Why is the conditional operator right associative?

Question

I can understand why the assignment operator is right associative. It makes sense that when

x = 4 + 3 

is evaluated, that 4 and 3 are added before being assigned to x.

I am unclear as to how ?: would benefit from being right associative. Does it only matter when two ?:s were used like this

z = (a == b ? a : b ? c : d); 

Then it is evaluated like this:

z = (a == b ? a : (b ? c : d)); 

Surely it would make more sense to evaluate from left to right?

Answer 1

If it evaluated from left to right, it'd look like this:

z = ((a == b ? a : b) ? c : d); 

That is, it would use the result of the first conditional (a or b) as the boolean condition of the second conditional. That doesn't make much sense: that's like saying:

int z, tmp; /* first conditional */ if(a == b) tmp = a; else       tmp = b; /* second conditional */ if(tmp) z = c; else    z = d; 

While perhaps one day you'll want to do exactly this, it's far more likely that each ?: that follows is meant to add more conditions, like if / else if / else if / else, which is what the right-associative binding yields:

int z; /* first conditional */ if(a == b)                          z = a; else /* second conditional */ if(b) z = c; else                                z = d; 

Answer 2

In any language with a right associative ternary operator, you can stack them and build an if-elseif-elseif-else expression, like this:

val = a == 0 ? 1:       a == 1 ? 2:                4; 

On the contrary, in languages with a left associative ternary operator (such as PHP, thanks @user786653) you need to explicitly enforce the aforementioned intent with parentheses:

<?php // This will output 't', not 'true'. echo (true ? 'true' : false ? 't' : 'f');  // the following is a more obvious version of the same code as above echo ((true ? 'true' : false) ? 't' : 'f');  // here, you can see that the first expression is evaluated to 'true', which // in turn evaluates to (bool)true, thus returning the true branch of the // second ternary expression. ?>