Difference between a MACRO and a FUNCTION

[MAx]

Hi,
Could any one list possible number of diferences between a function
and a macro?
which one will be faster and why??

ex : function and a macro ti find max of two nums
#define MAX(a,b) (a>b) ? a:b

and
int max(int a,int b)

[Army1987]

On Thu, 20 Sep 2007 11:04:55 +0000, Richard Heathfield wrote:

Army1987 said:

>On Thu, 20 Sep 2007 00:04:05 -0700, MAx wrote:

<snip>

>>#define MAX(a,b) (a>b) ? a:b

Please add enough parentheses:
((a) (b) ? a : (b))

You missed some.

Syntax
1 conditional-expression:
logical-OR-expression
logical-OR-expression ? expression : conditional-expression

(As someone pointed out on this newsgroup, ? and : can only occur
in pairs, so they are like [ and ], or ( and ) for this matter.)
--
Army1987 (Replace "NOSPAM" with "email")
If you're sending e-mail from a Windows machine, turn off Microsoft's
stupid “Smart Quotes� feature. This is so you'll avoid sprinkling garbage
characters through your mail. -- Eric S. Raymond and Rick Moen

[Richard Heathfield]

Philip Potter said:

Richard Heathfield wrote:

>Army1987 said:

>>On Thu, 20 Sep 2007 00:04:05 -0700, MAx wrote:

<snip>

>>>#define MAX(a,b) (a>b) ? a:b
Please add enough parentheses:
((a) (b) ? a : (b))

You missed some.

<snip>

Really?

No, not really - at least, I just tried three times to break his version
and failed each time. So I owe him an apology.

(((a)>(b)) ? a : (b))

has the same meaning, as does

((a)>(b) ? (a) : (b))

Though this does look nicer due to consistency.

Yes, it does, doesn't it? Emerson notwithstanding ...

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Army1987]

On Thu, 20 Sep 2007 11:44:58 +0000, Richard Heathfield wrote:

Philip Potter said:

>Richard Heathfield wrote:

>>Army1987 said:
Please add enough parentheses:
((a) (b) ? a : (b))
You missed some.

[snip]

>(((a)>(b)) ? a : (b))

has the same meaning, as does

((a)>(b) ? (a) : (b))

Though this does look nicer due to consistency.

Yes, it does, doesn't it? Emerson notwithstanding ...

It does (maybe whitespace around will add even more
consistency?), but who's Emerson?
--
Army1987 (Replace "NOSPAM" with "email")
If you're sending e-mail from a Windows machine, turn off Microsoft's
stupid “Smart Quotes� feature. This is so you'll avoid sprinkling garbage
characters through your mail. -- Eric S. Raymond and Rick Moen

[Eberhard Schefold]

Army1987 wrote:

It does (maybe whitespace around will add even more
consistency?), but who's Emerson?

"A foolish consistency is the hobgoblin of little minds."
-- Ralph Waldo Emerson (1803–1882)

Don't think I'm a literary person, but I remembered the line from
"What's Up, Doc". :-)

[John Bode]

On Sep 20, 2:04 am, MAx <mahesh1...@gma il.comwrote:

Hi,
Could any one list possible number of diferences between a function
and a macro?

Grab your handy C reference and read the section about the
preprocessor, focusing on the terms "definition " and "replacemen t".
After that, it should be quite obvious what the differences between a
function and a function-like macro are.

which one will be faster and why??

That depends on the operations involved. For your example below, all
things being equal, the macro should be faster. Again, once you've
read up on what exactly the preprocessor does with a function-like
macro definition, the reason why should be fairly evident.

>
ex : function and a macro ti find max of two nums
#define MAX(a,b) (a>b) ? a:b

and
int max(int a,int b)

[Keith Thompson]

MAx <ma********@gma il.comwrites:

Could any one list possible number of diferences between a function
and a macro?
which one will be faster and why??

ex : function and a macro ti find max of two nums
#define MAX(a,b) (a>b) ? a:b

and
int max(int a,int b)

Is this homework?

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Keith Thompson]

Richard Heathfield <rj*@see.sig.in validwrites:

Philip Potter said:

>Richard Heathfield wrote:

>>Army1987 said:
On Thu, 20 Sep 2007 00:04:05 -0700, MAx wrote:

<snip>

#define MAX(a,b) (a>b) ? a:b
Please add enough parentheses:
((a) (b) ? a : (b))

You missed some.

<snip>

Really?

No, not really - at least, I just tried three times to break his version
and failed each time. So I owe him an apology.

[...]

Nevertheless, I would certainly use parentheses myself:
#define MAX(a, b) ((a) (b) ? (a) : (b))
rather than
#define MAX(a, b) ((a) (b) ? a : (b))

Consistency is certainly part of it. Another part is that I'm *still*
not 100% convinced that the parentheses are unnecessary. I have a
nagging hunch that there *might* be an argument that would break the
first form, perhaps something involving the ?: operator in the operand
itself. Or perhaps there's some argument that should be a syntax
error, but that will cause the unparenthesized version to expand to a
legal expression that doesn't do what you want.

It's very likely that my concern is groundless, and that the
unparenthesized form is perfectly safe. But if it takes an
experienced C programmer like RH three tries, not to prove that it's
safe, but merely to fail to prove that it isn't, then it's likely to
waste some of the time of every attentive programmer who reads it. If
I'm tracking down a bug that manifests itself anywhere near any use of
this macro, I'm likely to waste time convincing myself that the macro
isn't the problem before persuing other possibilities.

We had something similar here recently, with a macro definition
something like:

#define N -42

Several people immediately complained that it *should* be

#define N (-42)

but it took a day or two before anyone was able to present an example
where it actually made a difference.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Richard Heathfield]

Keith Thompson said:

I have a
nagging hunch that there *might* be an argument that would break the
first form, perhaps something involving the ?: operator in the operand
itself.

That was my principal method of attack. I couldn't break it that way, but
that doesn't mean it can't be broken. (Nor does it mean it can,
obviously.)

I suspect that a grammatical analysis will bear more fruit than any number
of attempts at a crack.

<snip>

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[=?iso-2022-kr?q?=1B=24=29CHarald_van_D=0E=29=26=0F]

On Thu, 20 Sep 2007 12:39:57 -0700, Keith Thompson wrote:

Richard Heathfield <rj*@see.sig.in validwrites:

>Philip Potter said:

>>Richard Heathfield wrote:
Army1987 said:
On Thu, 20 Sep 2007 00:04:05 -0700, MAx wrote:

<snip>

>#define MAX(a,b) (a>b) ? a:b
Please add enough parentheses:
((a) (b) ? a : (b))

You missed some.

<snip>

Really?

No, not really - at least, I just tried three times to break his
version and failed each time. So I owe him an apology.

[...]

Nevertheless, I would certainly use parentheses myself:
#define MAX(a, b) ((a) (b) ? (a) : (b))
rather than
#define MAX(a, b) ((a) (b) ? a : (b))

Consistency is certainly part of it. Another part is that I'm *still*
not 100% convinced that the parentheses are unnecessary. I have a
nagging hunch that there *might* be an argument that would break the
first form, perhaps something involving the ?: operator in the operand
itself.

The relevant rules from the grammar are:

primary-expression:
( expression )

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

Assuming a is an expression, it is valid in both forms. If it's not
valid, it's not an expression. Even if a contains a nested conditional
expression, for example in MAX(1 ? 2 : 3, 4 ? 5 : 6), the compiler will
do the right thing, since you're allowed to write an expression

((1 ? 2 : 3) (4 ? 5 : 6) ? 1 ? 2 : 3 : (4 ? 5 : 6))

with no more a problem with the ?: nesting than with the () nesting.

Or perhaps there's some argument that should be a syntax error,
but that will cause the unparenthesized version to expand to a legal
expression that doesn't do what you want.

#define A 0)==(0

The parenthesised version will accept MAX(A, A), as it expands to

((0)==(0) (0)==(0) ? (0)==(0) : (0)==(0))

while the unparenthesised version will show that this is a syntax error.
I don't consider this a good reason to not parenthesise a. Similarly, if
a similar construct is found that's valid with the second definition, but
not with the first, I don't consider it a good reason to include the
parentheses. (For what it's worth, I tried finding one, but failed to.)

Personally, I'd probably parenthesise a too, so that preprocessor output
is a little bit easier to read, and because I don't see a downside to it,
but it's not necessary.

[John Bode]

On Sep 20, 3:49 am, MAx <mahesh1...@gma il.comwrote:

On Sep 20, 1:10 pm, Mark Bluemel <mark_blue...@p obox.comwrote:

MAx wrote:

Hi,
Could any one list possible number of diferences between a function
and a macro?
which one will be faster and why??

Do I smell homework?

obviously not a homework.
this was an interview question...
i am looking for an impressive answer.

If you don't already have a good answer for that particular question
(or know where to look it up), then you shouldn't be interviewing for
a job that involves C programming.