C Preprocessor and if-else condition statement

Question

I run my C code in vs2010 (win32 console application). It was compiled as C++ application.

#include "stdafx.h"

#define     YES     1;
#define     NO      0;

// function to determine if an integer is even
int isEven(int number)
{
    int answer;

    if ( number % 2 == 0)
        answer = YES;
    else
        answer = NO;    
    return answer;

}

int main()
{
    int isEven(int number);

    if (isEven(17) == YES)
        printf("yes "); 
    else
        printf("no ");


    if ( isEven(20) == YES)
        printf("yes\n");
    else
        printf("no\n");

     return 0;
}

Compiler error as below.

p300.cpp(18): error C2181: illegal else without matching if
p300.cpp(30): error C2143: syntax error : missing ')' before ';'
p300.cpp(30): error C2059: syntax error : ')'
p300.cpp(31): warning C4390: ';' : empty controlled statement found; is this the intent?
p300.cpp(33): error C2181: illegal else without matching if
p300.cpp(37): error C2143: syntax error : missing ')' before ';'
p300.cpp(37): error C2059: syntax error : ')'
p300.cpp(38): warning C4390: ';' : empty controlled statement found; is this the intent?
p300.cpp(40): error C2181: illegal else without matching if

Then I also tried to insert several of { } for each of if-else condition statement, but the code still compiled failed. What's wrong with my code?

Answer 1

The compile error is due to the semicolons on your #define statements. Remove them.

#define is a preprocessor macro, not c syntax. It doesn't need a semicolon. The preprocessor does straight substitution on YES and NO, which makes:

if ( number % 2 == 0)
    answer = YES;
else
    answer = NO;

Turn into:

if ( number % 2 == 0)
    answer = 1;;  // <-- Notice the two semicolons!
else
    answer = 0;;

That makes two statements between if and else, so compiler errors ensue. I suspect you get different compiler errors when you add { and } due to

if (isEven(17) == YES)

becoming

if (isEven(17) == 1;)

By the way, this question is tagged c, but your filename is .cpp, which is a common suffix for c++. If you are using c++, definitely use the bool type.

bool is_even = true;
bool is_odd = false;

Answer 2

to start with, remove the semicolons from the end of the defines

#define     YES     1
#define     NO      0