syntax - Assignment of two values in parentheses in C

Question

What does this piece of code in C do:

p = (1, 2.1);

What do we know about p?

Answer 1

The comma operator in C is a sequence point which means that the expressions separated by the comma are executed from left to right. The value of the whole expression is the value of the rightmost expression, in your case 2.1, which gets assigned to the variable p.

Since the expressions in your example don’t have side effects, the use of the comma separator here makes no sense whatsoever.

The parentheses on the other hand are important since the assignment operator (=) binds stronger than the comma operator (it has higher precedence) and would get evaluated before the comma operator without the parentheses. The result would thus be p == 1.