C语言-if/else,switch/case

if/else

#include<stdio.h>

int main() {
	int a = 10;
	scanf("%d", &a);
	if (a < 18)         //注意:if()后  与  else后面都默认控制一条语句,如果比一条语句多就加上  {}
		printf("青少年\n");
	else if (a >= 18 && a < 28)
		printf("青年\n");
	else if (a >= 28 && a < 40)
		printf("壮年\n");
	else if (a >= 40 && a < 60)
		printf("中年\n");
	else if (a>=60&&a<100)
		printf("老年\n");
	else
	{
		printf("老祖\n");
	}
}

else的就近原则

#include<stdio.h>

int main() {

	//else的就近原则
	int a = 0;
	int b = 2;
	if (a == 1)
		if (b == 2)
			printf("hehe\n");
		else//就近原则
			printf("haha\n");
    return 0;
}

switch/case

#include<stdio.h>

int main() {
	//switch  case 语句   (case后必须是整型表达式)
	int day = 0;
	scanf("%d", &day);
	switch (day) 
	{
	case 1:
		printf("星期一\n");
		break;
	case 2:
		printf("星期二\n");
		break;
	case 3:
		printf("星期三\n");
		break;
	case 4:
		printf("星期四\n");
		break;
	case 5:
		printf("星期五\n");
		break;
	case 6:
		printf("星期六\n");
		break;
	case 7:
		printf("星期日\n");
		break;
	}
    return 0;
}
    
#include<stdio.h>

int main() {
	int day = 0;
	scanf("%d", &day);
    	switch (day) 
    {
    case 1:
    case 2:
    case 3:
    case 4:
    case 5:
	printf("weekday\n");
	break;
    case 6:
    case 7:
	printf("weekend\n");
	break;
    }
    return 0;
}

#include<stdio.h>

int main() {
	//switch case 练习  注意:有无break;
	int n = 1;
	int m = 2;
	switch (n) {
	case 1:m++;
	case 2:n++;
	case 3:
		switch (n) {//switch允许嵌套使用
		case 1:n++;
		case 2:m++; n++;
			break;
		}
	case 4:
		m++;
		break;
	default:
		break;
	}
	printf("m=%d n=%d\n", m, n);
	return 0; 
}