?output

[jw]

what is its output and why?
#include<iostre am>

using namespace std;


#define abs(x) (x>0 ? x:-x)
void main()
{
int a=3,b=5;
cout<<abs(abs(a +b));
}

[Ivan Vecerina]

Re: ?output

"jw" <jackwht@gmail. com> wrote in message
news:1140101482 .397041.297420@ g14g2000cwa.goo glegroups.com.. .
: what is its output and why?
: #include<iostre am>
:
: using namespace std;
:
:
: #define abs(x) (x>0 ? x:-x)
: void main()
: {
: int a=3,b=5;
: cout<<abs(abs(a +b));
: }

We don't do homework.
Try substituting the macros, and see what happens.

--
http://ivan.vecerina.com/contact/?subject=NG_POST <- email contact form

[Dietmar Kuehl]

Re: ?output

jw wrote:
[color=blue]
> #include<iostre am>
>
> using namespace std;
>
>
> #define abs(x) (x>0 ? x:-x)
> void main()
> {
> int a=3,b=5;
> cout<<abs(abs(a +b));
> }[/color]

This program is not well-formed and should not compile in the first
place. If it compiles without diagnostic your compiler is broken and
thus its output is not a reliable source of information.
--
<mailto:dietmar _kuehl@yahoo.co m> <http://www.dietmar-kuehl.de/>
<http://www.eai-systems.com> - Efficient Artificial Intelligence

[Victor Bazarov]

Re: ?output

jw wrote:[color=blue]
> what is its output and why?
> #include<iostre am>
>
> using namespace std;
>
>
> #define abs(x) (x>0 ? x:-x)
> void main()
> {
> int a=3,b=5;
> cout<<abs(abs(a +b));
> }
>[/color]

What do you think the output is and why?

Read the FAQ 5.2.

[jw]

Re: ?output


Dietmar Kuehl wrote:[color=blue]
> jw wrote:
>[color=green]
> > #include<iostre am>
> >
> > using namespace std;
> >
> >
> > #define abs(x) (x>0 ? x:-x)
> > void main()
> > {
> > int a=3,b=5;
> > cout<<abs(abs(a +b));
> > }[/color]
>[/color]
it is not a homework am on holiday and studying c++,this code gives 8
as a result but i think it must give 2 i couldnt understand this

[Ben Pope]

Re: ?output

jw wrote:[color=blue]
> Dietmar Kuehl wrote:[color=green]
>> jw wrote:
>>[color=darkred]
>>> #include<iostre am>
>>>
>>> using namespace std;
>>>
>>>
>>> #define abs(x) (x>0 ? x:-x)
>>> void main()
>>> {
>>> int a=3,b=5;
>>> cout<<abs(abs(a +b));
>>> }[/color][/color]
> it is not a homework am on holiday and studying c++,this code gives 8
> as a result but i think it must give 2 i couldnt understand this[/color]

8 looks about right to me.

On paper, do a+b
then apply abs.
then apply abs.

What should "abs" conceptually do?
What does your abs above do?

Ben Pope
--
I'm not just a number. To many, I'm known as a string...

[jw]

Re: ?output

aww sorry it is (a-b) and gives 8

#define abs(x) (x>0 ? x:-x)
void main()
{
int a=3,b=5;
cout<<abs(abs(a-b));
}

[Dietmar Kuehl]

Re: ?output

jw wrote:[color=blue]
> Dietmar Kuehl wrote:[color=green]
>> jw wrote:[/color][/color]
[...][color=blue][color=green][color=darkred]
>> > void main()
>> > {[/color][/color][/color]
[...][color=blue][color=green][color=darkred]
>> > }[/color][/color][/color]
[color=blue]
> it is not a homework am on holiday and studying c++,this code gives 8
> as a result but i think it must give 2 i couldnt understand this[/color]

Maybe you should start of studying English first: the above is not
proper sentence. Of course, it doesn't claim to be as all periods
are absent. Second, note that I haven't claimed that it is a
homework assignment. I claimed that your code or even the above
excerpt is illegal C++ which is backed up by basic.start.mai n
(3.6.1), paragraph 2, second sentence which reads:

It shall have a return type of type int, [...]

The "It" refers to "main()". The stuff following the comma is
irrelevant to the return type of 'main()'. Your turn!

Apparently, you are more interested in the effect of your macros
than the answer to your apparent question (well, strictly speaking
there was no question; I assumed you would be interested in the
output of the program and as I stated, the C++ standard makes no
statement about the output of this malformed program). With
respect to your macros, the actual answer is this: Why bother?
There are uses for macros in C++ but this is none of them. Use a
[n inline] function instead!
--
<mailto:dietmar _kuehl@yahoo.co m> <http://www.dietmar-kuehl.de/>
<http://www.eai-systems.com> - Efficient Artificial Intelligence

[Ben Pope]

Re: ?output

jw wrote:[color=blue]
> aww sorry it is (a-b) and gives 8
>
> #define abs(x) (x>0 ? x:-x)
> void main()
> {
> int a=3,b=5;
> cout<<abs(abs(a-b));
> }
>[/color]

compare with:

#include <iostream>

#define abs(x) (x>0 ? x:-x)
int main() {
int a=3,b=5;
std::cout<<abs( (abs((a-b))));
}

macros are evil. Just use an inline function.

Ben Pope
--
I'm not just a number. To many, I'm known as a string...

[jw]

Re: ?output

i know i can use inline func. but i saw this code in a
competition,out put was asked and many of the competitors said 2 but the
ans. is 8 ?

[Dietmar Kuehl]

Re: ?output

jw wrote:[color=blue]
> aww sorry it is (a-b) and gives 8
>
> #define abs(x) (x>0 ? x:-x)
> void main()
> {
> int a=3,b=5;
> cout<<abs(abs(a-b));
> }[/color]

This program is still broken...

Of course, the macro use results in an expression evaluating to
"8"! What else should be result? Just write replace the macro
with the corresponding expression or, even better, have the
compiler do it for, usually using the "-E" switch. For example,
with gcc you can look at the preprocessed output using something
like

g++ -E tst.cpp

if you store the above code in tst.cpp (you will probably want
to omit the include directive to get concise output...).
--
<mailto:dietmar _kuehl@yahoo.co m> <http://www.dietmar-kuehl.de/>
<http://www.eai-systems.com> - Efficient Artificial Intelligence

[Dietmar Kuehl]

Re: ?output

jw wrote:[color=blue]
> i know i can use inline func. but i saw this code in a
> competition,out put was asked and many of the competitors said 2 but the
> ans. is 8 ?[/color]

Of course it is "8" (assuming the program is correct to have a
return type of 'int' for 'main()'). Macros just to textual
replacement and do not in any way affect operator precedence of
the resulting expression. Just do the textual replacement and you'd
see that the result is effectively "-(-a-b)" which, of course,
evaluates to "8" if "a" is "3" and "b" is "5".
--
<mailto:dietmar _kuehl@yahoo.co m> <http://www.dietmar-kuehl.de/>
<http://www.eai-systems.com> - Efficient Artificial Intelligence

[n2xssvv g02gfr12930]

Re: ?output

jw wrote:[color=blue]
> aww sorry it is (a-b) and gives 8
>
> #define abs(x) (x>0 ? x:-x)
> void main()
> {
> int a=3,b=5;
> cout<<abs(abs(a-b));[/color]
abs(a-b) expands to (a-b>0 ? a-b:-a-b)

abs(abs(a-b)) expands to

(a-b>0 ? a-b:-a-b)>0 ? (a-b>0 ? a-b:-a-b):-(a-b>0 ? a-b:-a-b)[color=blue]
> }
>[/color]

quite simply this is obviously a mess and not what was intended
but does give the result 8. Perhaps now you'll see why macros can
cause so much grief and should generally be avoided.

JB

[Arjun Thounaojam]

Re: ?output

...and if you still insist on using macros, follow macro rule no. 2:
Always enclose your args in parentheses!
so instead of:
#define abs(x) (x>0 ? x:-x)
use:
#define abs(x) ((x)>0 ? (x):-(x))

-80