C语言基础

• hello world
  编译gcc -o hello main.c

  #include <stdio.h>
  int main() {
  printf("Hello World!");
  return 0;
  }

• 基础知识
  c语言句末必须有分号
  区分大小写,变量申请可以不使用

  // 这是一行注释
  /*
  这是多行注释
  */

  打印用printf("")

  printf("%5d\n", 123); // 输出为 "  123" d是十进制
  printf("%+d\n", 123); 输出+123
  %f小数  %s字符串 %o八进制 %x十六进制 %p指针 %%显示%
  printf("%6.2f\n", 0.5); 输出0.50
  printf("%.5s", "hello world"); 输出hello

• 变量,常量,数据类型

int height, width;
height=12;
int a=1;
main函数之外变量当前文件有效
const float PI = 3.14; 不可修改
short a = 0b1010110;  // 2 进制数字 0b
int b = 02713;        // 8 进制数字0开头
long c = 0X1DAB83;    // 16 进制数字0x或0X
字符串,必须双引号
char s1[]="我是中国的";不可修改
char *s2="我是太空人";可修改
字符类型,必须单引号
char c = 'B';char c = 66;两个等价
unsigned int a;unsigned; 无符号类型>=0
unsigned char c;表示 0-255
signed char c; -128-127
short int;long int;long long int;可以省略int
long int      x = 1234L;等价
long long int x = 1234LL;
unsigned int           x = 1234U;
unsigned long int      x = 1234UL;
unsigned long long int x = 1234ULL;
float x       = 3.14f;
double x      = 3.14;
long double x = 3.14L;
float c = 10.5;
double x = 123.456e+3;浮点数
_Bool b=0; 1真0伪
类型显示转换
int b=(int)3.14;

• 流程控制

  ### if 
  多条语句用{}
  int x = 10;
  if (x == 10) printf("x is 10");
  if (x >1){
        printf("x >1\n");
    }else if(x<1){
        printf("x <1\n");
    }else{
        printf("x =1\n");
    }
  ### 三元运算
    int x = -10;
    char *y=(x>1)?"大于1":"小于1";
    printf("%s",y);
  ### switch
    int day = 4;
  switch (day) {
  case 3: printf("周三"); break;
  case 4: printf("周四"); break;
  default:
    printf("期待周末");
  }
  ### while
    int i = 0;
  while (i < 5) {
  printf("%d\n", i);
  i++;
  }
  ### for
  int i;
  for (i = 0; i < 5; i++) {
  printf("%d\n", i);
  }
  ### break,continue
  跳出循环用在switch,for
  终止本轮循环进入下一轮continue

• 数组

  int arr[]={1,2,3,4,5}; int arr1[2]={1,-1};
  int a[2];a[0]=1;a[1]=0;
  int a[2][5] = {
  {0, 1, 2, 3, 4},
  {5, 6, 7, 8, 9}
  }; 多维数组
  printf("%zd\n",sizeof(arr)); 长度 %ud 是size_t数据类型
  char* weekdays[] = {
  "Monday",
  "Tuesday",
  "Wednesday",
  "Thursday",
  "Friday",
  "Saturday",
  "Sunday"
  };

• 指针

  int arr[2]={1,2};
   int * a=&arr[0];
   int* b=&arr[1];
   int c=*a+*b;
   printf("%d\n",c); //3
   printf("%d\n",*a+1); //2
   printf("%p\n",&c); //000000e3b65ffb64
   printf("%d\n",*a+*b);//3
   printf("%d",*a-*b);//-1
   int a=2;
    printf("%d\n",*(&a));//2

• 函数

  int inx(int a){ //定义在main之前或者申明在前 int inx(int a);
    a++;
    return a;
  }
  int (*p)(int) = &inx;//函数指针调用等价于inx(1)
  printf("%d\n",(*p)(1));
  void swap(int* x, int* y) {//交换xy值,函数只能返回一个值,多个使用指针和地址传递
  int temp;
  temp = *x;
  *x = *y;
  *y = temp;
  }
  int a=1,b=2;
    swap(&a,&b);
    printf("%d %d\n",a,b);//2,1
  exit(0); 运行成功退出
  exit(1); 异常终止
  static int count = 1; 只初始化一次,赋值只能是常量
  static int fn(int num);只能当前文件使用的函数
  double average(int i, ...) 可变参数

• 结构体

  typedef struct car {  //设置别名
  char* name;
  float price;
  int speed;
  }st;
  st saturn ={"测试", 16000.99, 175};
  printf("车名:%s,价格:%f,速度:%d\n", saturn.name, saturn.price, saturn.speed);
  typedef unsigned char BYTE;//类型别名
  BYTE c = 'z';
  union foo {
  int a;
  float b;
  } x;
  x.a = 12;
  enum { ONE = 1, TWO = 2 }; 枚举

• 预处理器

  #define MAX 100
  #define SQUARE(X) X*X
  printf("%d\n", SQUARE(3 + 4));//19

• 输入

  int i;
  scanf("%d", &i); 键盘输入
  printf("%d\n", i);
  处理文件
  FILE* fp;char c;
  fp = fopen("hello.txt", "r");
  if (fp == NULL) {
    return -1;
  }
  c = fgetc(fp);
  printf("%c\n", c);
  fclose(fp);