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Глава 15. Операторы
Оператором называется нечто, состоящее из одного или более значений (выражений,
если говорить на жаргоне программирования), которое можно вычислить как новое
значение (таким образом, вся конструкция может рассматриваться как выражение).
Отсюда следует, что функции или любые другие конструкции, которые возвращают
значение (например, print()) являются операторами, в отличие
от всех остальных языковых конструкций (например, echo()),
которые ничего не возвращают.
Операторы бывают трех видов. Во-первых, это унарные операторы, которые работают
только с одним аргументом, например, !
(оператор отрицания) или ++ (инкримент). Вторую группу
составляют бинарные операторы: в нее входят большинство поддерживаемых в PHP операторов,
полный список которых вы можете найти в разделе
Порядок выполнения операторов.
И последнюю, третью группу составляет тернарный оператор ?:.
Он используется для условного выбора между двумя операторами, в зависимости
от результата вычисления третьего оператора. Говоря другими словами, он позволяет
определить две альтернативные ветви дальнейшего выполнения. Тернарный оператор
рекомендуется заключать в круглые скобки.
Приоритет операторов определяет, насколько "тесно" связанны между
собой два выражения. Например, выражение 1 +
5 * 3 вычисляется как 16, а не
18, поскольку операция умножения ("*") имеет
более высокий приоритет, чем операция сложения ("+"). В случае, если
операторы имеют одинаковый приоритет, они будут выполняться слева направо.
Круглые скобки могут использоваться для принудительного указания
необходимого порядка выполнения операторов. Например,
выражение (1 + 5) * 3 вычисляется как
18.
В следующей таблице приведен список операторов, отсортированный по
убыванию их приоритетов. Операторы, размещенные в одной строке имеют
одинаковый приоритет и порядок их выполнения опроделяется исходя из
их ассоциативности.
Таблица 15-1. Порядок выполнения операторов | Ассоциативность | Оператор |
|---|
| неассоциативна | new | | правая | [ | | неассоциативна | ++ -- | | неассоциативна | ! ~ - (int) (float) (string) (array) (object) @ | | левая | * / % | | левая | + - . | | левая | << >> | | неассоциативна | < <= > >= | | неассоциативна | == != === !== | | левая | & | | левая | ^ | | левая | | | | левая | && | | левая | || | | левая | ? : | | правая |
= += -= *= /= .= %= &= |= ^= <<= >>=
| | левая | and | | левая | xor | | левая | or | | левая | , |
Левая ассоциативность подразумевает, что выражение вычисляется слева направо,
правая ассоциативность соответственно подразумевает противоположный порядок.
Пример 15-1. Ассоциативность |
<?php
$a = 3 * 3 % 5; $a = true ? 0 : true ? 1 : 2; $a = 1;
$b = 2;
$a = $b += 3; ?>
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Вы можете использовать круглые скобки для повышения читабельности кода.
Замечание:
Несмотря на то, что оператор ! имеет
более высокий приоритет чем =, PHP позволяет
использовать следующую конструкцию: if (!$a = foo()),
которая присваивает переменной $a результат
выполнения функции foo().
Операторы
golotyuk at gmail dot com
09-Jul-2006 09:51
Simple POST and PRE incremnt sample:
<?php
$b = 5;
$a = ( ( ++$b ) > 5 ); echo (int)$a;
$b = 5;
$a = ( ( $b++ ) > 5 ); echo (int)$a;
?>
This will output 10, because of the difference in post- and pre-increment operations
dlyons at lyons42 dot com
26-Nov-2005 03:30
Re: Rick on 2-Sep-2005.
Actually, the C equivalent of "$a[$c++]=$b[$c++];" has undefined behavior, and the increments are by no means guaranteed to happen after the assignment in C. (Search for a discussion of C "sequence points" for details.)
sm
02-Sep-2005 05:15
Note the highly unfortunate difference from Java, which associates the trinary operator right-to-left.
---------------------------- source
function trinaryTest($foo){ // works as you think in Java, but not PHP
$bar = $foo > 20
? "greater than 20"
: $foo > 10
? "greater than 10"
: $foo > 5
? "greater than 5"
: "not worthy of consideration";
echo $foo." => ".$bar."\n";
}
echo "\n\n\n----trinaryTest\n\n";
trinaryTest(21);
trinaryTest(11);
trinaryTest(6);
trinaryTest(4);
function trinaryTestParens($foo){
$bar = $foo > 20
? "greater than 20"
: ($foo > 10
? "greater than 10"
: ($foo > 5
? "greater than 5"
: "not worthy of consideration"));
echo $foo." => ".$bar."\n";
}
echo "\n\n\n----trinaryTestParens\n\n";
trinaryTestParens(21);
trinaryTestParens(11);
trinaryTest(6);
trinaryTestParens(4);
---------------------------- output
----trinaryTest
21 => greater than 5
11 => greater than 5
6 => greater than 5
4 => not worthy of consideration
----trinaryTestParens
21 => greater than 20
11 => greater than 10
6 => greater than 5
4 => not worthy of consideration
rick at nomorespam dot fourfront dot ltd dot uk
02-Sep-2005 03:51
A quick note to any C developers out there, assignment expressions are not interpreted as you may expect - take the following code ;-
<?php
$a=array(1,2,3);
$b=array(4,5,6);
$c=1;
$a[$c++]=$b[$c++];
print_r( $a ) ;
?>
This will output;-
Array ( [0] => 1 [1] => 6 [2] => 3 )
as if the code said;-
$a[1]=$b[2];
Under a C compiler the result is;-
Array ( [0] => 1 [1] => 5 [2] => 3 )
as if the code said;-
$a[1]=$b[1];
It would appear that in php the increment in the left side of the assignment is processed prior to processing the right side of the assignment, whereas in C, neither increment occurs until after the assignment.
kit dot lester at lycos dot co dot uk
22-Aug-2005 05:38
D'oh! please ignore (& forgive) the first paragraph in my note yesterday - I said that a diadic operator had the same precedence & associativity as a bunch of monadics. A bit of early senility must have struck me.
When I find time I'll rework my test to find out what I should have said - but using instanceof monadically didn't cause any visible errors in the test, so it could take a while.
[I'd be delighted if someone else beats me to it/ spares me the difficulties of seeing what's wrong in something that shouldn't have worked.]
kit dot lester at lycos do co dot uk
21-Aug-2005 09:21
Table 15-1 omits the precedence of instanceof - testing suggests it to be of the same precedence as not, negate, casting, and @.
The table also omits the precedence (and associativity) of the "execution operator" - but since that's a sort of quoting, I don't think it meaningfully has a precedence or associativity - they explain what is to happen where there's an open-endedness in the sense of missing brackets, and the quoting is a sort of bracket. (At least: because of the execution operator's double-ended closedness, I can't figure out any code where it would matter, so I can't test it.)
webmaster AT cafe-clope DOT net
12-Aug-2005 12:47
I regularly use some syntax like :
<?php
if(!$myvar)
$myvar = $value ;
?>
and
<?php
if($myvar)
echo "myvar is $myvar today" ;
?>
(or <?php echo ($myvar ? "myvar is $myvar today" : "") ?>)
It's small, but can become heavy when used too much.
Isn't there some trick to better such syntaxes ?
I was wondering about using things like :
<?php $myvar ||= $value ; ?>
<?php $myvar ?= $value ; ?>
<?php echo ($myvar ? "myvar is $myvar today") ; ?>
edwardsbc at yahoo dot com
04-May-2005 10:26
In response to npeelman at cfl dot rr dot com
29-Dec-2004 06:22:
You have misunderstood the behaviour of the interpreter.
With out the curly braces and the single quoted key identifier, the interpreter "assumes" you meant your CONSTANT to be a string. This ONLY works within a parsed (double quoted) string. And it doesn't help you at all if your array is multi-dimensional. I consider this a very bad habbit because it will get you in trouble elsewhere. Try the following:
<?php
define('item','AnyOldThing');
define('b',12);
$arr['item']['b'] = 'string';
$arr['AnyOldThing'][12]= 'Maybe not what I intended.';
echo "This is a {$arr['item']['b']}"; echo "This is a $arr[item][b]"; echo $arr[item][b]; ?>
npeelman at cfl dot rr dot com
29-Dec-2004 03:22
Update to message by yasuo_ohgaki at hotmail dot com:
I know this is an old message but when using an Associative array in a string you do not have to use { and } to resolve ambiguity.
ex:
Associative Array in string:
$arr['item'] = 'string';
echo "This is {$arr['item']}"; //prints "This is string".
...does work but, so does:
echo "This is $arr[item]"; //prints "This is string".
... simply enclose the whole string with double quotes and leave out the single quotes from around the index name. This simplifies the code and makes things easier to read.
Stopping at the dot completely
01-Sep-2004 01:33
The low precedence of the OR operator is useful for error control statements such as this one:
$my_file = @file ('non_existent_file') or die ("Failed opening file: error was '$php_errormsg'");
Notice the good readability of the code.
22-Aug-2004 09:51
I think warhog's note about the differing precedence between && / AND and || / OR is worth repeating. Since && and || evaluate before the assignment operator (=) while AND and OR evaluate after it, you can get COMPLETELY different results if you don't fully parenthesise.
I cannot imagine when it would ever be important that those two pairs have differing precedence, but they do. And I just spent two hours discovering that the hard way because I broke my career-long rule:
*Always fully parenthesise!*
11-Jun-2004 07:22
Warhog wrote: "maybe usefull for some tricky coding and helpfull to prevent bugs :D"
I'm sure Warhog was being facetious, but for the new programmers in the audience I'd like to point out that 'tricky coding' and relying on precedence/order of evaluation are both well-known ways to *produce* bugs.
Use parentheses instead.
09-Jun-2004 05:58
of course this should be clear, but i think it has to be mentioned espacially:
AND is not the same like &&
for example:
<?php $a && $b || $c; ?>
is not the same like
<?php $a AND $b || $c; ?>
the first thing is
(a and b) or c
the second
a and (b or c)
'cause || has got a higher priority than and, but less than &&
of course, using always [ && and || ] or [ AND and OR ] would be okay, but than you should at least respect the following:
<?php $a = $b && $c; ?>
<?php $a = $b AND $c; ?>
the first code will set $a to the result of the comparison $b with $c, both have to be true, while the second code line will set $a like $b and THAN - after that - compare the success of this with the value of $c
maybe usefull for some tricky coding and helpfull to prevent bugs :D
greetz, Warhog
yasuo_ohgaki at hotmail dot com
26-Mar-2001 12:34
About "{" and "}".
Sometimes PHP programmers need to use "{" and "}" to resolve ambiguity. Here is some examples.
Variable Variables:
$foo = "test";
$$bar = "this is";
echo "${$bar} $foo"; // prints "this is test"
Note: it is the same as
echo "$test $foo";
Array in string:
$arr[10][10][10] = "string";
echo "This is {$arr[10][10][10]}"; // prints "This is string"
Associative Array in string:
$arr['item'] = 'string';
echo "This is {$arr['item']}"; //prints "This is string".
yasuo_ohgaki at hotmail dot com
25-Mar-2001 11:53
Other Language books' operator precedence section usually include "(" and ")" - with exception of a Perl book that I have. (In PHP "{" and "}" should also be considered also). However, PHP Manual is not listed "(" and ")" in precedence list. It looks like "(" and ")" has higher precedence as it should be.
Note: If you write following code, you would need "()" to get expected value.
$bar = true;
$str = "TEST". ($bar ? 'true' : 'false') ."TEST";
Without "(" and ")" you will get only "true" in $str.
(PHP4.0.4pl1/Apache DSO/Linux, PHP4.0.5RC1/Apache DSO/W2K Server)
It's due to precedence, probably.
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