Ternary operator left associativity [duplicate]

Question

In the PHP manual, I find the following 'user contributed note' under "Operators".

Note that in php the ternary operator ?: has a left associativity unlike in C and C++ where it has right associativity.

You cannot write code like this (as you may have accustomed to in C/C++):

<?php 
$a = 2; 
echo ( 
    $a == 1 ? 'one' : 
    $a == 2 ? 'two' : 
    $a == 3 ? 'three' : 
    $a == 4 ? 'four' : 'other'); 
echo "\n"; 
// prints 'four' 

I actually try it and it really prints four. However I could not understand the reason behind it and still feel it should print two or other.

Can someone please explain what is happening here and why it is printing 'four'?

Answer 1

In any sane language, the ternary operator is right-associative, such that you would expect your code to be interpreted like this:

$a = 2;
echo ($a == 1 ? 'one' :
     ($a == 2 ? 'two' :
     ($a == 3 ? 'three' :
     ($a == 4 ? 'four' : 'other'))));    # prints 'two'

However, the PHP ternary operator is weirdly left-associative, such that your code is actually equivalent to this:

<?php
$a = 2;
echo (((($a == 1  ? 'one' :
         $a == 2) ? 'two' :
         $a == 3) ? 'three' :
         $a == 4) ? 'four' : 'other');   # prints 'four'

In case it still isn't clear, the evaluation goes like this:

echo ((((FALSE    ? 'one' :
         TRUE)    ? 'two' :
         $a == 3) ? 'three' :
         $a == 4) ? 'four' : 'other');

echo ((( TRUE     ? 'two' :
         $a == 3) ? 'three' :
         $a == 4) ? 'four' : 'other');

echo ((  'two'    ? 'three' :
         $a == 4) ? 'four' : 'other');

echo (    'three' ? 'four' : 'other');

echo 'four';

Answer 2

Because your whole expression evaluates as if it was (......) ? 'four' : 'other'. Since the first element is probably something truthy, it gives you 'four'. In saner languages, where ?: has right associativity, the whole expression evaluates as if it was $a == 1 ? 'one' : (......), where if $a is not 1, you go on to test other things.

Answer 3

This is what I came up with to help myself understand left vs. right associativity for the ternary operator.

// PHP

$a = "T";
$vehicle =  $a == "B" ? "bus" :
            $a == "A" ? "airplane" :
            $a == "T" ? "train" :
            $a == "C" ? "car" :
            $a == "H" ? "horse" : "feet";

            // (as seen by the PHP interpreter)
            // INITIAL EXPRESSION: ((((($a == "B" ? "bus" : $a == "A") ? "airplane" : $a == "T") ? "train" : $a == "C") ? "car" : $a == "H") ? "horse" : "feet");
            // STEP 1:             (((((FALSE ? "bus" : FALSE) ? "airplane" : TRUE) ? "train" : FALSE) ? "car" : FALSE) ? "horse" : "feet")
            // STEP 2:             ((((FALSE ? "airplane" : TRUE) ? "train" : FALSE) ? "car" : FALSE) ? "horse" : "feet")
            // STEP 3:             (((TRUE ? "train" : FALSE) ? "car" : FALSE) ? "horse" : "feet")
            // STEP 4:             (("train" ? "car" : FALSE) ? "horse" : "feet")
            // STEP 5:             ("car" ? "horse" : "feet")
            // FINAL EVALUATION:   ("horse")

            // If you used the initial expression here (with the parenthesis) in a different language, it would also evaluate to "horse."

echo $vehicle; // gives us "horse"

This is as opposed to:

// EVERY OTHER LANGUAGE

var a = "T";
var vehicle =   a == "B" ? "bus" :
                a == "A" ? "airplane" :
                a == "T" ? "train" :
                a == "C" ? "car" :
                a == "H" ? "horse" : "feet";

                // (as seen by the other language's interpreter)
                // INITIAL EXPRESSION: (a == "B" ? "bus" : (a == "A" ? "airplane" : (a == "T" ? "train" : (a == "C" ? "car" : (a == "H" ? "horse" : "feet")))));
                // STEP 1:             (FALSE ? "bus" : (FALSE ? "airplane" : (TRUE ? "train" : (FALSE ? "car" : (FALSE ? "horse" : "feet")))))
                // STEP 2:             (FALSE ? "bus" : (FALSE ? "airplane" : (TRUE ? "train" : (FALSE ? "car" : "feet"))))
                // STEP 3:             (FALSE ? "bus" : (FALSE ? "airplane" : (TRUE ? "train" : "feet")))
                // STEP 4:             (FALSE ? "bus" : (FALSE ? "airplane" : "train"))
                // STEP 5:             (FALSE ? "bus" : "train")
                // FINAL EVALUATION:   ("train")

                // If you used the initial expression here (with the parenthesis) in PHP, it would also evaluate to "train."

console.log(vehicle); // gives us "train"

If you notice, in the PHP example, the innermost expression is on the left, and on the second example, the innermost expression is on the right. Each step evaluates the next innermost expression until there is a single result. Parenthesis are clearly very important if you're going to nest ternary operations in PHP!