What does the comma operator mean?

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?

It would in Python, but not in C. – Fred Larson May 31 '17 at 14:49 — Fred Larson, May 31 '17 at 14:49
that's not a *variable declaration in **C***. – Antti Haapala -- Слава Україні May 31 '17 at 14:52 — Antti Haapala -- Слава Україні, May 31 '17 at 14:52

Vlad from Moscow · Accepted Answer · 2017-05-31T15:37:03.447

6

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.

edited May 31 '17 at 15:37

answered May 31 '17 at 14:49

Vlad from Moscow

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Wouldn't B be 2? I believe that the result of a sequence separated by the comma operator is the _last_ element evaluated. Hence the value assigned will be 2. That is also what your quote says: "_the result has its type and value_". – Paul Ogilvie May 31 '17 at 14:55
@PaulOgilvie It would, if the , operator had higher precedence than = – Ctx May 31 '17 at 14:56
3

@PaulOgilvie The assignment operator has higher priority than the comma operator. – Vlad from Moscow May 31 '17 at 14:56
1

Thanks. Never too old to learn. – Paul Ogilvie May 31 '17 at 14:56
@PaulOgilvie I agree with you.:) – Vlad from Moscow May 31 '17 at 14:57

What does the comma operator mean?

1 Answers1