在什么情况下,switch会不执行里面的case,直接跳过整个switch Block

#include <stdio.h>
int main()
{
    int a;
    scanf("%d", &a);
    switch (a)
    {
    case1:printf("Monday\n"); break;
    case2:printf("Tuesday\n"); break;
    case3:printf("Wednesday\n"); break;
    default:printf("error\n");
    }
}