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Matt Chang

#define A 100

void main()
{ int b=0;
int c=30;

while(1)
{ b++;
if(b>1000) b=0; if(abs(b-c)>10) { #ifdef A #undef A #define A 200 #endif

print("A1= %d",A); }
else { #ifdef A #undef A #define A 300 #endif

print("A2= %d",A); }

}
}

taget result A1 =200 A2 = 300

I want to ask,
Dos the #undef rules meet The C standard specifies?
thank you

Parents
  • 0 in reply to Matt Chang

    But maybe you should spend some time and tell what you think - and more importantly why you think - this should, or shouldn't function.

    #define A 100

void main() {
    int b=0;
    int c=30;

    while (1) {
        b++;
        if (b > 1000) b=0;
        if (abs(b-c)>10) {
#ifdef A
    #undef A
    #define A 200
#endif
            print("A1= %d",A);
        } else {
#ifdef A
    #undef A
    #define A 300
#endif
            print("A2= %d",A);
        }
    }
}


    Note that the preprocessor does not care about any logical conditions or identations of the C/C++ source code. So the "if (abs(b-c)>10)" condition is irrelevant.

    The preprocessor just walks down the code text line for text line.
    taget result A1 =200 A2 = 300

    This is what the C compiler sees:

     <=== empty line where the preprocessor define was

void main() {
    int b=0;
    int c=30;

    while (1) {
        b++;
        if (b > 1000) b=0;
        if (abs(b-c)>10) {


  <==== empty lines where your #ifdef/#endif section was

            print("A1= %d",200);
        } else {


  <==== empty lines where your #ifdef/#endif section was

            print("A2= %d",300);
        }
    }
}

    Never think you can mix C flow control with preprocessor statements, since the preprocessor is just a glorious "search-replace" engine that performs the task before the C compiler starts to split the code into tokens for processing.

    Note, by the way, that it helps to use the proper tags when posting source code.

Reply
  • 0 in reply to Matt Chang

    But maybe you should spend some time and tell what you think - and more importantly why you think - this should, or shouldn't function.

    #define A 100

void main() {
    int b=0;
    int c=30;

    while (1) {
        b++;
        if (b > 1000) b=0;
        if (abs(b-c)>10) {
#ifdef A
    #undef A
    #define A 200
#endif
            print("A1= %d",A);
        } else {
#ifdef A
    #undef A
    #define A 300
#endif
            print("A2= %d",A);
        }
    }
}


    Note that the preprocessor does not care about any logical conditions or identations of the C/C++ source code. So the "if (abs(b-c)>10)" condition is irrelevant.

    The preprocessor just walks down the code text line for text line.
    taget result A1 =200 A2 = 300

    This is what the C compiler sees:

     <=== empty line where the preprocessor define was

void main() {
    int b=0;
    int c=30;

    while (1) {
        b++;
        if (b > 1000) b=0;
        if (abs(b-c)>10) {


  <==== empty lines where your #ifdef/#endif section was

            print("A1= %d",200);
        } else {


  <==== empty lines where your #ifdef/#endif section was

            print("A2= %d",300);
        }
    }
}

    Never think you can mix C flow control with preprocessor statements, since the preprocessor is just a glorious "search-replace" engine that performs the task before the C compiler starts to split the code into tokens for processing.

    Note, by the way, that it helps to use the proper tags when posting source code.

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