You know how C++ programmers are always bragging about how you can do things like compute factorials at run-time with templates? Well, C++'s template system is nothing but a glorified preprocessor, and here's evidence. Try it with something like gcc -w -DN=11 fact.c -o fact.

#ifndef INITIALIZED
    #define INITIALIZED
    #ifndef N
        #define N 11
    #endif
    #define A ((N)&1)
    #define B ((N)>>1&1)
    #define C ((N)>>2&1)
    #define D ((N)>>3&1)
int
main(void)
{
    printf("fact(%d) = %d", N,
    #include __FILE__
    );
}
#else /* INITIALIZED */
    (8*D + 4*C + 2*B + A)
    #if A
        #define A 0
    #else
        #define A 1
        #if B
            #define B 0
        #else
            #define B 1
            #if C
                #define C 0
            #else
                #define C 1
                #if D
                    #define D 0
                #endif
            #endif
        #endif
    #endif
    #if A || B || C || D
        *
        #include __FILE__
    #endif
#endif

Credit where credit is due: the idea comes from the IOCCC entry that does Towers of Hanoi with the preprocessor.

For our next example, we'll show how you can generate code for a 256-point FFT with the preprocessor.

(to be continued)