短路运算|字符串操作函数|内存mem操作函数

时间：2022-11-02 14:16:51

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2016—4—25

#include"string.h"
strlwr()把字符串中大写换成小写
strupr()把字符串中小写换成大写
#include"stdio.h"
#include"string.h"
#include"stdlib.h"
int main()
{
        char a1[]="Hello World";
        char a2[]="yun nan";
        char *b1=strlwr(a1);
        char *b2=strupr(a2);
        printf("%s\n",b1);
        printf("%s\n",b2);
        system("pause");
}
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2016—4—26
\r   换行，会到本行开头（Home）
\b   退格，光标后退一格
C语言只有字符串变量，没有常量
2016—4—27
float 有效位6-7位
double  15-16
long double  18-19
long long  8个字节
vs (ctrl+k,ctrl+f)可以自动控制代码格式对齐
~按位取反
^按位异或    1010       &按位与    1010
                      0111                        0111
             ------------                 ------------
                      1101                        0010
计算机中数都是按补码存放的，正数的二进制就是原码，反码和补码全都相同；负数的原码是二进制表示，反码为原码最高位不变其余各位取反、补码为反码+1；
#include <iostream>
using namespace std;
int main()
{
   int a=4,b=3;
   cout<<"~a="<<~a<<"   ~b"<<~b<<endl;
   cout<<"a^b="<<(a^b)<<endl;
   cout<<"a&b="<<(a&b)<<endl;
   system("pause");
  return 0;
}
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4-------0000 0100  
~-------1111 1011  负数，计算机中存的是补码
反码---- 1000 0100
补码-----1000 0101    -5
 -------------------------------------------------------------------
计算机中都是按补码运算的，int a = -11 ;  ~a = 10 ；
1、a 在计算机中是按补码存放的---> 1000 1011 ==》1111 0101
2、进行取反操作--->  1111 0101 ==》0000 1010  就是十进制的 10
短路运算|
短路运算符就是我们常用的“&&”、“||”，一般称为“条件操作”。
If（a>b||printf(“a小于b”)）
如果a>b就运行下面，否则输出printf;
#include"stdio.h"
#include"stdlib.h"
int main()
{
        int a=10,b=3;
        if(a>b||printf("a<b\n"));
        if(a<b||printf("a>b\n"));
        system("pause");
}
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2016—4—28
#include"stdio.h"
#include"stdlib.h"
#include"setjmp.h"
jmp_buf envbuf;  //定义变量
int ret;
void sum()
{
        printf("ret is %d\n",ret);
        printf("I am sum\n");
        longjmp(envbuf,1);
        printf("I am not here\n");
}
int main()
{
        ret=setjmp(envbuf); 
//跳转函数，ret=0；跳转后longjmp的第二个参数就为setjmp的返回值
        if(0==ret)
        {  sum();  }
        printf("ret is %d\n",ret);
        system("pause");
}

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2016—4—29
char *strncpy( char *to, const char *from, size_t count );
//将字符串from 中至多count个字符复制到字符串to中。如果字符串from 的长度小于count，其余部分用'\0'填补。返回处理完成的字符串。会覆盖 
char *strncat( char *to, const char *from, size_t count );
//将字符串from
中至多count个字符连接到字符串to中，追加空值结束符。返回处理完成的字符串。
int strncmp( const char *str1, const char *str2, size_t count );
//比较字符串str1 和 str2中至多count个字符。str1比str2少，返回小于0的数；str1等于str2，返回0；str1大于str2,返回大于0的数 。
eg:strncpy(str1,str2,count)    strncat(str1,str2,count)    strncmp(str1,str2,count) 
void *memset( void *buffer, int ch, size_t count );
//函数拷贝ch 到buffer 从头开始的count 个字符里, 并返回buffer指针。

memset() 可以应用在将一段内存初始化为某个值。
memset(str(int),'\0'(0),sizeof(int))   //字符串（整形）数组置0，不要置成其他字符
void *memcpy( void *to, const void *from, size_t count );
//函数从from中复制count 个字符到to中，并返回to指针。
如果to 和 from 重叠，则函数行为不确定，复制从第一个开始，会产生覆盖。
eg:  memcpy(str1(int),str2(int),size)         int a[10],int b[10]={1,2,……}，memcpy(a,b,sizeof(b))   ->   a[10]={1,2,……}；结构体拷贝
int memcmp( const void *buffer1, const void *buffer2, size_t count );
//函数比较buffer1 和 buffer2的前count 个字符
eg:memcmp(int,int,size)(这个只能比较内存（整形、浮点型之类）的) 
void *memmove( void *to, const void *from, size_t count );
//与mencpy相同，不同的是当to 和 from 重叠，函数正常仍能工作。
eg:memmove(str1,str2,size)  自己往自己身上拷贝  abcde->deabc
全局变量   int a;  外部文件引用extern int a;      //比如我的.c文件里有定义全局变量，头文件里要用，可以直接extern引入。
全局变量   int a;  想要只能自己文件引用  其他文件不能引用，static int a;
定义好一个局部变量的字符数组最好要把它初始化为空--》char sstr[10]="";  或者char sstr[10]={};
main函数中的 argv[0] 表示的是 .exe 文件

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