What does the comma operator mean? [duplicate]

Question

I've declared variables and their values before but I've never done this on a single line before.

If I write

A, B = 0.0, 2;

Does this mean

A = 0 

and

B = 2? 

Answer 1

This expression

A, B = 0.0, 2;

is an expression with the comma operator (here are two comma operators). It can be presented like

( A ), ( B = 0.0 ), ( 2 );

As result the variable B will get the value 0.0. The variable A will be unchanged.

From the C Standard (6.5.17 Comma operator)

2 The left operand of a comma operator is evaluated as a void expression; there is a sequence point between its evaluation and that of the right operand. Then the right operand is evaluated; the result has its type and value

So the value of the above expression is 2 and the type is int. The value of the expression is not used. So the only its side effect is assigning the value 0.0 to the variable B.