The result of:
var_dump(null != $a = 15);
var_dump($a);
is:
bool(true)
int(15)
Why is this script not triggering an error?
Since != (not equal operator) has a higher precedence than = (assignment operator), $a should be compared to null first?
The only reason I can find is that the documentation says that this is still legal: http://php.net/manual/en/language.operators.precedence.php#example-129
It seems to be an exception to what is shown in the table above.
This is not about operator precedence but about: operator precedence lists don't tell you the details (really, they never do), e.g. about the bison rules and the resulting pattern matching and stack reducing.
Let's take the statement null != $a = 15;, for simplicity without the var_dump.
This is how the parser "sees" this statement - or: sees that it is a statement.
(I hope this will be rendered with a fix-width font everywhere...)
null != $a = 15 ;
T_VARIABLE
identifier compound_variable T_LNUMBER
namespace_name reference_variable common_scalar
general_constant base_variable scalar
scalar base_variable_with_functions_calls expr_without_variable
expr_without_variable variable = expr
expr T_IS_NOT_EQUAL \______ expr_without_variable _________/
\__________________ expr ____________________________________________/ ;
\_________________ unticked_statement _______________________________________________/
statement
( You can look up the rules at https://github.com/php/php-src/blob/PHP-5.6.15/Zend/zend_language_parser.y )
There's no special rule for the assignment operator in this case; there simply isn't another way for the parser to match the statement, so precedence doesn't apply.
I have this piece of code:
<?=!empty($options["placeholder"]) ? $options["placeholder"]:'search...'?>
I was under the impression I could do like:
<?=!empty($options["placeholder"]) ?:'search...'?>
But when $options["placeholder"] is not empty then it returns 1, as it is a ternary operator.
Do I have to always issue the variable 2 times?
Yes. There is however been many requests wanting to change this:
https://wiki.php.net/rfc/ifsetor
https://wiki.php.net/rfc/isset-set-operator
If you can be sure that $options['placeholder'] will be set--if not you can prefix it with # to suppress the warning--you can drop the empty call:
<?= $options["placeholder"] ?: 'search...' ?>
The Elvis operator ?: evaluates to the left hand side if it is truthy (true when coerced to a boolean value such as 1 or a non-empty string 'foo') or the right hand side if not.
Can any one explain me following construct.
I do googling for this about 2 hours but can't understand.
public function __construct($load_complex = true)
{
$load_complex and $this->complex = $this->getComplex();
}
See: http://www.php.net/manual/en/language.operators.logical.php
PHP uses intelligent expression evaluation. If any of AND's operands evaluates to false, then there is no reason to evaluate other, because result will be false.
So if $load_complex is false there is no need to evaluate $this->complex = $this->getComplex();
This is some kind of workaround, but I do not suggest to use it, because it makes your code hard to read.
Specifically to your example $this->complex = $this->getComplex() if and only if $load_complex is set to true.
LIVE DEMO
NOTE: If any one of OPERAND result becomes 'false' in short
circuit AND evaluation means, the part of statement will be
OMITTED because there is no need to evaluate it.
Dont code like below line because, you may get probably logical
error while you are putting Expression instead of assigning values
to the variable on LEFT HAND SIDE...
$load_complex and $this->complex = $this->getComplex();
I have modified below with conditinal statement for your needs...
if($load_complex and $this->complex) {
$this->getComplex();
}
Here is the sample:
if(($test = array('key'=>true)) && $test['key']){
// works
}
if($test = array('key'=>true) && $test['key']){
// does not work
}
Why is the parenthesis required? My understanding is that it computes the first conditional then the second no matter what.
And is it "safe" to do an assignment like this?
It's a matter of operator precedence in the language. In your second statement, you're essentially writing this to be evaluated:
$test = array('key'=>true) && $test['key'];
Just about any language is going to take that to mean this:
$test = (array('key'=>true) && $test['key']);
It's assigning to $test the value of the evaluation of the logical && between the two other values. So $test will be either true or false when evaluated.
i don't think the parens are required in PHP. they are in JS.
depends what you mean by "safe". it works. but some would argue that this is bad style and makes for less understandable and less maintainable code. otoh, K&R positively recommended it. it doesn't worry me and sometimes makes for tidier code.
It's because it's forcing the interpreter to perform the assignment before it attempts to evaluate any of conditions within the if.
Without the parenthesis, you're simply assigning the results of array('key'=>true) && $test['key'] to $test.
It is safe to use, but in this case the parenthesis are required for disambiguation. The operator precedence rules of PHP mean that the second line will be executed as:
if ($test = (array('key' => true) && $test['key'])) { .. }
So test will not be an array but a bool.
The practice of doing assignment in if statements itself is not really encouraging readability though, so you probably want to avoid doing this too much.
In PHP, the assignment operator = has lower precedence that most other operators. (For more information, see the documentation). Furthermore, the result of an expression using the assignment operator is the value of the assignment. With this in mind, consider the expressions you posted:
First example:
($test = array('key'=>true)) && $test['key']
The first part of this expression, ($test = array('key'=>true)) evaluates to array('key'=>true) (by the rule above), and the second part evaluates to true since the key was just set. Thus the whole expression evaluates to true.
Second example:
$test = array('key'=>true) && $test['key'] By the rules of operator precedence, $test is going to get assigned the value of the expression array('key'=>true) && $test['key']. The first half of this is true, but $test['key'] hasn't been set yet, so true && false is false, so $test takes the value false, which is also the result of the if condition.
What are the ? and : operators in PHP?
For example:
(($request_type == 'SSL') ? HTTPS_SERVER : HTTP_SERVER)
This is the conditional operator.
$x ? $y : $z
means "if $x is true, then use $y; otherwise use $z".
It also has a short form.
$x ?: $z
means "if $x is true, then use $x; otherwise use $z".
People will tell you that ?: is "the ternary operator". This is wrong. ?: is a ternary operator, which means that it has three operands. People wind up thinking its name is "the ternary operator" because it's often the only ternary operator a given language has.
I'm going to write a little bit on ternaries, what they are, how to use them, when and why to use them and when not to use them.
What is a ternary operator?
A ternary ? : is shorthand for if and else. That's basically it. See "Ternary Operators" half way down this page for more of an official explanation.
As of PHP 5.3:
Since PHP 5.3, it is possible to leave out the middle part of the ternary operator. Expression expr1 ?: expr3 returns expr1 if expr1 evaluates to TRUE, and expr3 otherwise.
As of PHP 7.0
PHP 7 has new Null Coalesce Operator. This is the same as a ternary but is also called an "isset ternary". This also allows a set of chained ternaries that remove the need for isset() checks.
In PHP 5, if you wanted to use a ternary with a potentially non-existent variable then you would have to perform an isset() at the beginning of the ternary statement:
$result = isset($nonExistentVariable) ? $nonExistentVariable : ‘default’;
In PHP 7, you can now do this instead:
$result = $nonExistentVariable ?? ‘default’;
The Null Coalesce Operator does not work with an empty string, however, so bear that in mind. The great thing about this is you can also chain the operators for multiple checks for multiple variables, providing a sort of backup depending on whether or not each variable in the chain exists:
$user = $userImpersonatingAnotherUser ?? $loggedInUser ?? “Guest”;
In PHP, with systems where a user can login, it is not uncommon for an administrator to be able to impersonate another user for testing purposes. With the above example, if the user is not impersonating another user, and also a logged in user does not exist, then the user will be a guest user instead. Read on more if you don't understand this yet to see what ternaries are and how they are used, and then come back to this bit to see how the new PHP
How are ternaries used?
Here's how a normal if statement looks:
if (isset($_POST['hello']))
{
$var = 'exists';
}
else
{
$var = 'error';
}
Let's shorten that down into a ternary.
$var = isset($_POST['hello']) ? 'exists' : 'error';
^ ^ ^ ^ |
| then | else |
| | |
if post isset $var=this $var=this
Much shorter, but maybe harder to read. Not only are they used for setting variables like $var in the previous example, but you can also do this with echo, and to check if a variable is false or not:
$isWinner = false;
// Outputs 'you lose'
echo ($isWinner) ? 'You win!' : 'You lose';
// Same goes for return
return ($isWinner) ? 'You win!' : 'You lose';
Why do people use them?
I think ternaries are sexy. Some developers like to show off, but sometimes ternaries just look nice in your code, especially when combined with other features like PHP 5.4's latest short echos.
<?php
$array = array(0 => 'orange', 1 => 'multicoloured');
?>
<div>
<?php foreach ($array as $key => $value) { ?>
<span><?=($value==='multicoloured')?'nonsense':'pointless'?></span>
<?php } ?>
</div>
<!-- Outputs:
<span>
pointless
</span>
<span>
nonsense
</span>
-->
Going off-topic slightly, when you're in a 'view/template' (if you're seperating your concerns through the MVC paradigm), you want as little server-side logic in there as possible. So, using ternaries and other short-hand code is sometimes the best way forward. By "other short-hand code", I mean:
if ($isWinner) :
// Show something cool
endif;
Note, I personally do not like this kind of shorthand if / endif nonsense
How fast is the ternary operator?
People LIKE micro-optimisations. They just do. So for some, it's important to know how much faster things like ternaries are when compared with normal if / else statements.
Reading this post, the differences are about 0.5ms. That's a lot!
Oh wait, no it's not. It's only a lot if you're doing thousands upon thousands of them in a row, repeatedly. Which you won't be. So don't worry about speed optimisation at all, it's absolutely pointless here.
When not to use ternaries
Your code should be:
Easy to read
Easy to understand
Easy to modify
Obviously this is subject to the persons intelligence and coding knowledge / general level of understanding on such concepts when coming to look at your code. A single simple ternary like the previous examples are okay, something like the following, however, is not what you should be doing:
echo ($colour === 'red') ? "Omg we're going to die" :
($colour === 'blue' ? "Ah sunshine and daisies" :
($colour === 'green' ? "Trees are green"
: "The bloody colour is orange, isn't it? That was pointless."));
That was pointless for three reasons:
Ridiculously long ternary embedding
Could've just used a switch statement
It was orange in the first place
Conclusion
Ternaries really are simple and nothing to get too worked up about. Don't consider any speed improvements, it really won't make a difference. Use them when they are simple and look nice, and always make sure your code will be readable by others in the future. If that means no ternaries, then don't use ternaries.
It's called a ternary operator. If the first expression evaluates to true, HTTPS_SERVER is used, else HTTP_SERVER is chosen.
It's basically a shorthand if statement, and the above code could also be rewritten as follows:
if ($request_type == 'SSL') {
HTTPS_SERVER;
}
else {
HTTP_SERVER;
}
This is sometimes known as the ternary conditional operator. Ternary means that it has three arguments, as x ? y : z. Basically, it checks if x is true; if it is, then put y instead of this operation, otherwise z.
$hello = $something ? "Yes, it's true" : "No, it's false";
Conditional operator ? : is an operator which is used to check a condition and select a value depending on the value of the condition. It is expressed in the following form:
variable = condition ? expression1 : expression2;
It works as follows...
Firstly, condition is evaluated.
If the condition is true, then expression1 is evalauated. And the value of expression1 is assigned to the variable.
If the condition is false, then expression2 is evaluated. And the value of expression2 is assigned to the variable.
For example:
x = (a>b) ? 5 : 9
In this, for x, firstly the condition (a>b) is evaluated. If this condition becomes true, then x will become the value 5 (ie, x=5). But if the condition (a>b) becomes false, then x will attain the value 9 (ie, x=9).
Ternary Operator
Sometimes conditional operator ? : is also called a ternary operator. This is so because it involves three operands. For example:
x ? y : z
Here, x,y and z are the three operands. If condition x is true, then value y is assigned otherwise value z is assigned.
This is a short way of writing if sentences. It is also used in other languages like Java, JavaScript and others.
Your code,
$protocol = $request_type == 'SSL' ? HTTPS_SERVER : HTTP_SERVER;
can be written like this:
if ($request_type == 'SSL')
$protocol = HTTPS_SERVER;
else
$protocol = HTTP_SERVER;
That is a one line if statement:
condition ? true : false
Translated to an ordinary if statement in your case, that would be:
if($request_type == 'SSL') HTTPS_SERVER;
else HTTP_SERVER;
That's basically a fancy way of writing an if-else statement. Some say it's easier to read, some say not.
Ternary operator at Wikipedia
This works like an if statement it's very simple and easy once you get used to it.
(conditions_expressions) ? what_to_do_if_true : what_to_do_if_false.
As John T says, it is called a ternary operator and is essentially a shorthand version of an if /else statement. Your example, as a full if / else statement, would read;
if($request_type == 'SSL')
{
HTTPS_SERVER;
}
else
{
HTTP_SERVER;
}