#define output is not correct

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3.00/5 (2 votes)

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i have typed the following code but not able understand the output

#include<iostream.h>
#include<conio.h>
#define AREA(a) a*a;
int area(int a)
   {
   return a*a;
   }
void main()
   {
   cout<<AREA(5+2);
   cout<<"\t"<<area(5+2);
   }

</conio.h></iostream.h>

and the output is 17 and 49
im not able to understand why macro giving ans as 17 instead of 49
please solve this ques with your valueable comments

[edit]Code blocks added, formatting - OriginalGriff[/edit]
@griff
sir

why this code is replaced like this 5+2*5+2
can you please explain

Posted 20-Mar-11 5:47am

preet88

Updated 20-Mar-11 6:15am

v3

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Richard MacCutchan 20-Mar-11 16:32pm

Unless you really know what you are doing then avoid macros like the plague. The side effects catch even the most experienced developers.

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OriginalGriff · Accepted Answer · 2011-03-20T06:03:00

Solution 1

It's pretty simple - operator precidence and preprocessors:
When you use area(5+2) the compiler does the math, and changes it to a call to area(7) - which returns 7*7 or 49.
When you use AREA(5+2) the preprocessor does a text subsitution, and replaces it with 5+2*5+2
Operator precidence of the "*" over the "+" then gets it compiled as 5 + (2*5) + 2 or 17.

To fix it, replace your definition by

C#

#define AREA(a) ((a)*(a))

[edit]Geoff Williams correctly pointed out that you need brackets round the whole macro definition to be truely safe, added with thanks. I spotted that you definitely don't need the semicolon on the end, removed with blushes... - OriginalGriff[/edit]

Posted 20-Mar-11 6:03am

OriginalGriff

Updated 20-Mar-11 10:29am

v2

Comments

Geoff Williams 20-Mar-11 13:39pm

To be really safe - or at least safe as you can be when using macros - you should wrap the macro definition in brackets as well i.e.

#define AREA(a) ((a) * (a))

OriginalGriff 20-Mar-11 16:26pm

Good point! Next time I see one of your answers, I'll give you a five.
Corrected.

OriginalGriff 20-Mar-11 16:30pm

I am really glad you pointed that out: I had also missed that it really, really, should not have had a semicolon on the end of the macro definition. :blush:

Sergey Alexandrovich Kryukov 20-Mar-11 15:21pm

Correct, a 5.
Better yet, using preprocessor should be discouraged, especially in this case.
--SA