How is this conditional parsed?

For some context, the conditional is used in the normal type of algorithm for finding linked list cycles:

From link: http://vijayinterviewquestions.blog开发者_如何学JAVAspot.com/2007/05/how-would-you-detect-loop-in-linked.html

p=head;
q=head->next;

while(p!=NULL && q!=NULL) {
    if(p==q) { //Loop detected! exit(0); }

    p=p->next;
    q=(q->next)?(q->next->next):q->next;
}

// No loop.

What does the line:

q=(q->next)?(q->next->next):q->next;

get parsed as though? I'm a little confused about operator precedence in this conditional - does q take the value of the left hand side of the ? or of the whole conditional?

---

that can be expanded to:

if (q->next) {
  q = q->next->next;
} else {
  q = q->next;
}

---

Its called the ternary operator and it means

if(q->next)
  q->next->next;
else
  q->next;

---

You're seeing an example of the ternary operator.

q=(q->next)?(q->next->next):q->next;

can also be read as:

if(q->next)
{
    q = (q->next->next)
}
else
{
    q = q->next;
}

The ternary operator has precedence lower than most other operators (excepting the comma operator, "throw" and all assignment operators).

http://en.cppreference.com/w/cpp/language/operator_precedence

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If you have a look at an operator precedence table you'll see that the ternary operator ?: is near the bottom of the list. This means that the subexpressions within it will be evaluated first. The parenthesis within this grouping just reinforce the fact; they were probably added for readability.

The beginning of the expression (q->next) is evaluated to see if it is true or not by comparing to zero; in the case of a pointer, a NULL pointer is zero or false and any other value is true.

The result of a ternary operator will be the first expression after the ? if the part to the left of the ? is true, otherwise it will be the expression after the :.

---

It takes the value of the whole conditional expression, since the assignment operator has very low precedence (second only to throw and ,). The expression in your question is equivalent to:

q = q->next ? q->next->next : q->next;

In order for the assignment to take priority, you would have to write something like:

(q = q->next) ? q->next->next : q->next;

Which, of course, always assigns q->next to q, then proceeds to evaluate one of the remaining conditional operands into thin air.