[C语言]模拟实现strcpy/strncpy/strcat/strncat/strcmp/strncmp/strstr

缺乏、安全感 2022-09-21 05:35 189阅读 0赞

1.模拟实现strcpy

//strcpy
 #include<stdio.h>
 #include<stdlib.h>
 #include<assert.h>
 char* my_strcpy(char* dst, char* src)
 {
 	assert(dst!=NULL&&src!=NULL);
 	char* ret = dst;
 	while ((*dst++ = *src++) != '\0')
 		;
 	return ret;
 }
 int main()
 {
 	char str1[] = "Hello World";
 	char str2[40];
 	my_strcpy(str2, str1);
 	printf("%s\n", str2);
 	system("pause");
 	return 0;
 }

2.模拟实现strnpy

思路：利用count来控制复制次数，每次复制时，减减count

//strncpy
 #include<stdio.h>
 #include<assert.h>
 char* my_strncpy(char* dst, char* src, int count)
 {
 	assert(dst != NULL&&src != NULL);
 	char* ret = dst;
 	while ((*dst++ = *src++) != '\0'&&count)
 		count--;
 	return ret;
 }
 int main()
 {
 	char str1[] = "Hello World";
 	char str2[40] = "\0";
 	char str3[40] = "\0";
 	my_strncpy(str2, str1,sizeof(str1));
 	my_strncpy(str3, str1, 5);
 	printf("%s\n", str2);
 	printf("%s\n", str3);
 	system("pause");
 	return 0;
 }

3.模拟实现strcat

思路：先遍历一遍dst,将源字符串指针走到\\0,然后利用src作为循环条件，将src依次赋值给dst，就形成连接。

//strcat
 #include<stdio.h>
 #include<stdlib.h>
 #include<assert.h>
 char* my_strcat(char* dst, char* src)
 {
 	assert(dst != NULL&&src != NULL);
 	char* ret = dst;
 	while (*dst)
 		dst++;
 	while (*src)
 	*dst++ = *src++;
 	return ret;
 }
 int main()
 {
 	
 	char str1[50] = "Hello";
 	char str2[50] = " World";
 	my_strcat(str1, str2);
 	printf("%s\n", str1);
 	system("pause");
 	return 0;
 }

4.模拟实现strncat

//strncat
 #include<stdio.h>
 #include<stdlib.h>
 #include<assert.h>
 char* my_strncat(char* dst, char* src, int count)//利用count控制连接次数
 {
 	assert(dst != NULL&&src != NULL);
 	char* ret = dst;
 
 	while (*dst)
 		dst++;
 	while (*src&&count)
 	{
 		*dst++ = *src++;
 		count--;
 	}
 	return ret;
 }
 int main()
 {
 	char str1[50] = "Hello";
 	char str2[50] = " World";
 	my_strncat(str1, str2,sizeof(str2));
 	my_strncat(str1, str2, 3);
 	printf("%s\n", str1);
 	system("pause");
 	return 0;
 }

5.模拟实现strcmp

//strcmp
 #include<stdio.h>
 #include<stdlib.h>
 #include<assert.h>
 int my_strcmp(const char* str1, const char* str2)
 {
 	assert(str1 != NULL&&str2 != NULL);
 	while (*str1==*str2) //如果两个一直相等，两个指针就后移
 	{
 		str1++;
 		str2++;
 		if (*str1 == '\0'&&*str2 == '\0')//知道移动到结尾。
 			return 0;
 	}
 	if (*str1>*str2)//两字符不相等，就进行比较
 		return 1;//大于返回1
 	else //小于 返回-1
 		return -1;
 }
 
 int main()
 {
 	char* str1 = "aaaa";
 	char* str2 = "aaaa";
 	char* str3 = "aaaa";
 	char* str4 = "bbb";
 	char* str5 = "cc";
 	char* str6 = "aaaa";
 	int ret1 = my_strcmp(str1, str2);
 	int ret2 = my_strcmp(str3, str4);
 	int ret3 = my_strcmp(str5, str6);
 	printf("%d\n", ret1);
 	printf("%d\n", ret2);
 	printf("%d\n", ret3);
 	system("pause");
 	return 0;
 }

6.模拟实现strncmp

//strncmp
 #include<stdio.h>
 #include<assert.h>
 int my_strncmp(const char* str1, const char* str2,int count)//利用count控制
 {
 	assert(str1 != NULL&&str2 != NULL);
 	while (*str1 == *str2&&count--) //如果两个相等，并且count不为0
 	{
 		str1++;
 		str2++;
 		if (count==0) //如果count等于0，就说明比完了
 			return 0;
 	}
 	if (*str1 > *str2)
 		return 1;
 	else
 		return -1;
 }
 int main()
 {
 	char* str1 = "aaaa";
 	char* str2 = "aaab";
 	char* str3 = "aac";
 	char* str4 = "aaaa";
 	int ret1 = my_strncmp(str1, str2,3);
 	int ret2 = my_strncmp(str3, str4,2);
 	printf("%d\n", ret1);
 	printf("%d\n", ret2);
 	system("pause");
 	return 0;
 }

7.模拟实现strstr

//strstr
 #include <stdio.h>
 #include <assert.h>
 
 char *my_strstr(const char *str1, const char *str2)
 {
 	assert(str1);
 	assert(str2);
 	char *p = str1; 
 	char *q = str2; 
 	char *start; 
 	while (*p)
 	{
 		start = p; 
 		while (*p && *q && *p == *q) 
 		{
 			p++;
 			q++;
 		}
 		if (*q == '\0')
 		{
 			return start;
 		}
 		p++; //不匹配，str1由上一次的起始位置向后加1
 	}
 	return NULL; //整体遍历完没有找到，返回空
 }
 
 int main()
 {
 	char *str1 = "abbbcdef";
 	char *str2 = "bbcd";
 	char *ret = my_strstr(str1, str2);
 	printf("%s\n", ret);
 	system("pause");
 	return 0;
 }