strcpy strlen memcpy strcat strcmp strstr strrev函数的实现代码

/*
//strcpy函数的实现，注意命名要与原来的库函数有区别
#include <iostream>
#include <cstdio>
#include <assert.h>
using namespace std;

char *mystrcpy(char *strDest, const char *strSrc)
{
if ((strDest == NULL) || (strSrc == NULL))//保证strDest和strSrc的有效性
return NULL;
char *strDestCopy = strDest;//保存目标字符串的首地址，以便最后的返回
//注意：++的优先级要比*的要高
while ((*strDest++ = *strSrc++) != '\0');//将字符从strsrc中复制到strDest中
return strDestCopy;
}

int main()
{
char *strSrc = "Hello World!";
char strDest[20];
mystrcpy(strDest, strSrc);
printf("strDest:%s\n", strDest);

system("pause");
}
*/

/*
//strlen函数的实现，有两种方法实现
#include <iostream>
#include <cstdio>
#include <assert.h>
using namespace std;

//第一种方法就是通过一个变量len的自增，直至遇到字符串的结束字符
int mystrlen1(const char *strSrc)
{
assert(strSrc != NULL);//保证字符串首地址的有效性
int len = 0;//结果的保存
while (*strSrc++ != '\0'){
len++;
}
return len;
}

//第二种方法就是通过对字符串首地址的保存，再用字符串的末地址减去首地址就可以
int mystrlen2(char *strSrc)
{
int len;
char *tmpstrSrc = strSrc;//将字符串的首地址先保存下来
while (*strSrc++ != '\0'); //直至遇到字符串的结束符，strSrc已经为字符串的末地址
len = (strSrc - tmpstrSrc - 1);//末地址减去首地址，得出结果
return len;
}

int main()
{
char *strSrc = "Hello World!";
int len1, len2;
len1 = mystrlen1(strSrc);
len2 = mystrlen2(strSrc);
printf("len1 = %d\n", len1);
printf("len2 = %d\n", len2);

system("pause");
}
*/

/*
//memcpy函数的实现
#include <iostream>
#include <cstdio>
#include <assert.h>
using namespace std;

void *mymemcpy(void *memTo, const void *memFrom, int size)
{
assert((memTo != NULL) && (memFrom != NULL));//保证字符串的有效性
char *tmpmemTo = (char *)memTo;//对字符串首地址的保存
char *tmpmemFrom = (char *)memFrom;
//按照size的大小进行复制
while (size-- > 0){
*tmpmemTo++ = *tmpmemFrom++;
}
return memTo;
}

int main()
{
char *strFrom = "Hello World!";
char strTo[20];
mymemcpy(strTo, strFrom, 6);
strTo[6] = '\0';
printf("strTo:%s\n", strTo);

system("pause");
}
*/

/*
//strcat函数的实现
#include <iostream>
#include <cstdio>
#include <assert.h>
using namespace std;

char *mystrcat(char *strDest, char *strSrc)
{
char *tmpstrDest;
tmpstrDest = strDest;//保存目标字符串的首地址
while (*strDest++ != '\0');// 循环到字符串的末尾
strDest--;
while ((*strDest++ = *strSrc++) != '\0');//进行字符串的复制
return tmpstrDest; //字符串首地址的返回
}

int main()
{
char *strDest = NULL;
char *strSrc1 = "Hello ";
char *strSrc2 = "World!";
strDest = (char *)malloc(256);
*strDest = '\0';
mystrcat(mystrcat(strDest, strSrc1), strSrc2);
printf("strDest:%s\n", strDest);
free(strDest);
strDest = NULL;

system("pause");
}
*/

/*
//strcmp函数的实现
#include <iostream>
#include <cstdio>
using namespace std;

int mystrcmp(const char *strSrc, const char *strDest)
{
int ans = 0;//结果的保存
//取两个字符串的各个字符进行相减，即是它们的ASCII相减
while (!(ans = *(unsigned char *)strSrc - *(unsigned char *)strDest) && *strDest){
++strSrc;
++strDest;
}
if (ans > 0)
ans = 1;
else if (ans < 0)
ans = -1;
return ans;
}

int main()
{
char *str = "1234567";
char *str1 = "1234567";
char *str2 = "12345678";
char *str3 = "1234566";

int test1 = mystrcmp(str, str1);
int test2 = mystrcmp(str, str2);
int test3 = mystrcmp(str, str3);

cout << test1 << endl << test2 << endl << test3 << endl;

system("pause");
}
*/

/*
//strstr函数的实现
#include <iostream>
#include <cstdio>
using namespace std;

int mystrstr(const char *strSrc, const char *strSub)
{
int pos = 0;
//对两个字符串的首地址进行保存
const char *bp;
const char *sp;
if (strSrc == NULL || strSub == NULL)//保证字符串的有效性
return -1;
while (strSrc){
bp = strSrc;//得到其字符串的首地址，从首地址开始进行逐一比较字符串是否相等！
sp = strSub;
do{
if (!*sp)//如果子串到达了字符串的结束符，证明已经找到，返回其位置
return pos;
} while (*bp++ == *sp++);
pos++;
strSrc++;//如果没有找到，将主模式串的位置向前推进一位，继续比较，一直到达字符串的结尾！
}
return -1;
}

int main()
{
char *strSrc = "123456";
char *strSub = "345";
int pos = mystrstr(strSrc, strSub);
cout << pos << endl;

system("pause");
}
*/

//strrev函数的实现
#include <iostream>
#include <cstdio>
using namespace std;

//方法1：遍历字符串，将第一个字符和最后一个字符交换，第2个和倒数第2个字符交换，依次循环！
char *strrev1(const char* str)
{
int len = strlen(str);
char *tmp = new char[len+1];
strcpy(tmp, str);
for (int i = 0; i < len/2; i++){
char c = tmp[i];
tmp[i] = tmp[len-1-i];
tmp[len-1-i] = c;
}

return tmp;
}

//方法2：用指针进行直接的操作
char *strrev2(const char *str)
{
int len = strlen(str);
char *tmp = new char[len+1];
strcpy(tmp, str);
char *ret = tmp;//保存首地址
char *p = tmp + len - 1; //保存尾地址

while (p > tmp){
char t = *tmp;
*tmp = *p;
*p = t;
--p;
++tmp;
}

return ret;
}

//方法3：我们借助异或运算（^）完成两个字符的交换
char *strrev3(const char *str)
{
char *tmp = new char[strlen(str) + 1];
strcpy(tmp, str);
char *ret = tmp;
char *p = tmp + strlen(str)-1;

while (p > tmp){
*p ^= *tmp;
*tmp ^= *p;
*p ^= *tmp;
--p;
++tmp;
}

return ret;
}

char *strrev4(const char *str)
{
char *tmp = new char[strlen(str) + 1];
strcpy(tmp, str);
char *ret = tmp;
char *p = tmp + strlen(str)-1;

while (p > tmp){
*p = *p + *tmp;
*tmp = *p - *tmp;
*p = *p - *tmp;
--p;
++tmp;
}

return ret;
}

int main()
{
char *str = "123456";
cout << str << endl;

char *str2 = strrev1(str);
cout << str2 << endl;

char *str3 = strrev2(str);
cout << str3 << endl;

char *str4 = strrev3(str);
cout << str4 << endl;

char *str5 = strrev4(str);
cout << str5 << endl;

system("pause");
}