

Finally the loop expression specifies the action to perform on the counter variable. For example to increment by 1:

i++;

The body of statements to be executed on each iteration of the loop are contained within the code block defined by the opening ({) and closing (}) braces.

Bringing this all together we can create a for loop to perform the task outlined in the earlier example:

int j = 10;

for (int i=0; i<100; i++)
{
      j += j;
}

System.Console.WriteLine ("j = " + j);

C# Loop Variable Scope

A key point to note in creating loops is that any variables defined within the body of a loop are only visible to code within the loop. This is concept known as scope. If, for example, a variable myCounter is defined in a for loop, that variable ceases to exist once the loop terminates:

// variable myCounter does not yet exist

for (int i = 0; i < 10; i++)
{
       int myCounter = 0; //myCounter variable created in scope of for loop

       myCounter += i;
}

// after loop exit variable myCounter is now out of scope and ceases to exist

Creating an Infinite for Loop

A for loop which will execute an infinite number of times may be constructed using for (;;) syntax. For example, the following code sample will output Hello from C# until the program is manually terminated by the user (or the computer is turned off or rebooted):

for (;;)
{
    System.Console.WriteLine ("Hello from C#");
}

Breaking Out of a for Loop

Having created a loop it is possible that under certain conditions you might want to break out of the loop before the completion criteria have been met (particularly if you have created an infinite loop). One such example might involve continually checking for activity on a network socket. Once activity has been detected it will be necessary to break out of the monitoring loop and perform some other task.

For the purpose of breaking out of a loop C# provides the break statement which breaks out of the current loop and resumes execution at the code directly after the loop. For example:

int j = 10;

for (int i = 0; i < 100; i++)
{
     j += j;

     if (j > 100)
          break;
}

In the above example the loop will continue to execute until the value of j exceeds 100 at which point the loop will exit.

Nested for Loops

So far we have looked at only a single level of for loop. It is also possible to nest for loops where one for loop resides inside another for loop. For example:

for (int i = 0; i < 100; i++)
{
     System.Console.WriteLine ( "i = " + i);

     for (int j = 0; j < 10; j++)
     {
             System.Console.WriteLine ( "j = " + j);
     }
}

The above example will loop 100 times displaying the value of i on each iteration. In addition, for each of those iterations it will loop 10 times displaying the value of j.

Breaking From Nested Loops

An important point to be aware of when breaking out of a nested for loop is that the break only exits from the current level of loop. For example, the following C# code example will exit from the current iteration of the nested loop when j is equal to 5. The outer loop will, however, continue to iterate and, in turn execute the nested loop:

for (int i = 0; i < 100; i++)
{
     System.Console.WriteLine ( "i = " + i);

     for (int j = 0; j < 10; j++)
     {
             if (j == 5)
                   break;

             System.Console.WriteLine ( "j = " + j);
     }
}

Continuing for Loops

Another useful statement for use in loops in the continue statement. When the execution process finds a continue statement in any kind of loop it skips all remaining code in the body of the loop and begins execution once again from the top of the loop. Using this technique we can construct a for loop which outputs only even numbers between 1 and 9:

for (int i = 1; i < 10; i++)
{
          if ((i % 2) != 0)
               continue;

           System.Console.WriteLine ( "i = " + i);
     }
}

In the example, if i is not divisible by 2 with 0 remaining the code performs a continue sending execution to the top of the for loop, thereby bypassing the code to output the value of i. This will result in the following output:

2
4
6
8