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if condition problem
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See more: C 
#include<stdio.h>
int main(){
    int a=0;
    #if (a==0)
         printf("Equal");
    #else if
         printf("Not equal");
    #endif
    return 0;
}
what will be the output..
A) Equal
(B) Not equal
(C) Null
(D) Garbage
(E) Compilation error
 
is it (a) or (e)
 
please any body explain
Posted 20 Nov '12 - 3:18
alman hossain355
Edited 20 Nov '12 - 3:33
Jochen Arndt27.9K
v2
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Comments
joshrduncan2012 - 20 Nov '12 - 9:20
Have you tried running the code in a compiler to see what output you receive?

4 solutions

It is (a).
because:
  • My compiler told me. Smile | :)
  • Documentation[^] states: "The `#if' directive allows you to test the value of an arithmetic expression [...] expression is a C expression of integer type, subject to stringent restrictions. It may contain [...] Identifiers that are not macros, which are all considered to be the number zero.
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Posted 20 Nov '12 - 3:39
CPallini246K
Comments
nv3 - 20 Nov '12 - 9:59
You are so right! Just to clarify, #if (a == 0) does not turn out as true, because of the previous declaration of "int a = 0;". It looks for preprocessor symbol called a, doesn't find one, and accordingly assumes a default value of 0. To check, delete the "int a = 0;" line and the result will stay the same. +5
CPallini - 20 Nov '12 - 11:18
Yes. Thank you.
alman hossain - 21 Nov '12 - 2:15
Syntax of conditional preprocessor directive (if) is: #if #else #endif In this code (a==0) is not constant expression.so answer is (E).I am still confused!!
CPallini - 21 Nov '12 - 2:55
The key is (from the point of view of the preprocessor): "Identifiers that are not macros, which are all considered to be the number zero". The identifier a is not a macro hence it is 0.
Manfred R. Bihy - 20 Nov '12 - 16:21
Well, one never stops learning! Thanks for checking that out I hate stuff like that though: Oh let's see it's undefined so I'll just go on assuming it's zero. Stuff like that freaks me out. On a similar note: Isn't there some preprocessor directive like ifdev or ifdefined to test if a symbol is defined or not. My last C programming is quite a while back, but I think there was something like that, somewhere at least. Not sure which one though as I also did some embedded programmng (Keil), some Unix stuff (mostly GNU) and then there were some rare occasions with MS VC. A long, long, long time ago! :)
CPallini - 20 Nov '12 - 16:37
There is, e.g. the famous #ifdef __cpluplus However sometimes you need to evaluate expressions, like in, for instance #if (WINVER >= 0x0600)
Manfred R. Bihy - 20 Nov '12 - 16:44
I do understand, but it's still silly to assume somthing that is not defined is equal to zero when used in an expression. Me no like! I get pretty anal about stuff like that. :)
Sergey Alexandrovich Kryukov - 20 Nov '12 - 19:28
It's not just something you rightfully dislike, this is actually wrong assumption. Too bad compilers still allow it. --SA
Sergey Alexandrovich Kryukov - 20 Nov '12 - 19:26
Amazing mix of pre-compiler and compiler code. I don't know of IP got it or not, but that's a 5. --SA
You should not be using the #if preprocessor macros as these affect the generation of the code, and not the executable program. Your code should be:
int main(){
    int a=0;
    if (a==0)
         printf("Equal");
    else
         printf("Not equal");
    return 0;
}
Try building this and executing with different values of a.
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Posted 20 Nov '12 - 4:12
Richard MacCutchan295K
Comments
Sergey Alexandrovich Kryukov - 20 Nov '12 - 19:27
Sure, a 5. --SA
The answer should be E.
a is here a variable declared in the code and not a symbol used in conditional compilation as indicated by the #if token.
 
The answer is of course A. Thanks and credit goes to CPallini[^] for his well founded Solution 2[^]. (I tend to make assumptions how certain software products should work simply based on my own diligence. This, however, is an error as we easily observed in this sample.)
 

Cheers!
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Posted 20 Nov '12 - 3:31
Manfred R. Bihy119K
Edited 20 Nov '12 - 10:28
v4
Comments
CPallini - 20 Nov '12 - 9:40
You didn't try to compile it, did you?
Manfred R. Bihy - 20 Nov '12 - 12:54
Nope, just a wild guess. I was hoping for the best in those C compilers. Looks as if I was wrong then? BTW, it's hard trying code and compiling on my Samsung Galaxy Tablet. :) Cheers!
CPallini - 20 Nov '12 - 13:31
To be honest, before trying it with the compiler, I made your same hypothesis.
It would be (A) as #if statement will always thread a==0 as true
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Posted 20 Nov '12 - 4:37
serhiy.semenyuk1.1K
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