I have this PHP if statement that I am trying to firgure out.
I know what the result is, but I have not seen this before (probably my bad).
Who can help me understand the code below? Why is $a showing and not $b? Because $a is first in line?
<?php
$a = 'has a value';
$b = 'this one too!';
if (($href = $a) || ($href = $b)) {
echo $href;
//Result is 'has a value'.
}
?>
|| uses "short-circuit" evaluation. If the first part of the expression is true, then the second part will not be evaluated. In PHP,
The value of an assignment expression is the value assigned.
(quoted from the = documentation)
so in this case the expression ($href = $a) has a value of the assigned value, 'has a value'. That string evaluates to true (see "converting to boolean"), so the second assignment will not be executed.
FYI, for another way to write this that's a little less repetitive, you can do this:
if ($href = $a ?: $b) {
echo $href;
}
Related
In PHP if you have the following code does $b get evaluated since $a will cause the if statement to return false?
$a = false;
$b = true;
if ($a && $b) {
// more code here
}
also if $b does get evaluated are there ever circumstances where a portion of an if statement may not be evaluated as the processor already knows that the value to be false?
Evaluation of && is stopped as soon as it hits the false condition.
These (&&) are short-circuit operators, so they don't go to check second condition if the first one true (in case of OR) or false(in case of AND).
Reference: http://php.net/manual/en/language.operators.logical.php
From documentation:
<?php
// --------------------
// foo() will never get called as those operators are short-circuit
$a = (false && foo());
$b = (true || foo());
$c = (false and foo());
$d = (true or foo());
Evaluation of logical expressions is stopped as soon as the result is known.
If $a is false, $b will not get evaluated, as it won't change the ($a && $b) result.
A consequence of that is that if the evaluation of $b requires more ressources than the evaluation of $a, starts your test condition with $a.
But be aware that:
PHP does not (in the general case) specify in which order an expression is evaluated and code that assumes a specific order of evaluation should be avoided, because the behavior can change between versions of PHP or depending on the surrounding code. (Source php docs)
So you should not assume $b is never evaluated if $a is false, as it may change in the future (says php docs).
In my never-ending quest to optimise my line usage, I've just got a quick question about what exactly can go into as assignment statement in PHP (and other languages too, but I'm working on a PHP project).
In my program I have a certain boolean variable, which is toggled by a few things summarised by an if statement. I thought, hang on, that if statement will evaluate to a boolean value, can I just use that logic in one line, as opposed to wrapping a separate assignment statement inside the if. Basically my question is will:
$myVar = ($a == $b);
be equivalent to
if ($a == $b) { $myVar = true; }
else { $myVar = false; }
As you can see, this saves me one whole line, so it will impact my project hugely. /sarcasm
What you are looking for is a terinary operation. Something simialar to
$var = ($a === $b ? true : false);
echo $var;
Depending on the evaluation result of $a === $b the value of $var is then set.
Short answer, $myVar = ($a == $b); is the same as if ($a == $b) { $myVar = true; } else { $myVar = false; }.
And if you want to be even shorter, you can even remove the (...) and have it barely $myVar = $a == $b;
I have an array ...
$a= array(1,2,3,4);
if (expr)
{ echo "if";
}
else
{ echo 'else';
}
When expr is ( $a = '' || $a == 'false') , output is "if" ,
but when expr is ( $a == 'false' || $a = '' ) , output is "else"
Can anyone explain why & how ordering makes a difference ??
Edit : I understand that I am assigning '' to $a. That is not the problem. The real question is : What does the expression $a = '' return? And why does reversing the order of the 2 situations switch us from the IF section to the ELSE section?
AGAIN : I UNDERSTAND I AM ASSIGNING NOT COMPARING. PLEASE ANSWER THE QUESTION AS IS.
First, never use = as a comparison operator. It is an assignment operator.
The difference is that false (as a boolean) is not the same as 'false' as a string.
Certain expressions are type juggled by PHP to evaluate somewhat differently to how you would expect.
false==""
// TRUE.
false=="false"
// FALSE.
Additionally, when you try to compare numbers to strings, PHP will try to juggle the data so that a comparison will be performed. There is a lot to it (much more than I will post here) but you would do well to investigate type juggling and various operators. The docs are a great start for this. You should also have a read of the comparison operators which go into a lot of detail about how various comparisons will work (depending on whether you use == or === for example).
With $a = '' you are setting $a to an empty string. This is the same as:
$a = '';
if($a){
echo 'if';
}
The || operator checks if the first condition is true and if it is, it continues with the code in the brackets. In PHP, if $a is set to anything, it will return true. In the second case $a does not equal the string 'false' (you are not comparing it to a boolean false even!), so it executes the code in the else part.
And Fluffeh is not entirely correct. You can use the assignment operator in an if condition very effectively, you just have to be smart about it.
$a = '' is an assignment: you have, in error, used = in place of ==. Assignment is an expression which has the value of the thing your assigning.
A single equals sign = is the assignment opporator, so $a = '' is assigning an empty string to $a not checking if it is equal to.
In your 1st example you set the value of $a to an empty string, then check if it is false. An empty tring evalutes to false in php, so the conditional is true.
In your second example, you check if $a equals false 1st (when the value of $a is an array), so the conditional is false
I'm a bit of an optimization freak (at least by my definition) and this question has been bugging me for quite a while.
I'm wondering if PHP does some optimization on && and ||:
Take the following example:
$a = "apple";
$b = "orange";
if ($a == "orange" && $b == "orange") {
//do stuff
}
When that code executes, it will check if $a is equal to "orange." In this case it isn't. However, there is an && operator. Since the first part ($a == "orange") already returned false, will PHP still check if $b is equal to "orange?"
I have the same question for ||:
$a = "orange";
$b = "orange";
if ($a == "orange" || $b == "orange") {
//do stuff
}
When it checks if $a is equal to "orange," it returns true. Since that would make the || operator return true, will PHP even check the second part of the || (since we already know it will be true)?
Hopefully I am making sense here, and hopefully somebody has an answer for me. Thank you!
PHP uses short circuit evaluation with binary conditionals (such as &&, || or their constant equivalents), so if the result of evaluating the LHS means the RHS isn't necessary, it won't.
For example...
method_exists($obj, 'func') AND $obj->func();
...is an exploitation of this fact. The RHS will only be evaluated if the LHS returns a truthy value in this example. The logic makes sense here, as you only want to call a method if it exists (so long as you're not using __call(), but that's another story).
You can also use OR in a similar fashion.
defined('BASE_PATH') OR die('Restricted access to this file.');
This pattern is used often as the first line in PHP files which are meant to be included and not accessed directly. If the BASE_PATH constant does not exist, the LHS is falsy so it executes the RHS, which die()s the script.
Yes, PHP short-circuits the && and || operators, meaning that no, the right operand won't be evaluated if the value of the left operand means that it doesn't need to be evaluated. There's no need to optimize them. You can test it like this:
function one() {
echo "One";
return false;
}
function two() {
echo "Two";
return true;
}
one() && two(); // Outputs One
echo "\n";
two() || one(); // Outputs Two
Here's a demo. If there were no short-circuiting, you'd get:
OneTwo
TwoOne
Given the following code:
if (is_valid($string) && up_to_length($string) && file_exists($file))
{
......
}
If is_valid($string) returns false, does the php interpreter still check later conditions, like up_to_length($string)?
If so, then why does it do extra work when it doesn't have to?
Yes, the PHP interpreter is "lazy", meaning it will do the minimum number of comparisons possible to evaluate conditions.
If you want to verify that, try this:
function saySomething()
{
echo 'hi!';
return true;
}
if (false && saySomething())
{
echo 'statement evaluated to true';
}
Yes, it does. Here's a little trick that relies on short-circuit evaluation. Sometimes you might have a small if statement that you'd prefer to write as a ternary, e.g.:
if ($confirmed) {
$answer = 'Yes';
} else {
$answer = 'No';
}
Can be re-written as:
$answer = $confirmed ? 'Yes' : 'No';
But then what if the yes block also required some function to be run?
if ($confirmed) {
do_something();
$answer = 'Yes';
} else {
$answer = 'No';
}
Well, rewriting as ternary is still possible, because of short-circuit evaluation:
$answer = $confirmed && (do_something() || true) ? 'Yes' : 'No';
In this case the expression (do_something() || true) does nothing to alter the overall outcome of the ternary, but ensures that the ternary condition stays true, ignoring the return value of do_something().
Bitwise operators are & and |.
They always evaluate both operands.
Logical operators are AND, OR, &&, and ||.
All four operators only evaluate the right side if they need to.
AND and OR have lower precedence than && and ||. See example below.
From the PHP manual:
// The result of the expression (false || true) is assigned to $e
// Acts like: ($e = (false || true))
$e = false || true;
// The constant false is assigned to $f before the "or" operation occurs
// Acts like: (($f = false) or true)
$f = false or true;
In this example, e will be true and f will be false.
Based on my research now, PHP doesn't seem to have the same && short circuit operator as JavaScript.
I ran this test:
$one = true;
$two = 'Cabbage';
$test = $one && $two;
echo $test;
and PHP 7.0.8 returned 1, not Cabbage.
No, it doesn't anymore check the other conditions if the first condition isn't satisfied.
I've create my own short-circuit evaluation logic, unfortunately it's nothing like javascripts quick syntax, but perhaps this is a solution you might find useful:
$short_circuit_isset = function($var, $default_value = NULL) {
return (isset($var)) ? : $default_value;
};
$return_title = $short_circuit_isset( $_GET['returntitle'], 'God');
// Should return type 'String' value 'God', if get param is not set
I can not recall where I got the following logic from, but if you do the following;
(isset($var)) ? : $default_value;
You can skip having to write the true condition variable again, after the question mark, e.g:
(isset($super_long_var_name)) ? $super_long_var_name : $default_value;
As very important observation, when using the Ternary Operator this way, you'll notice that if a comparison is made it will just pass the value of that comparison, since there isn't just a single variable. E.g:
$num = 1;
$num2 = 2;
var_dump( ($num < $num2) ? : 'oh snap' );
// outputs bool 'true'
My choice: do not trust Short Circuit evaluation in PHP...
function saySomething()
{
print ('hi!');
return true;
}
if (1 || saySomething())
{
print('statement evaluated to true');
}
The second part in the condition 1 || saySomething() is irrelevant, because this will always return true. Unfortunately saySomething() is evaluated & executed.
Maybe I'm misunderstood the exact logic of short-circuiting expressions, but this doesn't look like "it will do the minimum number of comparisons possible" to me.
Moreover, it's not only a performance concern, if you do assignments inside comparisons or if you do something that makes a difference, other than just comparing stuff, you could end with different results.
Anyway... be careful.
Side note: If you want to avoid the lazy check and run every part of the condition, in that case you need to use the logical AND like this:
if (condition1 & condition2) {
echo "both true";
}
else {
echo "one or both false";
}
This is useful when you need for example call two functions even if the first one returned false.