Loops and Arrays

for Loops

Sometimes in your programming, you will need to repeat certain operations more than once. In this section we will look at how to use for loops to control these repeating operations.

Here is a sample of a for loop:

  for ($i = 0; $i < 10; $i++)
  {
    (statement);
    (statement);
  }

Now let's talk about some of the syntax requirements of a for loop:

There are 3 parts to the for loop's statement:
1. Initializing a loop variable. This variable is sometimes called a sentry variable. The code
```
$i = 0;
```
  does this initializing.
2. Setting a condition to finish the loop. When the condition evaluates to TRUE the loop continues for at least one more iteration. When the condition evaluates to FALSE the loop stops. The code
```
$i < 10;
```
  sets this loop's condition. In this example, as long as $i is less than 10 the loop will continue.
3. Changing the sentry variable. If $i never becomes greater than or equal to 10 in this sample code, the loop will continue effectively forever. The code
```
$i++
```
  increments the variable $i by 1 each time the loop iterates. The symbol ++ is called an increment operator.
  
  It is important to NOTE that this third part of the for loop's statement is run AFTER the first two parts run once, meaning AFTER the statements in the curly braces run once.
There is a set of curly braces around the code that is run during each iteration (looping) of the loop.

This Simple for Loop page demonstrates the basics of using a for loop.

The code for the Simple for Loop page is shown below. This is file simpleFor.php:

<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <link type="text/css" rel="stylesheet" href="PHPstyles.css">
  <title>A Simple for Loop</title>
</head>

<body>
  <div id="mainDiv">
    <h1>A Simple for Loop</h1>
    
    <?php
    
    for ($i = 0; $i < 10; $i++){
      print "$i <br />\n";
    } // end for loop
    
    ?>
  </div>
</body>
</html>

Please note these points about the above code:

There is only one print statement in the page.
The print statement is executed 10 times.
The "magic" that makes this code work is in the for statement.
The code $i++ can also be written as
```
$i = $i + 1
```
but is usually written with the shorthand increment operator.

You can count by values other than 1 in a for loop.

The Counting by Fives page demonstrates how to count by five in a for loop.

The code for the Count by Fives page is shown below. This is file countByFive.php:

<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <link type="text/css" rel="stylesheet" href="PHPstyles.css">
  <title>Counting By Fives</title>
</head>

<body>
  <div id="mainDiv">
    <h1>Counting By Fives</h1>
    <p>
    <?php
    
    for ($i = 5; $i <= 50; $i += 5){
      print "$i <br />\n";
    } // end for loop
    
    ?>
    </p>
  </div>
</body>
</html>

Please note these points about the above code:

The initial value of the sentry variable is set to 5 in this code. This value simply seems to make more sense than starting at 0.
The code that increments the sentry variable,
```
$i += 5
```
uses another shortcut operator, +=. This code is equivalent to
```
$i = $i + 5
```

You can even count backwards in a for loop.

The Counting Backwards page demonstrates how to count backwards in a for loop.

The code for the Counting Backwards page is shown below. This is file countingBackwards.php:

<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <link type="text/css" rel="stylesheet" href="PHPstyles.css">
  <title>Counting Backwards</title>
</head>

<body>
  <div id="mainDiv">
    <h1>Counting Backwards</h1>
    
    <?php
    
    for ($i = 10; $i > 0; $i--){
      print "$i <br />\n";
    } // end for loop
    
    ?>
  </div>
</body>
</html>

Please note these points about the above code:

The code that decrements the sentry variable,
```
$i--
```
uses another shortcut operator, --. This code is equivalent to
```
$i = $i - 1
```

Shortcut Operators

There are other types of shortcut operators in PHP. The following table shows some shortcut operators that you can use:

Operator	Description
++	Increment (add 1)
--	Decrement (subtract 1)
+=	Add and Assign
-=	Subtract and Assign
*=	Multiply and Assign
/=	Divide and Assign
.=	Concatenate and Assign

while Loops

There is another kind of loop in PHP, which is more flexible than the for loop. This loop is a while loop.

Here is a sample of a while loop:

  $i = 0;
  while ($i < 10)
  {
    (statement);
    (statement);
    ($i++);
  }

Now let's talk about some of the syntax requirements of a while loop:

The while loop requires only one parameter, which is a condition.
If the condition initially evaluates to FALSE, the loop will never run. Sometimes in your coding, this non-action is what you want; just make sure it is the kind of behavior you want, if it occurs!
The loop will continue as long as the condition evaluates to TRUE.
When the condition evaluates to FALSE, the loop will exit.
If the condition is initially TRUE but it never evaluates to FALSE, the loop will effectively run forever. (Practically speaking, though, most PHP servers are set up to detect a runaway/infinite loop, and to stop it after a specified time period has elapsed.)
Therefore, you should always check to make sure that the code inside the loop is somehow changing the condition's variable(s) so the condition eventually evaluates to FALSE.

The Repeating Code with a while Loop page demonstrates how to use a while loop.

The code for the Repeating Code with a while Loop page is shown below. This is file while3a.php:

<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <link type="text/css" rel="stylesheet" href="PHPstyles.css">
  <title>Another Simple while Loop</title>
</head>

<body>
  <div id="mainDiv">
    <h1>A Simple while Loop 3</h1>
    
    <form method="post" action="while3b.php">
      Please type your name:
      <input type="text" name="theName">
      <br />
      <input type="submit" value="Go">
    </form>
  </div>
</body>
</html>

The action page is file while3b.php:

<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <link type="text/css" rel="stylesheet" href="PHPstyles.css">
  <title>Another Simple while Loop</title>
</head>

<body>
  <div id="mainDiv">
    <h1>A Simple while Loop 3</h1>
    
    <?php
      if ((!isset($_POST["theName"])) || ($_POST["theName"] == ""))
      {
        echo "<a href='while3a.php'>Go back and enter your name...</a>";
      }
      else
      {
        $name2 = $_POST["theName"];
        echo "The letters in $name2 are:<br />\n";
        while ($name2 != "")
        {
          $singleChar = substr($name2,0,1);
          echo "$singleChar<br />\n";
          $name2 = substr($name2,1);
        }
      }
    ?>
  </div>
</body>
</html>

Please note these points about the above while-loop pages:

The first page is a simple HTML form page. It doesn't even need to be a PHP page, technically. It could be a plain HTML page.
The PHP processing is in the second page, which is the action page for the form page.
The first thing that the PHP code does is to determine if the information from the first page's form is useable. It does this determination with the statement
```
if ((!isset($_POST["theName"])) || ($_POST["theName"] == ""))
```
and here are some notes about this statement:
- The variable $_POST is a built-in PHP array which contains information that was sent to this page from an HTML form with an attribute of method="post" in it. Go back and look at the <form> tag in the first page of this example.
- The PHP function isset() is used to test if a variable is in the memory of the PHP server. It returns true if so, and returns false if not. The main reason why $_POST would not be in memory would be if the user somehow displayed the second page, while3b.php, directly instead of using the first page's submit button.
- $_POST["theName"] will contain the text that the user typed into the form's input tag
```
<input type="text" name="theName">
```
- If the user doesn't type anything into the input field, the text in $_POST["theName"] will be an empty string, which is an empty sequence of characters, represented in PHP by two quotes next to each other, like this: "" or this: ''. (The first empty string in this sentence is two double quotes next to each other, and the second empty string is two single quotes next to each other.) If the user's input is empty, the action page puts a link back to the form page, into the display.
The text that the user typed into the input field, is assigned to the variable $name2 to make it easier for the following code to refer to this text.
The condition in the while statement is that the text in $name2 is not an empty string. Since we checked on an empty string before we got to this while loop, we are sure that the loop will run through at least one iteration (looping).
The PHP substr() built-in function extracts some characters from a string. (The name of this function means "sub-string".) In this example, we first extract the first character from the $name2 variable. In this expression:
```
substr($name2,0,1)
```
the 0 (zero) tells substr() to start with the first character in the string, and the 1 (one) tells substr() to extract only one character. (We will see more about string positions later in this handout, but for now, note that the first character position in PHP is at location 0 (zero).)
After the echo() statement puts the character into the display, we then throw away the first character from the variable $name2 by using substr() a second time. We accomplish this trimming by telling substr() to start with the second character (at string index 1), and to extract characters all the way through the end of the string (because we left off the number that tells substr() how many characters to extract, and it defaults to everything that is still in the string).

do while Loops

do while loops are very similar to while loops, except that the condition is checked at the end of each iteration (looping) instead of at the beginning.

The main difference from regular while loops is that the first iteration of a do while loop is guaranteed to run (since the condition is only checked at the end of the iteration).

Here is a sample of a do while loop:

  $i = 0;
  do
  {
    (statement);
    (statement);
    ($i++);
  }
  while ($i < 10);

Now let's talk about some of the syntax requirements of a do while loop:

The code inside the curly braces will run at least once.
The do while loop requires only one parameter, which is a condition.
The condition is checked after the code inside the curly braces runs.
The loop will continue as long as the condition evaluates to TRUE.
When the condition evaluates to FALSE, the loop will exit.
If the condition is initially TRUE but it never evaluates to FALSE, the loop will effectively run forever. (Practically speaking, though, most PHP servers are set up to detect a runaway/infinite loop, and to stop it after a specified time period has elapsed.)
Therefore, you should always check to make sure that the code inside the loop is somehow changing the condition's variable(s) so the condition eventually evaluates to FALSE.

The Repeating Code with a do while Loop page demonstrates how to use a do-while loop.

The code for the Repeating Code with a do while Loop page is shown below. This is file do_while.php:

<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <link type="text/css" rel="stylesheet" href="PHPstyles.css">
  <title>A Simple while Loop</title>
</head>

<body>
  <div id="mainDiv">
    <h1>A Simple do while Loop</h1>
    
    <?php
    
    $i = 0;
    do
    {
      print "$i <br />\n";
      $i++;
    }
    while ($i < 10);
    
    ?>
  </div> 
</body>
</html>

Arrays

Now you know something about various ways of making your PHP code run in loops.

So now it is time to show you arrays, which by design work very well with loops.

Arrays are special variables that are made to hold lists of information.

You can create an array in PHP simply by naming/creating a variable and putting an optional number in square brackets after the variable name. Here is an example:

$teams[0] = "Cowboys";
$teams[1] = "Giants";
$teams[2] = "Eagles";
$teams[3] = "Redskins";

You can also leave the square brackets empty. PHP will automatically supply the next number in the sequence as the index. Here is an example:

$teams[] = "Cardinals";
$teams[] = "Seahawks";
$teams[] = "Chargers";

You can also use the array( ) built-in function to make an array. Here is an example:

$binary = array("000", "001", "010", "011");

Here is a very important point: The first index in a PHP array is 0 (zero), not 1. This is how most computer languages index their arrays.

Now here is a sample page, the Basic Array page, which demonstrates two arrays.

The code for the Basic Array page is shown below. This is file basicArray.php:

<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <link type="text/css" rel="stylesheet" href="PHPstyles.css">
  <title>Basic Array</title>
</head>

<body>
  <div id="mainDiv">
    <h1>Basic Arrays</h1>
    
    <?php
    
    //Simply assign values to an array
    $teams[0] = "Cowboys";
    $teams[1] = "Giants";
    $teams[2] = "Eagles";
    $teams[3] = "Redskins";
    $teams[] = "Cardinals";
    $teams[] = "Seahawks";
    $teams[] = "Chargers";
    
    //output array values
    print "<h3>Some NFL Teams</h3>\n";
    for ($i = 0; $i < 6; $i++){
      $realNum = $i + 1;
      print "$realNum: $teams[$i]<br>\n";
    } // end for loop
    
    //Use the array() function to make an array
    $binary = array("000", "001", "010", "011");
    
    print "<h3>Binary numbers</h3>\n";
    for ($i = 0; $i < count($binary); $i++){
      print "$i: $binary[$i]<br>\n";
    } // end for loop
    
    ?>
  </div>
</body>
</html>

Please note these points about the above code:

There are two loops in the code that are used to display the contents of the array variables.
The first loop has a logic bug in it. Can you spot it? (HINT: Look again at the page's display in a browser. How many items are displayed in the first list? How many items are in the array?)
The second loop uses the count( ) function to get the number of items (also called elements) in the array. I highly recommend that you use count() when you can. It eliminates logic bugs such as that in the first loop.
When you use the count( ) function to limit a for loop, you almost always use the less-than operator (<) to compare the loop variable with the value that count( ) returns. The reason for using < is that you don't want the array's index to go beyond the end of the array. The count( ) function returns the number of elements in the array, but the index of the last element in the array is one less than the number of elements in the array.

The Fancy Old Man page demonstrates some of the power of using arrays.

The code for the Fancy Old Man page is shown below. This is file FancyOldMan.php:

<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <link type="text/css" rel="stylesheet" href="PHPstyles.css">
  <title>Fancy Old Man</title>
</head>
<body>
  <div id="mainDiv">
    <h1>Fancy Old Man (with Arrays)</h1>
    <pre>
    <?php
    $place = array(
      "on my thumb",
      "on my shoe",
      "on my knee",
      "on my door");
      
    //print out song
    for ($verse = 0; $verse < count($place); $verse++)
    {
      $versenum = $verse + 1;
      print <<<HERE

      This old man, He played $versenum
      He played knick-knack $place[$verse]
      ...with a knick, knack, paddy-whack
      give a dog a bone
      This old man came rolling home 

HERE;
      } // end for loop
    ?>
    </pre>
  </div>
</body>
</html>

Please note these points about the above code:

The variable $versenum is used to "translate" the array index $verse (which, as a reminder, starts with 0 (zero) for the first item in the array) into the "ordinal" number of each verse.
The print statement uses a multi-line string to put each verse into the page.

Handling Images with Arrays

Sometimes you need to display images in a Web page in a situation where an array and a loop of some kind would be very handy.

It is not efficient to store the image files themselves in an array. But it is efficient to store the names of the image files in an array. You can even store just a portion of the file names if you want to save a little bit of memory storage on the server.

Here is an example of storing image file names in an array, and using the array to display the images in the page.

And now here is the code for this page. This is file imageArray.php:

<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <link type="text/css" rel="stylesheet" href="PHPstyles.css">
  <title>An Array of Images</title>
</head>

<body>
  <div id="mainDiv">
    <h3>Some Springtime Images</h3>
    <?php
      $imageArray = array("SpringFlowersPink_th","AlleySpring_th",
                          "AlleySpringMill_th","buds2_th");
      
      for ($i = 0; $i < count($imageArray); $i++)
      {
        $fileName = "images/" . $imageArray[$i] . ".jpg";
        print "<img src='$fileName' width='140' height='105' alt='Spring Images' />\n";
        print "   \n";
      }
    ?>
  </div>
</body>
</html>

Please note these points about the above code:

All of the images in this page have the same file extension, .jpg
The "_th" portion of the image file names is not required, but it is a convention among some designers to name small, "thumbnail" images with "_th" at the end of the file name (and before the file name extension, of course).
The PHP concatenation (string addition) operator is the period (.).
This code makes use of PHP's string interpolation feature, also, in the print statement that puts the <img> tags into the page.

Using an Array as Index Storage

Sometimes you need to refer to the elements in an array in some other order than lowest to highest index. One way to use a different order is to store the indexes in another array, and then use a normal for loop on this additional array to get the indexes into the first array.

It gets even more interesting if you need to refer to the first array in random index order.

Here is a page which shows how to use an additional array to store indexes into the first array. The indexes will be in random order.

Here is the code: This is page indirectIndex.php:

<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <link type="text/css" rel="stylesheet" href="PHPstyles.css">
  <title>Indirect Array Indexes</title>
</head>

<body>
  <div id="mainDiv">
    <h3>Some random images:</h3>
    <?php
      $indexArray = array(0,0,0,0,0);
      $imageArray = array("SpringFlowersPink_th","AlleySpring_th",
                          "AlleySpringMill_th","buds2_th");
      
      // Make an array of indexes:
      for ($i = 0; $i < count($indexArray); $i++)
      {
        $indexNumber = rand(0,count($imageArray)-1);
        $indexArray[$i] = $indexNumber;
      }
      
      // Display the images in the order of the indexes in the $indexArray:
      for ($i = 0; $i < count($indexArray); $i++)
      {
        $imageIndex = $indexArray[$i];
        $imageName = "images/" . $imageArray[$imageIndex] . ".jpg";
        print "<img src='$imageName' width='140' height='105' alt='Spring Images' />\n";
        print "   \n";
      }
    ?>
    <h3>(Refresh the page for a new set of images.)</h3>
  </div>
</body>
</html>

Please note these points about the above code:

There are 4 images in the $imageArray array. But we want to display 5 images at a time. So the $indexArray has 5 elements.
The $indexArray starts off with all zeroes. I needed to put something in the array to start with so the count( ) function would return the number of images that we want to display.
The rand( ) function has arguments (0,count($imageArray)-1) (note the -1) because the highest index in the $imageArray is one less than the number of images.
This code does not guarantee that there will be no index duplicates in $indexArray.

Richland College Multimedia Learning Center

Loops and Arrays

for Loops

Shortcut Operators

while Loops

do while Loops

Arrays

Handling Images with Arrays

Using an Array as Index Storage