if condition problem

Question

1.00/5 (1 vote)

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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 3:18am

alman hossain

Updated 20-Nov-12 3:33am

Jochen Arndt

v2

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Comments

joshrduncan2012 20-Nov-12 9:20am

Have you tried running the code in a compiler to see what output you receive?

4 solutions

Solution 3

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:

C++

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.

Posted 20-Nov-12 4:12am

Richard MacCutchan

Comments

Sergey Alexandrovich Kryukov 20-Nov-12 19:27pm

Sure, a 5.
--SA

Solution 1

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!

Posted 20-Nov-12 3:31am

Manfred Rudolf Bihy

Updated 20-Nov-12 10:28am

v4

Comments

CPallini 20-Nov-12 9:40am

You didn't try to compile it, did you?

Manfred Rudolf Bihy 20-Nov-12 12:54pm

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 13:31pm

To be honest, before trying it with the compiler, I made your same hypothesis.

Solution 4

It would be (A) as #if statement will always thread a==0 as true

Posted 20-Nov-12 4:37am

serhiy.semenyuk

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CPallini · Accepted Answer · 2012-11-20T03:39:00

Solution 2

It is (a).
because:

My compiler told me. :-)
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.

Posted 20-Nov-12 3:39am

CPallini

Comments

nv3 20-Nov-12 9:59am

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 11:18am

Yes.
Thank you.

alman hossain 21-Nov-12 2:15am

Syntax of conditional preprocessor directive (if) is:
#if <constant expression="">
#else
#endif
In this code (a==0) is not constant expression.so answer is (E).I am still confused!!

CPallini 21-Nov-12 2:55am

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 Rudolf Bihy 20-Nov-12 16:21pm

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 16:37pm

There is, e.g. the famous
#ifdef __cpluplus
However sometimes you need to evaluate expressions, like in, for instance
#if (WINVER >= 0x0600)

Manfred Rudolf Bihy 20-Nov-12 16:44pm

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 19:28pm

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 19:26pm

Amazing mix of pre-compiler and compiler code. I don't know of IP got it or not, but that's a 5.
--SA