BOLD

To view the results of the defines, you can look at the output of the preprocessor in a file:

http://www.keil.com/support/man/docs/c51/c51_cm_ifile.htm

That way you can see what the results of the #ifdefs.

Nothing strange with #undef.

The defined symbols are just a text-based cut and paste in your source code, while the #undef keeps the preprocessor happy that you aren't accidentally trying to clash preprocessor symbol names.

But how is "BOLD" a summary of your post, that may help other readers to understand what this thread is about?

Sorry for the summary type error, the summary should be "Question of Keil C complier Preprocessor"

but the Preprocessor I put it in if…else statement

I think the complier Preprocessor won't know what the variable it is . so I think the code will not print correct target data , but In a fact it's work fine , so I don't understand why?

Many Thanks for your answer ...

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.