[C语言] 预处理

Posted 2021-11-21 咸菜萝卜干

预处理

定义宏

--定义宏要套两层括号  ((x)+(x))

--#

#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>

#define SQUARE(x) x*x
#define PRINT(X) printf("the value of "#X" is %d\\n",X)
#define CAT(X,Y) X##Y
int main()
{
  printf("%d\\n",SQUARE(3));  //3*3
  printf("%d\\n",SQUARE(3+1));  // 3+1*3+1

  int a = 10;
  PRINT(a);
  //printf("the value of "a" is %d\\n",X);  三段字符拼成一段
  int b = 20;
  PRINT(b);
  int c = 30;
  PRINT(c);

  int yyds666 = 10;
  printf("%d",CAT(yyds,666));  //字符串拼接

  return 0;
}

宏的副作用

#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#define MAX(X,Y) ((X)>(Y))?(X):(Y)
int main()
{
  int a = 5;
  int b = 8;
  int m = MAX(a++,b++);
  printf("a = %d b = %d\\n",a,b);   // a =6 b = 10
  
  printf("m = %d\\n",m);  // m = 9



  return 0;
}

取消定义

atoi

#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <assert.h>
#include <limits.h>
#include <ctype.h>

enum State
{
  INVALID,
  VALID
};

enum State state = INVALID;

int my_atoi(const char* s)
{
  int flag = 1;
  //空指针
  if (NULL == s)
  {
    return 0;
  }
  //空字符
  if (*s == \\0)
  {
    return 0;
  }
  //跳过空白字符
  while (isspace(*s))
  {
    s++;
  }
  //+ / -
  if (*s == +)
  {
    flag = 1;
    s++;
  }
  else if (*s == -)
  {
    flag = -1;
    s++;
  }
  //处理数字字符的转换
  long long n = 0;
  while (isdigit(*s))
  {
    n = n * 10 + flag * (*s - 0);
    if (n > INT_MAX || n < INT_MIN)
    {
      return 0;
    }
    s++;
  }
  if (*s == \\0)
  {
    state = VALID;
    return (int)n;
  }
  else
  { 
    //非数字字符的情况
    return (int)n;
  }
    


}

int main()
{
  
  const char* p = "     -1234";
  int ret = my_atoi(p);
  if(state == VALID)
    printf("合法的转换:%d\\n",ret);
  else
    printf("不合法的转换:%d\\n", ret);

  return 0;
}