More on Loops

In lesson 2, we have learned about one of the most common loops being used in the programming world, the for loop. Now, there are 4 more types of loops other than the for loop. One is the foreach loop. This is similar to the for loop but it was made specially to iterate over arrays. The next type of loop is the do..while loop. This construct executes the statements within its scope for one to a defined number of times. The third type is called the while construct. This construct is similar to the do..while construct except that in the while loop, the statements in its scope can be bypassed. The fourth one is more advanced and will not be discussed further later on. It is called recursive procedures/functions. This is not actually a loop but a function or procedure which calls itself, thus simulating a loop. Recursive procedures/functions are used by more complex programs in almost all programming languages.

FOREACH

As stated above, foreach is used to iterate over arrays. This is very similar to the same control structure in C++. The basic syntax for a foreach statements are:

       foreach (array_expression as $value) statement;

and

       foreach (array_expression as $key => $value) statement;

When a foreach loop is used, the array pointer being used for the loop is automatically repositioned to the first index position in the array. It would also be worthwhile to consider that foreach operates on a copy of the array, not the array itself. If the first syntax is used, it would take in the value of the array and store it in $value upon its initial pass through the loop. It is then up to the programmer to decide on how to use the value in $value because in the next iteration, the $value would change to the value of the next item in the array. Here is an example:

       // This loop will traverse through the array $MyArray and display
       // the value of each element in the array.
       $MyArray = array ("PHP", "MySQL", "Apache");
       foreach ($MyArray as $value)
       {
         echo "Value: $value \n"; // this will print "Value: PHP" on its first iteration
       }

The second syntax is almost the same with the first except that the current element's key is stored in the variable $key. For example:

       // This loop will traverse through the array $MyArray and display
       // the value of each key-value pair in the array.
       $MyArray = array ("Webserver" => "Apache",
                         "Language" => "PHP",
                         "Database" => "MySQL");
       foreach ($MyArray as $key => $value)
       {
         echo "Key: $key <-> Value: $value"; // this will print:
                                             // "Key: Webserver <-> Value: Apache"
                                             // on its first iteration
       }

DO..WHILE

In do..while loops, truth expressions are checked at the end of each iteration. This guarantees the loop to execute at least once. The syntax for a do..while loop is:

       do
       {
         ..statements..
       } while (expr);

This is a fairly simple construct. Look at the example below to fully get a grasp on do..while.

       $x = 1; // an initial value should be present for the variable that
               // will be incremented
       do
       {
         $y = $x + 1;  // let's make $y always have a value of one greater than $x
         $x++; // if this is not incremented, this will be an endless loop 
         echo "$x : $y"; // display values for $x and $y
       } while ($x <= 3); // this expression sets the loop to execute continiously
                          // until $x reaches 3

WHILE

The only difference this construct has with do..while is the possibility of bypassing the loop if the loop condition is not met. This is because truth expressions are checked even before an iteration takes place. The basic syntax is:

       while (expr) statement;

As you can see, while loop is very simple. In fact, this is the simplest type of loop in PHP. An example of this loop in action is:

       $i = 1; // initialize $i
       while ($i <= 3) // set expression where truth will be checked
       {
         echo "Value = $i"; // display current value
         $i++; // increment $i to prevent endless loop
       }

There! This ends our section on PHP loops. Remember that loops can be a powerful ally but it can also be a destructive enemy if used in the wrong sense. The general rule on loops is: "Avoid endless loops". As we have seen in the examples above, a possible way to get into this dilema is forgetting to increment (or decrement) a variable which happens to be the control variable for the loop. With your knowledge on loops, you can now create shorter codes and traverse through database tables more effectively. Of course that would be tackled on later lessons.