Using increment in ternary operator in C

Question

For the example, after I use

int a = 5, b = 6;
x = (a < b) ? a++ : b++;

x gets the value of a, which is 5, and a increments to 6, which is expected.

When I use

a = (a < b) ? a++ : b++;

After this line, a still remains 5.

But

a = (a++ < b++) ? a : b;

a is now 6.

Why is this happening and why isn't increment operator executed in the first case?

EDIT: Just to clarify, I'm asking why this happens when I'm using these lines separately, one by one, not all three in the same time.

Answer 1

a = (a < b) ? a++ : b++;

here, we stored a in a, and then incremented it. But it is like

a = a++; // as a<b

which shows undefined behaviour.

a = (a++ < b++) ? a : b;

here, a is being incremented at the time of comparison, so now a is 6, which is stored in a.

Answer 2

Both cases involve undefined behaviour of some sort because you are incrementing a, returning a and assigning to a on the left side within 2 sequence points. 3 would be required for a clearly defined result.

In this case : a = (a < b) ? a++ : b++;

1. if a is smaller than b
2. a is returned (value = 5) as the result of the ternary operator
3. a is incremented (value = 6).
4. the result of the ternary operator (5) is assigned to the left hand side variable a (over-writing 6)

The order of steps 3 and 4 is not defined. It is equivalent to a = a++;

In this case : a = (a++ < b++) ? a : b;

1. If a is smaller that b
2. a and b are incremented (regardless which is smaller)
3. a is returned as the result of the ternary operator
4. it is assigned to the left hand side variable a

The order of steps 2 and 3 is not clearly defined.

It's important to keep track of sequence points in such cases. Relevant rules :

• The 1st expression of ternary operator on the left of ? is sequenced before the 2nd or 3rd expressions. And either of them is sequenced before assignment.
• The comparison is sequenced before the ?
• In expressions like a++ the value is returned before incrementing

Undefined behaviour:

• In expressions like a = a++; there is no sequence point between (post)incrementing a and assigning to a on the left side. Both happen after the original value of a is returned
• In expressions like a++ ? a : b there is no sequence point between (post)incrementing a and returning a from the ternary operator. Both happen after the ?