c - 我需要理解这段C代码

Question

我开始学习 C，所以我需要有人可以解释以下程序，该程序应将摄氏度转换为华氏度，反之亦然。一步一步。

#include <stdio.h>

#define TF2TC

     main() {
     double tc, tf, offset, conv;

     offset = 32.;

#ifdef TF2TC
     conv = 5. / 9.;
     printf("Fahrenheit Value= ");
     scanf("%lf",&tf);
     tc = (tf - offset) * conv;
     printf("celsius value= %f\n",tc);
#endif
#ifndef TF2TC
     conv = 9. / 5.;
     printf("celsius value= ");
     scanf("%lf",&tc);
     tf = tc * conv + offset;
     printf("fahrenheit value= %f\n",tf);
#endif
}

Answer 1

double tc, tf, offset, conv; // --> declaring 4 doubles

     offset = 32.; // --> assigning 32.0 to offset

#ifdef TF2TC // --> if the macro TF2TC was defined do this:
     conv = 5. / 9.; // --> assigning 5. / 9. to conv
     printf("Fahrenheit Value= "); // --> printing "Fahrenheit Value= "
     scanf("%lf",&tf); // --> scanning user input and storing it in tf
     tc = (tf - offset) * conv; // --> assigning (tf - offset) * conv to tc
     printf("celsius value= %f\n",tc); // --> printing "celsius value= {value of tc}"
#endif
#ifndef TF2TC // --> if the macro TF2TC wasn't defined do this:
     conv = 9. / 5.; // --> assigning 5. / 9. to conv
     printf("celsius value= "); // --> printing "celsius value= "
     scanf("%lf",&tc); // --> scanning user input and storing it in tf
     tf = tc * conv + offset; // assigning tc * conv + offset to tf
     printf("fahrenheit value= %f\n",tf); // --> printing "fahrenheit value= {value of tf} "
#endif