C Programming Macros multiplication

Question

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C Macros and use of arguments in parentheses

I found this macro question to be very interesting.

If this following code is defined as a Macro

#define MULT(x, y) x * y

And the function call is made as int z = MULT(3 + 2, 4 + 2);. The Desired output is 3+2=5 and 4+2=6 And 5*6 to be 30.

But the returned output was 13. It takes it as 3+2*4+2. Hence according to the precedence of operators it evaluates 2*4 first.

What is the fix here? In case of smaller functions like these which one is efficient? Defining the function or using the macros?

How do you measure efficiency? Coding time, execution time, maintainability... — Jens Gustedt, Jun 01 '12 at 10:00
The macro is probably more efficient. If the optimiser inlines the function they will probably be equivalent. — Klas Lindbäck, Jun 01 '12 at 11:20

score 12 · Accepted Answer · answered Jun 01 '12 at 09:53

12

Try something like:

#define MULT(x, y) ((x)*(y))

answered Jun 01 '12 at 09:53

aztaroth

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Thank you. But the question was more concerned about efficiency. – Katti Jun 01 '12 at 09:57
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@Katti: how is this in any way related to efficiency ? – Paul R Jun 01 '12 at 09:58
Why not just use `x * y` and skip the macro or function, then? – aztaroth Jun 01 '12 at 09:58
@PaulR, fewer parentheses, smaller compile time. Bam! (Just kidding) – Shahbaz Jun 01 '12 at 09:59
Macros are direct substitution and functions are called and the data is sent to them. So there has to be some difference is what I felt. – Katti Jun 01 '12 at 09:59
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@Katti: bad assumption - an inline function is just as efficient as a macro - there is rarely a reason to use a macro for this kind of thing with modern compilers. – Paul R Jun 01 '12 at 10:00
A function would be slower (unless you inline it/the call gets optimized away). – aztaroth Jun 01 '12 at 10:00
@Katti, you must believe in the intelligence of compilers nowadays. – Shahbaz Jun 01 '12 at 10:08

score 0 · Answer 2 · answered Jun 01 '12 at 09:54

0

Try this:

#define MULT(x, y) ((x) * (y))

Personally, I think it's more readable (and fewer keystrokes) to not use the macro. This is the kind of thing that someone will start adding to their code in the rush of learning new things and regret later on.

answered Jun 01 '12 at 09:54

duffymo

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You are still missing parentheses. Imagine `z/MULT(x, y)` – Shahbaz Jun 01 '12 at 09:54

C Programming Macros multiplication

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