A little bit confused with using conditional compilation

February 19, 2022

While I was learning the concept of conditional compilation, i encountered some problems.
I was trying to define a Symbolic constant and change its value if the value wasn’t something i expected. And facing some errors

SO this is the code

// Demonstrating conditional compilation

#include <stdio.h>
#define PI 3.14

int main()
{
    #if (PI==3.14)
        printf("Correct value!");
    #else
        #undef PI
        #define PI 3.14
        printf("Correct value assigned!");
    #endif
}

and this is the error(s)

Starting build...
/usr/bin/gcc -fdiagnostics-color=always -g "/home/karthik/karthik/Learning-C-Lang/Learning_C/10. Preprocessor/Preprocessor DIrectives/programs/programs/conditional_compilation_demo.c" -o "/home/karthik/karthik/Learning-C-Lang/Learning_C/Misc/Binaries (compiled on Linux)/Binaries/conditional_compilation_demo"
/home/karthik/karthik/Learning-C-Lang/Learning_C/10. Preprocessor/Preprocessor DIrectives/programs/programs/conditional_compilation_demo.c: In function ‘main’:
/home/karthik/karthik/Learning-C-Lang/Learning_C/10. Preprocessor/Preprocessor DIrectives/programs/programs/conditional_compilation_demo.c:4:12: error: floating constant in preprocessor expression
    4 | #define PI 3.14
      |            ^~~~
/home/karthik/karthik/Learning-C-Lang/Learning_C/10. Preprocessor/Preprocessor DIrectives/programs/programs/conditional_compilation_demo.c:8:10: note: in expansion of macro ‘PI’
    8 |     #if (PI==3.14)
      |          ^~
/home/karthik/karthik/Learning-C-Lang/Learning_C/10. Preprocessor/Preprocessor DIrectives/programs/programs/conditional_compilation_demo.c:8:14: error: floating constant in preprocessor expression
    8 |     #if (PI==3.14)
      |              ^~~~

Build finished with error(s).

But this code works

// Write a program to calculate the area of a circle. Make a symbolic constant for PI.

#include <stdio.h>

// Define a symbolic constant PI
// It is a convention to name symbolic constants in uppercase
#define PI 3.14 

int main()
{
    int r;
    printf("Enter radius: ");
    scanf("%d",&r);
    printf("Area = %f", PI*r*r);
}