enum in C

In C, the keyword “enum” is used to declare an enumeration.
```
#include<stdio.h>
enum day{
  Mon, 
  Tue, 
  Wed, 
  Thur, 
  Fri=18, 
  Sat=11, 
  Sun
};

int main() {
    printf("Enum day: %d %d %d %d %d %d %d", Mon, Tue, Wed, Thur, Fri, Sat, Sun);
    // Enum day: 0 1 2 3 18 11 12    
    return 0;
}
```
Anonymous enum is a enum trick for declaring a compile-time integer constant. It guarantees that no variable is instantiated and therefore there's no runtime overhead. 
```
enum { color = 1 };
```
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