Couldn't understand the ternary operator behavior

Question

Output of the following program is : 3 1 3

int main()
{
    int a = 0, b = 1, c = 3;
    *((a) ? &b : &a) = a ? b : c; // Couldn't understand expression
    printf("%d %d %d \n", a, b, c);
    return 0;
}

How ternary operator is working here for this output?

Which part is unclear? `a` is `0` so it should be obvious which option is chosen in both cases. — JJJ, Aug 13 '13 at 15:18
Tell us what you think the output should be along with your reasoning and we will tell you if you are right or wrong. — Colin D, Aug 13 '13 at 15:19
I think the question is about the references being used in the first ternary operator. It is a bit confusing. — Reinstate Monica -- notmaynard, Aug 13 '13 at 15:20
@iamnotmaynard especially confusing since they're not references; they're variable addresses, of which one (being returned from the ternary condition) is being *dereferenced*. Methinks you're on to something, however. Perhaps the OP just doesn't do pointers. — WhozCraig, Aug 13 '13 at 15:29
@WhozCraig Stupid question here: what is the difference between a reference and a variable address? (I haven't used C in a while; I mostly do Java, which doesn't really do pointers, except when it kind of does.) — Reinstate Monica -- notmaynard, Aug 13 '13 at 15:38
@iamnotmaynard Addresses are the fundamental "values" of pointers, where-as references, (the C++ term) a different, more restrictive (and thats a good thing) beast. See [this discussion](http://stackoverflow.com/questions/57483/what-are-the-differences-between-pointer-variable-and-reference-variable-in-c). — WhozCraig, Aug 13 '13 at 15:40

score 5 · Accepted Answer · answered Aug 13 '13 at 15:30

5

 *( (a) ? &b : &a) =     a ? b : c ;
     ^                   ^
     = 0                 = 0
     ==false             ==false

so the above expression:

 *( &a) =  c ;

that is:

     a =  c ;

because c = 3;, so it give:

     a =  3 ;

b is 1 and c is 3 in your declaration(remains unchanged).

answered Aug 13 '13 at 15:30

Grijesh Chauhan

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read: [how Conditional operator works?](http://en.wikipedia.org/wiki/Conditional_%28programming%29#If_expressions) – Grijesh Chauhan Aug 13 '13 at 15:38
So if a=1, then result would be 1,1,3? – Jiminion Aug 13 '13 at 15:47
@Jim suppose if `a = 1` then expression would be == to `b = b`, and yes then o/p will be 1, 1, 3 – Grijesh Chauhan Aug 13 '13 at 15:57

Reinstate Monica -- notmaynard · Answer 2 · 2013-08-13T15:40:47.363

My C is a little weak, so if I'm off-base here, someone let me know.

Assuming you're talking about the expression *((a) ? &b : &a) (and that you know how the ternary operator works generally), here's how it breaks down:

The condition (a) is evaluated: when using an int in a boolean, 0 evaluates to false, while positive (or non-zero?) values evaluate to true — in this case, a == 0 is false.

The ternary then returns the value &b if true and &a if false (in this case it's false, so it returns &a).

The value thus returned is a reference to a variable; the asterisk before the surrounding parentheses dereferences this reference, so the expression *((a) ? &b : &a) evaluates to the variable — but not the value of — a (if a where not 0, the expression would evaluate to b).

That line then assigns to this variable (in this case a) the value of the second ternary expression, which I assume you can figure out.

The final effect is that it assigns the value of c to the variable a, which explains the output.

score 0 · Answer 3 · answered Aug 13 '13 at 15:41

Note that the ternary conditional operator has higher precedence than the assignment operator. The ternary conditional has the form:

condition ? true-result : false-result

If condition is true, the result is true-result, otherwise the result is false-result. The operator is short-circuiting in the sense that only one of the results is ever evaluated.

This operator can be used for conditional assignment. For example:

int days_of_year = is_leap_year ? 366 : 365;

The result of the ternary conditional is an r-value. This means that the result is a value that cannot be the target of an assignment. However, there is a trick that can be used to use the ternary conditional with pointers and dereference so as to get the behavior of conditionally assigning to one variable or another.

int x;
int y;
*(need_x ? &x : &y) = 0;

While the result of a ternary condition is an r-value, the result of a dereference is an l-value. This means the result of the dereference can be used as the target of an assignment.

Your code uses both of these ideas in combination.

Couldn't understand the ternary operator behavior

3 Answers3