1、实现strlen()函数的三种方式
(1)计数器方式
#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <stdlib.h>
#include<assert.h>
int my_strlen(char *str)
{
int count = 0;
while (*str)
{
str++;
count++;
}
return count;
}
int main()
{
char str[] = "abcdef";
printf("%d\n", my_strlen(str));
system("pause");
return 0;
}
(2)递归方式
#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <stdlib.h>
#include<assert.h>
int my_strlen(char *str)
{
if (*str == '\0')
return 0;
else
return 1 + my_strlen(str + 1);
}
int main()
{
char str[] = "abcdef";
printf("%d\n", my_strlen(str));
system("pause");
return 0;
}
(3)指针-指针的方法
两指针相减规则详见指针(上)
#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <stdlib.h>
#include<assert.h>
int my_strlen(char *str)
{
char *pdest = str;
while (*pdest)
{
pdest++;
}
return pdest - str;
}
int main()
{
char str[] = "abcdef";
printf("%d\n", my_strlen(str));
system("pause");
return 0;
}
2、C语言实现strcpy()函数
#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <stdlib.h>
#include<assert.h>
char *my_strcpy(char *pdest, char *pstr)
{
assert(pdest);
assert(pstr);
char *start = pdest;
while (*pdest++ = *pstr++)
{
;
}
return start;
}
int main()
{
char str1[] = "abcdef";
char str2[30] = { 0 };
printf("%s\n", my_strcpy(str2, str1));
system("pause");
return 0;
}
3、C语言实现strcat()函数
#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <stdlib.h>
#include<assert.h>
char *my_strcat(char *start, char *end)
{
assert(start);
assert(end);
char *pdest = start;
while (*start)
{
start++;
}
while (*start++ = *end++)
{
;
}
*start = *end;
return pdest;
}
int main()
{
char str1[30] = "ancdef";
char str2[] = "aaaaa";
printf("%s\n", my_strcat(str1, str2));
system("pause");
return 0;
}
4、C语言实现strstr()函数
#pragma warning(disable:4996)
#include<stdio.h>
#include<stdlib.h>
#include<assert.h>
char *my_strstr(const char *str,const char *sub_str)
{
assert(str);
if (*sub_str == '\0')
{
return NULL;
}
//char *cp = (char *)str;
//char *sub_cp = (char *)sub_str;
const char *cp = str;
const char *sub_cp = sub_str;
while (*str)
{
sub_str = sub_cp;
str = cp;
while (*str && *sub_str && *str == *sub_str)
{
str++;
sub_str++;
}
if (*sub_str == '\0')
{
return cp;
}
else if (*str == '\0')
return NULL;
cp++;
}
return NULL;
}
int main()
{
char str1[] = "abcdef1234";
char str2[] = "abc";
char *ret = my_strstr(str1, str2);
if (ret)
printf("%s\n", ret);
else
printf("NULL\n");
system("pause");
return 0;
}
5、C语言实现strchr()函数
#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <stdlib.h>
#include<assert.h>
#include<string.h>
char *my_strchr(const char *psrc, char ch)
{
assert(psrc);
while (*psrc)
{
if (*psrc == ch)
{
return psrc;
}
else
psrc++;
}
return NULL;
}
int main()
{
char arr[] = "abcdeg";
char ch = 'b';
const char *ret = my_strchr(arr, ch);
printf("%s\n", ret);
system("pause");
return 0;
}
6、C语言实现strcmp()函数三种方式
(1)
#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <stdlib.h>
#include<assert.h>
int my_strcmp(char *str1, char *str2)
{
assert(str1);
assert(str2);
while (*str1 || *str2)
{
if (*str1 > *str2)
{
return 1;
}
else if (*str1 < *str2)
{
return -1;
}
else
{
str1++;
str2++;
}
}
return 0;
}
int main()
{
char str1[] = "bcd";
char str2[] = "abcde";
int num = my_strcmp(str1, str2);
printf("%d\n", num);
system("pause");
return 0;
}
(2)
#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <stdlib.h>
#include<assert.h>
int my_strcmp(char *str1, char *str2)
{
int ret = 0;
while (!(ret = *(unsigned char*)str1 - *(unsigned char *)str2) && *str2 )
{
str1++;
str2++;
}
if (ret < 0)
return -1;
else if (ret>0)
return 1;
return ret; //最后一个字符相等且至少有个为\0
}
int main()
{
char str1[] = "abcde";
char str2[] = "abcd";
printf("%d\n", my_strcmp(str1, str2));
system("pause");
return 0;
}
(3)
#pragma warning(disable:4996)
#include<stdio.h>
#include<stdlib.h>
#include<assert.h>
int my_strcmp(const char *str1, const char *str2)
{
assert(str1);
assert(str2);
while (*str1 && *str2 && *str1 == *str2)
{
str1++;
str2++;
if (*str1 > *str2)
return 1;
else if(*str1 < *str2)
return -1;
}
if (*str1 == '\0' && *str2!='\0')
return -1;
else if (*str2 == '\0' && *str2 != '\0')
return 1;
else
return 0;
}
int main()
{
char str1[] = "abce";
char str2[] = "abae";
printf("%d\n", my_strcmp(str1, str2));
system("pause");
return 0;
}