C-switch语句, 逻辑运算符, 三元运算符, 指针与函数

switch (month) {
            case 2:
                printf("%d年的%d月有29天\n", year, month);
                break;
            case 4:
            case 6:
            case 9:
            case 11:
                printf("%d年的%d月有30天\n", year, month);
                break;
            case 1:
            case 3:
            case 5:
            case 7:
            case 8:
            case 10:
            case 12:
                printf("%d年的%d月有31天\n", year, month);
                break;
            default:
                break;

1 if(表达式){
2      A;
3 }else{
4      B;
5 }