case后用常量代替int值为什么会报错？

Question

#include <stdio.h>
int red = 0;
int yellow=1;
int green=2;

int main(int argc,char const *argv[])
{
    int color=-1;
    char *output=NULL;
    
    scanf("%d",&color);
    switch(color)
    {
        case red: output="red";break;
        case yellow: output="yellow";break;
        case green: output="green";break;
        default: output="unknown";
    }
    
    printf("%s\n",output);
    
    
    return 0;
}

这段代码在case处会报错[Error] case label does not reduce to an integer constant，但是red，yellow，green的值已经在之前定义为常量了，为什么会报错呢，求教。

Answer 1

定义#define 就行

#include <stdio.h>
#define red 0
#define yellow 1
#define green 2
int main(int argc,char const *argv[])
{
    int color=-1;
    char *output=NULL;
    scanf("%d",&color);
    switch(color)
    {
        case red: output="red";break;
        case yellow: output="yellow";break;
        case green: output="green";break;
        default: output="unknown";
    }
    printf("%s\n",output);
    
    return 0;
}