Simple test questions about php operator precedence
Someone in the group happened to be asking a question about the priority of PHP operators today. I happened to have encountered similar problems before. Let’s sort them out here so that everyone can understand and prevent mistakes:)
Let’s start with a simple one:
$a = 3;
if ($a = 5) {
$a++;
}
var_dump($a);// (int)6
So a good coding practice is to put the number in front and write it as 5 == $a. In this way, even if you write 5 = $a, an error will be reported at this time and it will not be an implicit assignment
Come on, let’s move on to the next pit.
$x = 2;
echo $x == 2 ? 'I' : $x == 1 ? 'you' : 'it'; //Your
classmates who fell into the trap may think that the result is me, because $x is indeed equal to 2 according to the rules of the ternary operator ?If the former is true, it will output me.
The knowledge point here lies in the operator priority and the left-association characteristics of the ternary operator. In fact, the real execution process is like this:
$x = 2;//Assign 2 to variable x Nothing to say
echo $x == 2 ? 'I' : $x == 1 ? 'You ' : 'It'; //You
//Because == has a higher priority than the ternary operator, it is converted into the following code
#echo true? 'I' : false ? 'You' : 'It';
//Due to the left associative nature of the ternary operator, the above code is equivalent to
# echo (true ? 'I' : false) ? 'You' : 'It';
//First calculate the
#echo 'I' in the left bracket ? 'You' : 'It'; //Everyone should understand by now
Well, continue to watch the pit
$a = 3;
$b = 5;
if ($a = 3 || $b = 7) {
$a++;
$b++;
}
var_dump($a);//(bool)true
var_dump($ b);//(int)6
I believe many students will have encountered this question when doing interview questions. I was also fooled the first time I did it.
The students who were tricked believe that there are probably several answers (4,6), (2,6)
$bThere is basically no controversy, the key is$ What is the situation? Let’s explain this pitfall.
#There is nothing to say about assignment
$a = 3;
$b = 5;
if ($a = 3 || $b = 7) {
$a++;
$b++;
}
//Knowledge point 1: || has a higher priority than
//So $a = 3 || $b = 7 is equivalent to $a = (3 || $b = 7)
//Because 3 || $b = 7, the entire expression is true, so it returns true and assigns it to $a. At this time, $a = true, it will be executed. Content in brackets
//Knowledge point 2: Increment/decrement operators do not affect Boolean values. Decreasing the NULL value has no effect, but the result of incrementing NULL is 1
//which means $a is still true, not 2 after 1++ $b is still++
//So the result is $a is true, $b is 6
Students who fall into the trap here will think it is 3. Note that the = sign is an assignment == comparison operator
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