Order of precedence with nested ternary operators [duplicate] - php

Question

This question already has answers here:
Stacking Multiple Ternary Operators in PHP
(11 answers)
Closed 2 years ago.
I was in class the other day and this snippet of code was presented:
<?php
//Intialize the input
$score=rand(50,100);
//Determine the Grade
$grade=($score>=90)?'A':(
($score>=80)?'B':(
($score>=70)?'C':(
($score>=60)?'D':'F')));
//Output the Results
echo "<h1>A score of $score = $grade</h1>";
?>
At the time I questioned the order of operations within the nested ternary operators, thinking that they would evaluate from the inside out, that is it would evaluate if $score were >= 60 first, then if $score >= 70, etc -- working through the whole stack every time regardless of the score.
To me it seems that this construct should follow the same order of precedence given to mathematical operators -- resolving the inner-most set of parentheses first and then working out, unless there is some order of operations unique to the ternary.
Unfortunately the discussion in class quickly became about winning an argument, when I really just wanted to understand how it works. So my questions are two:
(1)How would this statement me interpreted and why?
and
(2)Is the some sort of stack trace or step through tool that would allow me to watch how this code executes?

Answer 1

PHP respects brackets. Expressions inside the innermost ( ... ) are evaluated first, like we are taught in elementary school.
PHP is unusual in that ternary operators are left-associative. This means without brackets, the ternary expression is evaluated left to right.
But in this particular case, the brackets force the expression to be evaluated right to left. This code is equivalent to:
if ($score >= 90) {
$grade = 'A';
}
elseif ($score >= 80) {
$grade = 'B';
}
elseif ($score >= 70) {
$grade = 'C';
}
...

Answer 2

The ternary operator is left associative, but with brackets applied, it evaluates right to left.
You might use xdebug or phpdbg as a step debugger to step through your code and see how it evaluates.
There is also VulcanLogicDumper around, which shows the instructions:
http://3v4l.org/QeF9i/vld#tabs compared to an if-elseif-else structure http://3v4l.org/bZE6M/vld#tabs
line # * op fetch ext return operands
---------------------------------------------------------------------------------
3 0 > SEND_VAL 50
1 SEND_VAL 100
2 DO_FCALL 2 $0 'rand'
3 ASSIGN !0, $0
5 4 IS_SMALLER_OR_EQUAL ~2 90, !0
5 > JMPZ ~2, ->8
6 > QM_ASSIGN ~3 'A'
7 > JMP ->24
6 8 > IS_SMALLER_OR_EQUAL ~4 80, !0
9 > JMPZ ~4, ->12
10 > QM_ASSIGN ~5 'B'
11 > JMP ->23
7 12 > IS_SMALLER_OR_EQUAL ~6 70, !0
13 > JMPZ ~6, ->16
14 > QM_ASSIGN ~7 'C'
15 > JMP ->22
8 16 > IS_SMALLER_OR_EQUAL ~8 60, !0
17 > JMPZ ~8, ->20
18 > QM_ASSIGN ~9 'D'
19 > JMP ->21
20 > QM_ASSIGN ~9 'F'
21 > QM_ASSIGN ~7 ~9
22 > QM_ASSIGN ~5 ~7
23 > QM_ASSIGN ~3 ~5
24 > ASSIGN !1, ~3
10 25 ADD_STRING ~11 '%3Ch1%3EA+score+of+'
26 ADD_VAR ~11 ~11, !0
27 ADD_STRING ~11 ~11, '+%3D+'
28 ADD_VAR ~11 ~11, !1
29 ADD_STRING ~11 ~11, '%3C%2Fh1%3E'
30 ECHO ~11
31 > RETURN 1
How to read these Opcodes
I will try to explain the first JMPZ in the opcode, in order to understand how it evaluates:
Of interest is Line 5, Opcode Number 5:
5 > JMPZ ~2, ->8
This means: If compare with 90 (opcode 4) is false, then JUMP to Opcode 8.
Warning: ->8 doesn't mean jump to Line 8.
Now, what is Opcode 8? The comparison with 80
6 8 > IS_SMALLER_OR_EQUAL ~4 80, !0
And now it's safe to say that this doesn't evaluate, like you expected from inside out (90->60->70), but like an if-elseif-else structure (90->80->70).

Answer 3

The ternary operator short-circuits. Only the appropriate operand is evaluated. This means that the parens do not matter until they are actually tested.
echo false ? (crash() / 0) : "Worked.";

Answer 4

ternary are left to right:
http://php.net/manual/en/language.operators.comparison.php#language.operators.comparison.ternary
So it will evaluate what is to the left of the ? and based on that, evaluate either the 1st or 2nd side of the :.
You could put in a function call that has a side effect to demonstrate this:
function p($a,$b) { echo $a . " >= " . $b; return $a>=$b; }
$grade=(p($score,90))?'A':(
p($score,80)?'B':(
p($score,70)?'C':(
p($score,60)?'D':'F')));