1.标识符常量和宏函数（宏函数是简单替换所以需要把括号加到位）

#include<stdio.h>

#define MAX 1000//标识符常量
#define num 10
//#define SUM(X,Y) X+Y //不对
#define SUM(X,Y) ((X)+(Y))

int max(int a, int b)
{
  return a>b?a:b;
}

int main()
{
  int m = MAX;
  //NUM = 20;标识符常量不能修改
  printf("%d\n",m);
  int a=10,b=20;
  int sum = 10*SUM(a,b);//打印120宏替换是单纯替换
  printf("%d\n",sum);
  return 0;
}

 2.指针语法

#include<stdio.h>

//32跟地址线\数据线，高低电平对应01二进制位，所有2^32个地址编号
//一个编号管理一个字节的空间，所以产生了内存地址的概念

int main()
{
  int a = 10;
  int* pa = &a;

  char ch='w';
  char* pch = &ch;//取地址
  *pa = 20;//解引用
  printf("%d\n",a);
  printf("%d\n",sizeof(a));//系统a开辟的空间，首字节地址的编号即为a的地址
  printf("%p\n",&a);//打印a的地址
  return 0;
}

#include<stdio.h>
#include<unistd.h>

int main()
{
  int a = 10;
  char ch  ='C';
  int* pa = &a;
  char* pc = &ch;

  printf("sizeof(pa)=%p\n",(int) sizeof(pa));
  printf("sizeof(pc)=%p\n",(int)sizeof(pc));
  
  return 0;
}

3. 结构体变量和结构体指针：

#include<stdio.h>
#include<unistd.h>

struct Stu
{
  char name[20];
  short age;
  char sex[5];//一个汉字需要两个字符空间
  char id[13];
};

int main()
{
  int a = 10;
  struct Stu s={"张三",20,"男","20221111"};
  struct Stu* ps = &s;
  printf("%s,%d,%s,%s\n",ps->name,ps->age,ps->sex,ps->id);
 //printf("%s,%d,%s,%s\n",s.name,s.age,s.sex,s.id);
  return 0;
}

 4.指针的使用

#include<stdio.h>

void Swap(int* x, int* y)
{
 int temp = *x;
 *x=*y;
 *y=temp;
}

int main()
{
  int a=10,b=20;
  Swap(&a,&b);
  printf("a=%d,b=%d\n",a,b);
  return 0;
}

 5.if else

#include<stdio.h>

int main()
{
int age = 60;
//if(age>=18)
 // printf("成年\n");
 // else
 // printf("未成年\n");
if(age<18)
  printf("未成年\n");
 else if(18<=age && age<30)
   printf("青年\n");
 else if(30<=age && age<50)
   printf("中年\n");
  return 0;
}

6.switch case

#include<stdio.h>

int main()
{
  printf("please input day\n");
  int day;
  scanf("%d",&day);
  switch(day)
  {
    case 1:
      printf("星期1\n");
      break;
    case 2:
      printf("星期2\n");
      break;
    case 3:
      printf("星期3\n");
      break;
    case 4:
      printf("星期4\n");
      break;
    case 5:
      printf("星期5\n");
      break;
    case 6:
      printf("星期6\n");
      break;
    case 7:
      printf("星期日\n");
      break;
    default:
      break;
  }
  return 0;
}

#include<stdio.h>
   
   int main()
   {
     printf("please input day\n");
     int day;
     scanf("%d",&day);
     switch(day)
     {
      case 1:
      case 2:
      case 3:
      case 4:
      case 5:
        printf("星期5\n");
        break;
      case 6:
      case 7:
        printf("星期日\n");
        break;
      default:
        printf("日期输入错误\n");
        break;
    }
    return 0;
  }

7.循环

#include<stdio.h>

int main()
{
  int num=1;
  while(num<=100)
  {
    if(num%2==1)
    {
      printf("%d ",num);
    }
    num++;
  }
  printf("\n");
  return 0;
}