Are "arithmetic expressions" separated by commas allowed in C or we need separate statements for each?

Question

In C, is the following statement

a+=3,b=c*2,d=a+b;

equivalent to the following block of statements:

a+=3;
b=c*2;
d=a+b;

I am sure you got my point. Can we safely use multiple mathematical expressions separated by commas in the same statement in C? And in what cases this can pose problems?

Answer 1

It might be easier if you think of the comma-expression list you present like this:

((a += 3, b = c * 2), d = a + b)

First the innermost comma-expression is evaluated:

a += 3, b = c * 2

That expression will be evaluated in two steps:

a += 3
b = c * 2

The result of a += 3 will be thrown away by the compiler, but the assignment still happens, it's just that the returned result is thrown away. The result of the first comma expression is b (which will be c * 2 (whatever that is)).

The result of the first comma expression is then on the left-hand side of the next comma expression:

b = c * 2, d = a + b

Which will then be sequenced as

b = c * 2
d = a + b

The result of the expression b = c * 2 is thrown away (but as it is still evaluated the assignment still happens), and the result of the complete expression is d (which is a + b).

The result of the whole expression will be d.