Simple but specific array compiling error (C)

Question

This is what I write:

   const int MAX=100;

   int main (){
       int notas [MAX]={0};

The compiler says the following:

[Error] variable-sized object may not be initialized
[Warning] excess elements in array initializer

When I write MAX with #define MAX 100, it works. But I don´t understand what's the matter with doing it this way?

Answer 1

Despite the const in the declaration

const int MAX = 100;

MAX is not a constant expression (i.e., something whose value is known at compile time). Its value isn't known until run time, so the declaration of notas is treated as a variable-length array declaration, and a VLA declaration may not have an initializer (nor may a VLA be declared at file scope, nor may it be a member of a struct or union type).

With the preprocessor macro

#define MAX 100

all instances of the symbol MAX are replaced with the literal 100 after preprocessing, so it's effectively the same as writing

int notas[100] = {0};

which is why using the preprocessor macro works.