Comma operator precedence while used with ? : operator

Question

I have no idea why the result of the two sub programs below are different:

    int a , b;

    a = 13, b=12;
    (a > b)? (a++,b--):(a--,b++); // Now a is 14 and b is 11

    a = 13, b=12;
    (a > b)? a++,b-- : a--,b++;   // Now a is 14 but b is 12

However for these cases, the results are identical:

    a = 13, b=12;
    (a < b) ? a++,b-- : a--,b++; // Now a is 12 and b is 13

    a = 13, b=12;
    (a < b) ? (a++,b--) : (a--,b++); // Again a is 12 and b is 13

Why parentheses make difference for the statement after "?", but make no difference for the statement after ":"? Do you have any idea?

One more reason not to use the comma in this way. – TobiMcNamobi May 31 '13 at 09:31 — TobiMcNamobi, May 31 '13 at 09:31

score 24 · Accepted Answer · answered May 31 '13 at 09:28

This one:

(a > b)? a++,b-- : a--,b++;

is equivalent to:

((a > b) ? (a++, b--) : a--), b++;

so b is always incremented and only sometimes decremented. There is no way to parse the comma operator between ? and : other than as parenthesized in the 'equivalent to' expression. But after the :, the unparenthesized comma terminates the ternary ?: operator and leaves the increment as unconditionally executed. The precedence of the comma operator is very, very low.

CB Bailey · Answer 2 · 2013-05-31T09:44:27.947

The relevant parts of the C++ grammar are:

conditional-expression:
    logical-or-expression
    logical-or-expression ? expression : assignment-expression

assignment-expression:
    conditional-expression
    logical-or-expression assignment-operator assignment-expression
    throw-expression

expression:
    assignment-expression
    expression, assignment-expression

In summary, while the 'middle' of a conditional expression can be a full expression extending up to the :, the last sub-expression can only be an assignment-expression which excludes expressions using the comma operator (other than where the comma operator appears as part of valid sub-expression of an assignment-expression such as a parenthesized primary-expression or as the second operand of another conditional-expression).

In C, the last sub-expression is more restricted, it cannot even be an assignment-expression although this is not a concern in your example.

conditional-expression:
    logical-OR-expression
    logical-OR-expression ? expression : conditional-expression

This has to come from the C++ standard since 'throw-expression' is not a part of C. However, the question is dual-tagged, so it is valid. — Jonathan Leffler, May 31 '13 at 09:40
@JonathanLeffler: Yes, it does. There are important differences between the C and C++ conditional operator grammars; I was browsing C++ questions and didn't notice the C tag. — CB Bailey, May 31 '13 at 09:44

score 3 · Answer 3 · edited May 31 '13 at 09:33

3

In this case

(a > b)? a++,b-- : a--,b++;

It is equivalent to

((a > b)? a++,b-- : a--),b++;

edited May 31 '13 at 09:33

Jonathan Leffler

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answered May 31 '13 at 09:28

Naruil

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score 2 · Answer 4 · answered May 31 '13 at 09:29

2

I guess it's because x ? y cannot be considered a valid expression, therefore the comma can't split the operator there. x ? y : z is a valid expression, the the comma after the colon can split into two expressions.

answered May 31 '13 at 09:29

Medinoc

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Comma operator precedence while used with ? : operator

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