程序代写代做代考 Java data structure javascript COMP284 Scripting Languages – Handouts

COMP284 Scripting Languages – Handouts

COMP284 Scripting Languages
Lecture 10: PHP (Part 2)

Handouts

Ullrich Hustadt

Department of Computer Science
School of Electrical Engineering, Electronics, and Computer Science

University of Liverpool

Contents

1 Scalar types
Integers and Floating-point numbers
Exceptions and error handling
Booleans
Strings

2 Compound types
Arrays
Foreach-loops
Array functions

3 Printing

COMP284 Scripting Languages Lecture 10 Slide L10 – 1

Scalar types Integers and Floating-point numbers

Integers and Floating-point numbers

• PHP distinguishes between
• integer numbers 0 2012 -40 1263978
• floating-point numbers 1.25 256.0 -12e19 2.4e-10

• PHP supports a wide range of pre-defined mathematical functions
abs(number) absolute value
ceil(number) round fractions up
floor(number) round fractions down
round(number [,prec,mode]) round fractions
log(number [,base]) logarithm
rand(min,max) generate an integer random number
sqrt(number) square root

• PHP provides a range of pre-defined number constants including
M_PI 3.14159265358979323846
NAN ‘not a number’
INF ‘infinity’

COMP284 Scripting Languages Lecture 10 Slide L10 – 2

Scalar types Integers and Floating-point numbers

Integers and Floating-point numbers: NAN and INF

The constants NAN and INF are used as return values for some applications
of mathematical functions that do not return a number

• log(0) returns -INF (negative ‘infinity’)
• sqrt(-1) returns NAN (‘not a number’)

In contrast

• 1/0 returns FALSE and produces an error message
• 0/0 returns FALSE and produces an error message
and execution of the script continues!

In PHP 7

• 1/0 returns INF and produces an error message
• 0/0 returns NAN and produces an error message
and execution of the script continues!

COMP284 Scripting Languages Lecture 10 Slide L10 – 3

Scalar types Integers and Floating-point numbers

Integers and Floating-point numbers: NAN and INF

NAN and INF can be compared with each other and other numbers using
equality and comparison operators:

NAN == NAN ; FALSE NAN === NAN ; FALSE NAN == 1 ; FALSE
INF == INF ; FALSE INF === INF ; TRUE INF == 1 ; FALSE
NAN < NAN ; TRUE INF < INF ; TRUE 1 < INF ; TRUE NAN < INF ; TRUE INF < NAN ; TRUE INF < 1 ; FALSE NAN < 1 ; TRUE 1 < NAN ; TRUE In PHP 5.3 and earlier versions, INF == INF returns FALSE In PHP 5.4 and later versions, INF == INF returns TRUE COMP284 Scripting Languages Lecture 10 Slide L10 – 4 Scalar types Integers and Floating-point numbers Integers and Floating-point numbers: NAN and INF • PHP provides three functions to test whether a value is or is not NAN, INF or -INF: • bool is_nan(value) returns TRUE iff value is NAN • bool is_infinite(value) returns TRUE iff value is INF or -INF • bool is_finite(value) returns TRUE iff value is neither NAN nor INF/-INF • In conversion to a boolean value, both NAN and INF are converted to TRUE • In conversion to a string, NAN converts to ’NAN’ and INF converts to ’INF’ COMP284 Scripting Languages Lecture 10 Slide L10 – 5 Scalar types Exceptions and error handling Exceptions and error handling PHP distinguishes between exceptions and errors • A possible way to perform exception handling in PHP is as follows: try { ... run code here ... // try } catch (Exception $e) { ... handle the exception here using $e // catch } • Errors must be dealt with by an error handling function (‘Division by zero’ produces an error not an exception) One possible approach is to let the error handling function turn errors into exceptions function exception_error_handler($errno , $errstr , $errfile , $errline ) { throw new ErrorException($errstr , $errno , 0, $errfile , $errline ); } set_error_handler("exception_error_handler"); http://www.php.net/manual/en/class.errorexception.php COMP284 Scripting Languages Lecture 10 Slide L10 – 6 http://www.php.net/manual/en/class.errorexception.php Scalar types Booleans Booleans • Unlike Perl, PHP does have a boolean datatype with constants TRUE and FALSE (case insensitive) • PHP offers the same short-circuit boolean operators as Java and Perl: && (conjunction) || (disjunction) ! (negation) • Alternatively, and and or can be used instead of && and ||, respectively • However, not is not a PHP operator • The truth tables for these operators are the same as for Perl • Remember that && and || are not commutative, that is, (A && B) is not the same as (B && A) (A || B) is not the same as (B || A) COMP284 Scripting Languages Lecture 10 Slide L10 – 7 Scalar types Booleans Type conversion to boolean When converting to boolean, the following values are considered FALSE: • the boolean FALSE itself • the integer 0 (zero) • the float 0.0 (zero) • the empty string, and the string ’0’ • an array with zero elements • an object with zero member variables (PHP 4 only) • the special type NULL (including unset variables) • SimpleXML objects created from empty tags Every other value is considered TRUE (including any resource) COMP284 Scripting Languages Lecture 10 Slide L10 – 8 Scalar types Strings Strings • PHP supports both single-quoted and double-quoted strings • PHP also supports heredocs as a means to specify multi-line strings The only difference to Perl is the use of <<< instead of << in their definition: <<

’;

print <<Some text

EOF;

COMP284 Scripting Languages Lecture 10 Slide L10 – 9

Scalar types Strings

Strings

• Variable interpolation is applied to double-quoted strings
(with slight differences to Perl)

• The string concatenation operator is denoted by ‘.’ (as in Perl)
• Instead of Perl’s string multiplication operator ‘x’ there is
string str_repeat(string_arg, number)

• There are no built-in HTML shortcuts in PHP

$title = “String Multiplication”;

$string = “

I shall not repeat myself.

”;

print ”

“.str_repeat($string ,3).’’;

I shall not repeat myself.

I shall not repeat myself.

I shall not repeat myself.

COMP284 Scripting Languages Lecture 10 Slide L10 – 10

Compound types Arrays

Arrays

• PHP only supports associative arrays (hashes), simply called arrays
• PHP arrays are created using the array construct or,

since PHP 5.4, [ … ]:

array(key => value , … )

[key => value , …]

where key is an integer or string and value can be of any type,
including arrays

$arr1 = [1 => “Peter”, 3 => 2009, “a” => 101];

$arr2 = array (200846369 => array(“name” => “Jan Olsen”,

“COMP101” => 69,

“COMP102” => 52));

• The size of an array can be determined using the count function:
int count(array [, mode])

print count($arr1); // prints 3

print count($arr2); // prints 1

print count($arr2 ,1); // prints 4

COMP284 Scripting Languages Lecture 10 Slide L10 – 11

Compound types Arrays

Arrays

• It is possible to omit the keys when using the array construct:
$arr3 = array(“Peter”, “Paul”, “Mary”);

The values given in array will then be associated with the
natural numbers 0, 1, . . .

• All the keys of an array can be retrieved using
array_keys($array1)

; returns a natural number-indexed array containing
the keys of $array1

• All the values of an array can be retrieved using
array_values($array1)

; returns a natural number-indexed array containing
the values stored in $array1

COMP284 Scripting Languages Lecture 10 Slide L10 – 12

Compound types Arrays

Arrays

• An individual array element can be accessed via its key
• Accessing an undefined key produces an error message

and returns NULL

$arr1 = array(1 => “Peter”, 3 => 2009, “a”=> 101);

print “’a’: “.$arr1[“a”].”
”;

’a’: 101

print “’b’: “.$arr1[“b”].”
”;

PHP Notice: Undefined index: b in on line

’b’: // $arr1[“b”] returns NULL

$arr1[’b’] = 102;

print “’b’: “.$arr1[“b”].”
”;

’b’: 102

COMP284 Scripting Languages Lecture 10 Slide L10 – 13

Compound types Arrays

Arrays

• PHP allows the construct
$array[] = value;

PHP will determine the maximum value M among the integer indices in
$array and use the key K = M + 1; if there are no integer indices in
$array, then K = 0 will be used
; auto-increment for array keys

$arr4 [] = 51; // 0 => 51

$arr4 [] = 42; // 1 => 42

$arr4 [] = 33; // 2 => 33

• A key-value pair can be removed from an array using the
unset function:

$arr1 = array(1 => “Peter”, 3 => 2009, “a” => 101);

unset($arr1 [3]); // Removes the pair 3 => 2009

unset($arr1 ); // Removes the whole array

COMP284 Scripting Languages Lecture 10 Slide L10 – 14

Compound types Foreach-loops

Arrays: foreach-loop

• PHP provides a foreach-loop construct to ‘loop’ through the elements
of an array

• Syntax and semantics is slightly different from that of the corresponding
construct in Perl

foreach (array as $value)

statement

foreach (array as $key => $value)

statement

• array is an array expression
• $key and $value are two variables, storing a different key-value pair in
array at each iteration of the foreach-loop

• We call $value the foreach-variable
• foreach iterates through an array in the order in which elements were

defined

COMP284 Scripting Languages Lecture 10 Slide L10 – 15

Compound types Foreach-loops

Arrays: foreach-loop

foreach iterates through an array in the order in which elements were
defined

Example 1:
foreach (array(“Peter”, “Paul”, “Mary”) as $key => $value)

print “The array maps $key to $value
”;

The array maps 0 to Peter

The array maps 1 to Paul

The array maps 2 to Mary

Example 2:
$arr5 [2] = “Marry”;

$arr5 [0] = “Peter”;

$arr5 [1] = “Paul”;

// 0 => ’Peter ’, 1 => ’Paul ’, 2 => ’Marry ’

foreach ($arr5 as $key => $value)

print “The array maps $key to $value
”;

The array maps 2 to Mary

The array maps 0 to Peter

The array maps 1 to Paul

COMP284 Scripting Languages Lecture 10 Slide L10 – 16

Compound types Foreach-loops

Arrays: foreach-loop

Does changing the value of the foreach-variable change the element of the
list that it currently stores?

Example 3:

$arr6 = array(“name” => “Peter”, “year” => 2009);

foreach ($arr6 as $key => $value) {

print “The array maps $key to $value
”;

$value .= ” – modified”; // Changing $value

}

print “
”;

foreach ($arr6 as $key => $value)

print “The array now maps $key to $value
”;

The array maps name to Peter

The array maps year to 2009

The array now maps name to Peter

The array now maps year to 2009

COMP284 Scripting Languages Lecture 10 Slide L10 – 17

Compound types Foreach-loops

Arrays: foreach-loop

• In order to modify array elements within a foreach-loop we need use a
reference

foreach (array as &$value)

statement

unset($value);

foreach (array as $key => &$value)

statement

unset($value);

• In the code schemata above, &$value is a variable whose value is stored at
the same location as an array element

• Note that PHP does not allow the key to be a reference
• The unset statement is important to return $value to being a ‘normal’

variable

COMP284 Scripting Languages Lecture 10 Slide L10 – 18

Compound types Foreach-loops

Arrays: foreach-loop

In order to modify array elements within a foreach-loop we need use a
reference

Example:

$arr6 = array(“name” => “Peter”, “year” => 2009);

foreach ($arr6 as $key => &$value) { // Note: reference!

print “The array maps $key to $value
”;

$value .= ” – modified”;

}

unset($value ); // Remove the reference from $value

print “
”;

foreach ($arr6 as $key => $value)

print “The array now maps $key to $value
”;

The array maps name to Peter

The array maps year to 2009

The array now maps name to Peter – modified

The array now maps year to 2009 – modified

COMP284 Scripting Languages Lecture 10 Slide L10 – 19

Compound types Array functions

Array functions

PHP has no stack or queue data structures,
but has stack and queue functions for arrays:

• array_push($array, value1, value2,…)
appends one or more elements at the end of the end of an array variable;
returns the number of elements in the resulting array

• array_pop($array)
extracts the last element from an array and returns it

• array_shift($array)
shift extracts the first element of an array and returns it

• array_unshift($array, value1, value2,…)
inserts one or more elements at the start of an array variable;
returns the number of elements in the resulting array

Note: $array needs to be a variable

COMP284 Scripting Languages Lecture 10 Slide L10 – 20

Printing

Printing

In PHP, the default command for generating output is echo

• void echo(arg1)
void echo arg1, arg2, …

• Outputs all arguments
• No parentheses are allowed if there is more than one argument
• More efficient than print (and therefore preferred)

Additionally, PHP also provides the functions print, and printf:

• int print(arg)

• Outputs its argument
Only one argument is allowed!

• Returns value 1
• Parentheses can be omitted

COMP284 Scripting Languages Lecture 10 Slide L10 – 21

Printing

Printing

• string sprintf(format, arg1, arg2, ….)
• Returns a string produced according to the formatting string format
• Parentheses are necessary

See http://www.php.net/manual/en/function.sprintf.php
for details

• int printf(format, arg1, arg2, …)

• Produces output according to format
• Parentheses are necessary
• Returns the length of the outputted string

• Important: In contrast to Perl, a PHP array cannot take the place
of a list of arguments

printf(“%2d apples %2d oranges
”,array (5 ,7));

produces an error message

COMP284 Scripting Languages Lecture 10 Slide L10 – 22

http://www.php.net/manual/en/function.sprintf.php

Printing

Printing

• string vsprintf(format, array)
• Returns a string produced according to the formatting string format
• Identical to sprintf but accepts an array as argument
• Parentheses are necessary

• int vprintf(format, array)
• Produces output according to format
• Identical to printf but accepts an array as argument
• Parentheses are necessary

vprintf(“%2d apples %2d oranges
”,array (5 ,7));

5 apples 7 oranges

COMP284 Scripting Languages Lecture 10 Slide L10 – 23

Printing

Revision

Read

• Chapter 6: PHP Arrays

of

R. Nixon:
Learning PHP, MySQL, and JavaScript.
O’Reilly, 2009.

• http://uk.php.net/manual/en/language.types.boolean.php
• http://uk.php.net/manual/en/language.types.integer.php
• http://uk.php.net/manual/en/language.types.float.php
• http://uk.php.net/manual/en/language.types.string.php
• http://uk.php.net/manual/en/language.types.array.php
• http://uk.php.net/manual/en/control-structures.foreach.php

COMP284 Scripting Languages Lecture 10 Slide L10 – 24

http://uk.php.net/manual/en/language.types.boolean.php
http://uk.php.net/manual/en/language.types.integer.php
http://uk.php.net/manual/en/language.types.float.php
http://uk.php.net/manual/en/language.types.string.php
http://uk.php.net/manual/en/language.types.array.php
http://uk.php.net/manual/en/control-structures.foreach.php

Lecture 10
Scalar types
Integers and Floating-point numbers
Exceptions and error handling
Booleans
Strings

Compound types
Arrays
Foreach-loops
Array functions

Printing