How does C evaluate 2 minuses with 2 operands?

Question

Consider:

    int a;
    a = -3--3;
    printf("%d", a);

The 2nd line (a = -3--3) produces a compile time error.
On adding an extra pair of brackets as follows:

    int a;
    a = -3 - (-3);
    printf("%d", a);

the error disappears.

Since the brackets remove the error I believe that some ambiguity must have caused it.

But I don't see why the compiler is confused. I try to subtract -3 from -3 .

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All help is appreciated.

Answer 1

C has a unary decrement operator that's spelled -- , and a "maximal munch rule" that makes it recognize that operator rather than a minus sign and a negative sign even where the latter would make more sense. For about half a dozen accompanying reasons, 3 -- 3 doesn't make sense, and you get an error.

If you'd said a = -3 - -3; , instead of cramming everything together like your space bar's broken, you'd have been fine. :PA space between the two operators keeps C from seeing a -- .

Answer 2

Taking for granted that you know that "--" represents a unary operator for pre/post decrement, it is easy to understand the reason why the compilers throw an error.

The parser of the C compiler reads the first minus sign, then, it expects either a number (meaning that your intention is to perform a regular substraction) or another minus sign (meaning that your intention is to perform a pre-decrement unary operation. In your case, you are basically telling the C compiler that you want to perform a pre-decrement. It doesn't make sense for two reasons:

a) If your intention was actually to perform a pre-decrement operation, then you are missing an extra binary operator (eg "+"), just like this: a = -3 + (--3).

b) The other reason (which makes my previous example also to throw a compile time error, is that you are trying to perform a unary operation on a constant. Totally non-sense, since the compiler reserves memory for a constant that you are, later on, trying to modify.

Hope this helps to get the concept of expressions in "C", a source of many headaches for beginners (and even for experts sometimes!).

My advice: keep the code clean, meaningful and use braces as much as common sense tells, which is the contrary that you did, in my opinion.