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eg-259:lecture17 [2008/11/13 18:58] – external edit 127.0.0.1eg-259:lecture17 [2013/03/06 19:22] – [Introduction to PHP (Part 2)] eechris
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 +
 +~~SLIDESHOW~~
 +====== Introduction to PHP (Part 2) ======
 +
 +**Contact Hour 22**: To be discussed on Wednesday 20th March, 2013.
 +
 +**Lecturer**: [[C.P.Jobling@Swansea.ac.uk|Dr Chris P. Jobling]].
 +
 +More on the language features of PHP
 +
 +
 +
 +
 +===== Introduction to PHP (Part 2) =====
 +
 +We continue our review of the Basics of PHP with a discussion of //arrays//, //functions// and //regular expressions//.
 +
 +----
 +
 +Based on Chapter 12 of Robert W. Sebasta, //Programming the World-Wide Web//, 3rd Edition, Addison Wesley, 2006. and Chapter 12 of Chris Bates, //Web Programming: Building Internet Applications//, 3rd Edition, John Wiley, 2006.
 +
 +===== Contents of this Lecture =====
 +
 +//Further features of the PHP language with examples//
 +
 +  * [[eg-259:lecture17#arrays|Arrays]] 
 +  * [[eg-259:lecture17#functions|Functions]]
 +  * [[eg-259:lecture17#pattern_matching|Pattern Matching]]
 +
 +The [[eg-259:examples:php1#examples_from_lecture_17|Examples for PHP Part 2]].
 +
 +
 +===== Learning Outcomes ====
 +
 +//At the end of this lecture you should be able to answer these questions//:
 +
 +  - In what two ways can arrays in PHP be created? 
 +  - What keys are used when an array is created but no keys are specified? 
 +  - Must all of the values of an array be of the same type? 
 +  - Must all of the keys of an array be of the same type? 
 +  - What exactly do the ''array_keys'' and ''array_values'' functions do? 
 +
 +
 +
 +
 +
 +===== Learning Outcomes (continued) =====
 +
 +//At the end of this lecture you should be able to answer these questions//:
 +
 +  - What exactly does the ''in_array'' function do? 
 +  - Explain the actions of the ''implode'' and ''explode'' functions. 
 +  - Describe the actions of the ''next'', ''reset'', and ''prev'' functions. 
 +  - What are the syntax and semantics of the two forms of the ''foreach'' statement? 
 +  - Describe the result of using the ''sort'' function on an array that has both string and numeric values. 
 +
 +
 +===== Learning Outcomes (continued) =====
 +
 +//At the end of this lecture you should be able to answer these questions//:
 +
 +  - What is the difference between the ''sort'' and ''asort'' functions? 
 +  - What happens if a script defines the same function more than once? 
 +  - Are function names case sensitive? 
 +  - What value is returned by a function if its execution does not end by executing a ''return'' statement? 
 +
 +
 +
 +===== Learning Outcomes (continued) =====
 +
 +//At the end of this lecture you should be able to answer these questions//:
 +
 +  - What are the two ways you can specify that a parameter is to be passed by reference? 
 +  - How can a variable used outside a function be accessed by the function? 
 +  - How can you define a variable in a function so that its lifetime extends beyond the time the function is in its first execution? 
 +
 +
 +===== Arrays =====
 +
 +  * Not like the arrays of any other programming language 
 +  * A PHP array is a //generalization// of the arrays of other languages 
 +  * A PHP array is really a mapping of keys to values, where the keys can be numbers (to get a //traditional// array) or strings (to get a //associative// array) 
 +
 +
 +===== Array creation =====
 +
 +  * Use the ''array()'' construct, which takes one or more ''key => value'' pairs as parameters and returns an array of them 
 +  * The keys are non-negative integer literals or string literals 
 +  * The values can be anything 
 +  * //Example//:
 +<code php>
 +    $list = array(0 => "apples", 1 => "oranges", 2 => "grapes")
 +</code>
 +  * This is a "regular" array of strings
 +
 +
 +===== Array creation (continued) =====
 +
 +  * If a key is omitted and there have been integer keys, the default key will be the largest current key + 1 
 +  * If a key is omitted and there have been no integer keys, 0 is the default key 
 +  * If a key appears that has already appeared, the new value will overwrite the old one 
 +
 +
 +
 +===== Array elements =====
 +
 +  * Arrays can have mixed kinds of elements
 +  * //Some examples//:
 +<code php>
 +    $list = array("make" => "Cessna", "model" => "C210", 
 +                  "year" => 1960, 3 => "sold");
 +
 +    $list = array(1, 3, 5, 7, 9);
 +
 +    $list = array(5, 3 => 7, 5 => 10, "month" => "May");
 +
 +    $colours = array('red', 'blue', 'green', 'yellow');
 +</code>
 +
 +
 +===== Accessing array elements =====
 +
 +  * //Use brackets//:
 +<code php>
 +    $list[4] = 7;
 +    $list["day"] = "Tuesday";
 +    $list[] = 17;
 +</code>
 +  * If an element with the specified key does not exist, it is created
 +  * If the array does not exist, the array is created
 +
 +
 +===== Accessing array elements =====
 +
 +  * The keys or values can be extracted from an array:
 +<code php>
 +    $highs = array("Mon" => 74, "Tue" => 70,
 +                   "Wed" => 67, "Thu" => 62,
 +                   "Fri" => 65);
 +    $days = array_keys($highs);
 +    $temps = array_values($highs);
 +</code>
 +
 +
 +
 +===== Dealing with Arrays =====
 +
 +  * An array can be deleted with ''unset'':
 +<code php>
 +    unset($list);
 +    unset($list[4]);  # No index 4 element now
 +</code>
 +  * ''is_array($list)'' returns ''true'' if ''$list'' is an array
 +  * ''in_array(17, $list)'' returns true if 17 is an element of ''$list''
 +
 +
 +===== Dealing with Arrays (continued) =====
 +
 +  * ''explode(" ", $str)'' creates an array with the values of the words from ''$str'', split on a space 
 +  * ''implode(" ", $list)'' creates a string of the elements from ''$list'', separated by a space 
 +
 +
 +
 +
 +===== Internal structure of arrays  =====
 +
 +{{eg-259:l17-logical-structure.png|The logical internal structure of arrays in PHP}}
 + 
 +
 +
 +===== Sequential access to array elements =====
 +
 +  * Use ''current'' and ''next'':
 +<code php>
 +    $colours = array("blue", "red", "green", "yellow");
 +    $colour = current($colours);
 +    print("$colour <br />");
 +    while ( $colour = next($colours) ) {
 +      print ("$colour <br />");
 +    }
 +</code>
 +  * This does not always work -- for example, if the value in the array happens to be ''FALSE''
 +
 +
 +
 +===== Sequential access to array elements (continued) =====
 +
 +  * //Alternative//: use ''each'', instead of ''next'':
 +<code php>
 +    while ( $element = each($colours) ) {
 +      print ("$element['value'] <br />");
 +    }
 +</code>
 +  * The ''prev'' function moves ''current'' backwards
 +
 +===== Sequential access to array elements (continued) =====
 +
 +  * To implement stacks with an array use ''array_push($list, $element)'' and ''array_pop($list)'' 
 +
 +
 +===== Sequential access to array elements (continued) =====
 +
 +  * To visit every element of an array use ''foreach (array_name as scalar_name) { ... }:''
 +<code php>
 +    foreach ($colours as $colour) {
 +      print "Is $colour your favourite colour?<br />";
 +    }
 +</code>
 +  * //produces//:
 +  
 +  Is red your favourite colour?
 +  Is blue your favourite colour?
 +  Is green your favourite colour?
 +  Is yellow your favourite colour?
 +
 +
 +
 +===== Sequential access to array elements (continued) =====
 +
 +  * ''foreach'' can iterate through both //keys// and //values//:
 +<code php>
 +    foreach ($colours as $key => $colour) { ... }
 +</code>
 +  * Inside the compound statement, both ''$key'' and ''$colour'' are defined
 +  * //Example//:
 +<code php>
 +    $ages = array("Bob" => 42, "Mary" => 43);
 +    foreach ($ages as $name => $age) {
 +      print("$name is $age years old <br />");
 +    }
 +</code>
 +
 +
 +===== Sorting Arrays =====
 +
 +  * ''sort'' sorts the values of an array, leaving the keys in their present order -- intended for traditional arrays
 +    *e.g., ''sort($list);''
 +  * The ''sort'' function does not return anything
 +  * Works for both strings and numbers, even mixed strings and numbers:
 +<code php>
 +    $list = ('h', 100, 'c', 20, 'a');
 +    sort($list);
 +    // Produces ('a', 'c', 'h'‘, 20, 100)
 +</code>
 +
 +
 +===== Sorting Arrays (continued) =====
 +
 +  * In PHP 4+, the sort function can take a second parameter, which specifies a particular kind of sort:
 +<code php>
 +    sort($list, SORT_NUMERIC);
 +</code>
 +
 +
 +===== More sorting functions =====
 +
 +  * ''asort'' sorts the values of an array, while keeping the key/value relationships -- intended for associative arrays
 +  * ''rsort'' sorts the values of an array into reverse order
 +  * ''ksort'' sorts the elements of an array by the keys, maintaining the key/value relationships e.g.:
 +<code php>
 +    $list("Fred" => 17, "Mary" => 21, "Bob" => 49, "Jill" => 28);
 +    ksort($list);
 +    // $list is now ("Bob" => 49, "Fred" => 17, "Jill" => 28, "Mary" => 21)
 +</code>
 +  * ''krsort'' sorts the elements of an array by the keys into reverse order
 +
 +
 +
 +
 +
 +
 +
 +===== Example of Array Sorting =====
 +
 +  * An example to illustrate several of the sorting functions: [[/eg-259/examples/lecture17/sorting.php|sorting.php]] ([[http://localhost/eg-259/examples/lecture17/sorting.php|sorting.php @ localhost]])
 +
 +----
 +
 +   * //Code//:
 +<code php>
 +<<!DOCTYPE html>
 +
 +<!-- sorting.php - An example to illustrate several of the
 +     sorting functions -->
 +<html lang="en">
 +  <head> 
 +    <meta charset="utf-8" />
 +    <title> sorting.php - An example to illustrate several of the
 +     sorting functions </title>
 +  </head>
 +  <body>
 +    <?php
 +       $original = array("Fred" => 31, "Al" => 27,
 +                         "Gandalf" => "wizzard",
 +                         "Betty" => 42, "Frodo" => "hobbit");
 +    ?>
 +    <h4> Original Array </h4>
 +    <?php
 +      foreach ($original as $key => $value) {
 +         print("[$key] => $value <br />");
 +      }
 +    ?>
 +    <h4> Array sorted with sort </h4>
 +    <?php
 +      $new = $original;
 +      sort($new);
 +       foreach ($new as $key => $value) {
 +         print("[$key] = $value <br />");
 +       }
 +    ?>
 +    <h4> Array sorted with asort </h4>
 +    <?php
 +       $new = $original;
 +       asort($new);
 +       foreach ($new as $key => $value) {
 +         print("[$key] = $value <br />");
 +       }
 +    ?>
 +    <h4> Array sorted with ksort </h4>
 +    <?php
 +       $new = $original;
 +       ksort($new);
 +       foreach ($new as $key => $value) {
 +         print("[$key] = $value <br />");
 +       }
 +    ?>
 +  </body>
 +</html> 
 +</code>
 +  * //Result//:
 +
 + {{eg-259:l17-sorting.png|Result of running the sort.php example}}
 +
 +
 +
 +===== User-Defined Functions =====
 +
 +  * //Syntactic form//:
 +<code php>
 +    function function_name( [formal_parameters] ) {
 +      ...
 +    }
 +</code>
 +
 +
 +===== General Characteristics of Functions =====
 +
 +  * Functions need not be defined before they are called (in PHP 3, they must) 
 +  * Function overloading is not supported((PPHP 5 supports classes, so it may be possible to use overloading for polymorphism)) 
 +  * If you try to redefine a function, it is an error 
 +  * Functions can have a variable number of parameters 
 +  * Default parameter values are supported 
 +  * Function definitions can be nested 
 +  * Function names are not case sensitive 
 +  * The ''return'' function is used to return a value; if there is no ''return'', there is no returned value 
 +
 +
 +===== Function Parameters =====
 +
 +  * If the caller sends too many actual parameters, the subprogram ignores the extra ones 
 +  * If the caller does not send enough parameters, the unmatched formal parameters are ''unbound'' 
 +  * The default parameter passing method is //pass by value// (one-way communication) 
 +
 +===== Function Parameters (continued) =====
 +
 +  * To specify //pass-by-reference//, prepend an ampersand to the formal parameter:
 +<code php>
 +    function addOne(&$param) {
 +       $param++;
 +    }
 +
 +    $it = 16;
 +    addOne($it);  // $it is now 17
 +</code>
 +
 +===== Function Parameters (continued) =====
 +
 +  * If the function does not specify its parameter to be pass by reference, you can prepend an ampersand to the actual parameter and still get pass-by-reference semantics:
 +<code php>
 +    function subOne($param) {
 +      $param--;
 +    }
 +    
 +    $it = 16;
 +    subOne(&$it);   // $it is now 15
 +</code>
 +
 +===== Return Values =====
 +
 +  * Any type may be returned, including objects and arrays, using the ''return''
 +  * If a function returns a reference, the name of the function must have a prepended ampersand:
 +<code php>
 +   function &newArray($x) { ... }
 +</code>
 +
 +===== The Scope of Variables =====
 +
 +  * An undeclared variable in a function has the scope of the function
 +  * To access a nonlocal variable, it must be declared to be //global//, as in:
 +<code php>
 +   global $sum;
 +</code>
 +
 +
 +===== The Lifetime of Variables =====
 +
 +  * Normally, the lifetime of a variable in a function is from its first appearance to the end of the function's execution:
 +<code php>
 +    static $sum = 0;  # $sum is static
 +</code>
 +
 +
 +===== Pattern Matching =====
 +
 +  * PHP has two kinds:
 +    * POSIX 
 +    * Perl-compatible (''preg'') 
 +<code php>
 +        preg_match(regex, str [,array])
 +</code>
 +  * The optional ''array'' is where to put the matches 
 +
 +
 +===== Example =====
 +
 +The [[/eg-259/examples/lecture17/word_table.php|word_table.php]] ([[http://localhost/eg-259/examples/lecture17/word_table.php|word_table.php @ localhost]]) example illustrates the use of functions and regular expressions
 +
 +----
 +
 +  * //Code//:
 +<code php>
 +<!DOCTYPE html>
 +<!--    word_table.php
 +
 +Uses a function to split a given string of text into
 +its constituent words. It also determines the frequency of
 +occurrence of each word. The words are separated by
 +whitespace or punctuation, possibly followed by whitespace.
 +The punctuation can be a period, a comma, a semicolon, a
 +colon, an exclamation point, or a question mark.
 +The main driver program calls the function and displays
 +the results.
 +-->
 +<html lang="en">
 +  <head>
 +    <meta charset="utf-8" />
 +    <title> Word Table </title>
 +  </head>
 +  <body>
 +    <?php
 +
 +    // Function splitter
 +    //  Parameter: a string of text containing words and punctuation
 +    //  Returns: an array in which the unique words of the string are
 +    //           the keys and their frequencies are the values.
 +
 +    function splitter($str) {
 +
 +      // Create the empty word frequency array
 +
 +      $freq = array();
 +
 +      // Split the parameter string into words
 +
 +      $words = preg_split("/[ .,;:!?]\s*/", $str);
 +
 +      // Loop to count the words (either increment or initialize to 1)
 +
 +      foreach ($words as $word) {
 +        $keys = array_keys($freq);
 +        if (in_array($word, $keys))
 +          $freq[$word]++;
 +        else
 +          $freq[$word] = 1;
 +      }
 +
 +      return $freq;
 +    }#** End of splitter
 +
 +    // Main test driver
 +
 +    $str = "apples are good for you, or don't you like apples?
 +or maybe you like oranges better than apples";
 +
 +    // Call splitter
 +
 +    $tbl = splitter($str);
 +
 +    // Display the words and their frequencies
 +
 +    print "<br /> Word Frequency <br /><br />";
 +    $sorted_keys = array_keys($tbl);
 +    sort($sorted_keys);
 +    foreach ($sorted_keys as $word) {
 +      print "$word $tbl[$word] <br />";
 +    }
 +  ?>
 +  </body>
 +</html>
 +</code>
 +
 +  * Output of the example script
 +
 +  Word Frequency
 +  
 +  apples 3
 +  are 1
 +  better 1
 +  don't 1
 +  for 1
 +  good 1
 +  like 2
 +  maybe 1
 +  or 2
 +  oranges 1
 +  than 1
 +  you 3
 +
 +===== Summary of this Lecture =====
 +
 +//Further features of the PHP language with examples//
 +
 +  * [[eg-259:lecture17#arrays|Arrays]] 
 +  * [[eg-259:lecture17#functions|Functions]]
 +  * [[eg-259:lecture17#pattern_matching|Pattern Matching]]
 +
 +The [[eg-259:examples:php1#examples_from_lecture_17|Examples for PHP Part 2]].
 +
 +===== Learning Outcomes ====
 +
 +//At the end of this lecture you should be able to answer these questions//:
 +
 +  - In what two ways can arrays in PHP be created? 
 +  - What keys are used when an array is created but no keys are specified? 
 +  - Must all of the values of an array be of the same type? 
 +  - Must all of the keys of an array be of the same type? 
 +  - What exactly do the ''array_keys'' and ''array_values'' functions do? 
 +
 +
 +
 +
 +
 +===== Learning Outcomes (continued) =====
 +
 +//At the end of this lecture you should be able to answer these questions//:
 +
 +  - What exactly does the ''in_array'' function do? 
 +  - Explain the actions of the ''implode'' and ''explode'' functions. 
 +  - Describe the actions of the ''next'', ''reset'', and ''prev'' functions. 
 +  - What are the syntax and semantics of the two forms of the ''foreach'' statement? 
 +  - Describe the result of using the ''sort'' function on an array that has both string and numeric values. 
 +
 +
 +===== Learning Outcomes (continued) =====
 +
 +//At the end of this lecture you should be able to answer these questions//:
 +
 +  - What is the difference between the ''sort'' and ''asort'' functions? 
 +  - What happens if a script defines the same function more than once? 
 +  - Are function names case sensitive? 
 +  - What value is returned by a function if its execution does not end by executing a ''return'' statement? 
 +
 +
 +
 +===== Learning Outcomes (continued) =====
 +
 +//At the end of this lecture you should be able to answer these questions//:
 +
 +  - What are the two ways you can specify that a parameter is to be passed by reference? 
 +  - How can a variable used outside a function be accessed by the function? 
 +  - How can you define a variable in a function so that its lifetime extends beyond the time the function is in its first execution? 
 +
 +
 +===== Homework Exercises =====
 +
 +Write the following PHP functions and code to test them
 +
 +  - //Parameter//: An array of strings. //Return value//: A list of the unique strings in the parameter array.
 +  - //Parameter//: an array of numbers. //Return value//: The average and median of the parameter array.
 +  - //Parameter//: A string of numbers separated by spaces. //Return value//: The first four-digit number in the string.
 +
 +
 +===== Homework Exercises (continued) =====
 +
 +  - Modify the sample script ''word_table.php'' discussed in this lecture to place the output table in an XHTML table. 
 +
 +These and other PHP exercises are also available on the [[eg-259:homework:17|homework page]]. 
 +
 +
 +
 +
 +
 +
 +
 +
 +===== What's Next? =====
 +
 +**PHP for Web Applications**
 +
 +    * [[eg-259:lecture18#queries_and_query_strings|Queries and Query Strings]]
 +    * [[eg-259:lecture18#form_handling|Form Handling]]
 +    * [[eg-259:lecture18#files|Files]]
 +    * [[eg-259:lecture18#storing_information_about_client_state|Storing Information about Client State]]
 +    * [[eg-259:lecture18#session_tracking|Session Tracking]]
 +
 +
 +
 +[[eg-259:lecture16|Previous Lecture]] | [[eg-259:home]] | [[eg-259:lecture18|Next Lecture]]
eg-259/lecture17.txt · Last modified: 2013/03/08 18:04 by eechris