Confused by squaring macro SQR in c

Question

This question was asked to me in a mock interview...Really got surprised to find awkward answers...

consider a macro:

#define SQR(x) (x*x)

Example 1:

SQR(2) //prints 4

Example 2:

If SQR(1+1) is given it doesn't sum (1+1) to 2 but rather ...

SQR(1+1) //prints 3

Awkward right? What is the reason? How does this code work?

NOTE: I searched SO but couldn't find any relevant questions. If there are any kindly please share it!

This is the C-preprocessor. You could achieve the same behavior if you used the C-preprocessor with any language. — user7116, Jun 12 '13 at 17:22
see this link for why macros can totally suck. Also, see this comment to see why I suck at html. (seriously, wtf, why doesnt my link work?) — ash, Jun 12 '13 at 17:28
Thank u all!!! It was a piece of info for beginner lik me!!! I love SO!!! — abu, Jun 12 '13 at 17:35
@ash because is html rather than the mini markdown... click the help button below or format like: `[link](http://www.iso-9899.info/wiki/Why_not_macros)` — Grady Player, Jun 12 '13 at 17:38
ah ha! in which case, I meant to do this: [why not macros?](http://iso-9899.info/wiki/Why_not_macros) — ash, Jun 12 '13 at 17:41
@abu Interesting post for you: [MAX using typeof extension of gcc](http://stackoverflow.com/questions/17039325/max-using-typeof-extension-of-gcc?answertab=votes#tab-top) — Grijesh Chauhan, Jun 12 '13 at 17:44
The title is about `sqrt`, but nothing int he question is about `sqrt`. — Kaz, Jun 12 '13 at 18:04

Luchian Grigore · Accepted Answer · 2013-06-12T17:30:22.423

47

SQR(1+1) expands to 1+1*1+1 which is 3, not 4, correct?

A correct definition of the macro would be

#define SQR(x) ((x)*(x))

which expands to (1+1)*(1+1) and, more important, shows you one of the reasons you shouldn't use macros where they aren't needed. The following is better:

inline int SQR(int x)
{
    return x*x;
}

Furthermore: SQR(i++) would be undefined behavior if SQR is a macro, and completely correct if SQR is a function.

edited Jun 12 '13 at 17:30

answered Jun 12 '13 at 17:20

Luchian Grigore

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Of course, even the "corrected" macro can still surprise you when it evaluates the argument twice. – Mike Seymour Jun 12 '13 at 17:28
@MikeSeymour this what you had in mind? – Luchian Grigore Jun 12 '13 at 17:30
Hi Luchian. It's simple to see that `x*x` is wrong. Also, `(x*x)` just puts that in parenthses. Can you come up with an example where `(x)*(x)` is more wrong than the final solution `((x)*(x))`? I think we need an operator that has higher precedence than `*`. I can think of `sizeof SQR(x)` go wrong. Is there a more realistic example than that? – Thomas Weller May 17 '23 at 11:59
Also, do you know why SQR(i++) is UB? A reference to the C++ standard would be nice. – Thomas Weller May 17 '23 at 12:21

Devolus · Answer 2 · 2023-07-20T04:35:29.300

5

The problem is that macros are doing textual substition before it is compiled, so the macro expands to 1+1*1+1 which means 1+(1*1)+1 because of the mathematical rules, which in turn evaluates to 1+1+1 = 3 which is the correct result. That's why you should put that in brackets.

#define SQR(x) ((x)*(x))

edited Jul 20 '23 at 04:35

answered Jun 12 '13 at 17:21

Devolus

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score 5 · Answer 3 · answered Jun 12 '13 at 17:23

5

That is why you always put arguments to macros into ():

#define SQR(x) ((x)*(x))

answered Jun 12 '13 at 17:23

Philipp T.

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Confused by squaring macro SQR in c

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