字符函数和字符串函数的模拟实现

模拟实现strlen（字符串长度）

#include <stdio.h>
#include <assert.h>

int my_strlen(const char* str)
{
    
    
	int count = 0;
	assert(str != NULL);
	while (*str != '\0')
	{
    
    
		count++;
		str++;
	}
	return count;
}

int main()
{
    
    
	char arr[] = "abcdefgh";
	int len = my_strlen(arr);
	printf("%d\n", len);
	return 0;
}

模拟实现strcpy（字符串拷贝）

#include <stdio.h>
#include <assert.h>

char* my_strcpy(char* dest, const char* src)
{
    
    
	assert(dest != NULL);
	assert(src != NULL);
	char* ret = dest;
	while (*dest++ = *src++)
	{
    
    
		;
	}
	return ret;
}

int main()
{
    
    
	char arr1[] = "abcdefh";
	char arr2[] = "xyz";
	my_strcpy(arr1, arr2);
	printf("%s\n", arr1);
	return 0;
}

模拟实现strcmp（字符串比较）

#include <stdio.h>
#include <assert.h>

int my_strcmp(const char* str1, const char* str2)
{
    
    
	assert(str1 != NULL);
	assert(str2 != NULL);
	while (*str1 == *str2)
	{
    
    
		if (*str1 == '\0')
		{
    
    
			return 0;
		}
		str1++;
		str2++;
	}
	if (*str1 > *str2)
		return 1;
	else
		return -1;
}

int main()
{
    
    
	char* p1 = "abc";
	char* p2 = "xyz";
	int ret = my_strcmp(p1, p2);
	printf("ret = %d\n", ret);
	return 0;
}

模拟实现strcat（字符串追加）

#include <stdio.h>
#include <assert.h>

char* my_strcat(char* dest, const char* src)
{
    
    
	assert(dest != NULL);
	assert(src != NULL);
	char* ret = dest;
	while (*dest != '\0')
	{
    
    
		dest++;
	}
	while (*dest++ = *src++)
	{
    
    
		;
	}
	return ret;
}

int main()
{
    
    
	char arr1[10] = "abc";
	char arr2[] = "xyz";
	my_strcat(arr1, arr2);
	printf("%s\n", arr1);
	return 0;
}

模拟实现strstr（查找字符串）

#include <stdio.h>
#include <assert.h>

char* my_strstr(const char* p1, const char* p2)
{
    
    
	assert(p1 != NULL);
	assert(p2 != NULL);
	char *s1 = NULL;
	char *s2 = NULL;
	char *cur = (char*)p1;
	if (*p2 == '\0')
	{
    
    
		return (char*)p1;
	}
	while (*cur)
	{
    
    
		s1 = cur;//当前地址
		s2 = (char*)p2;
		while (*s1 && *s2 && (*s1 == *s2))
		{
    
    
			s1++;
			s2++;
		}
		if (*s2 == '\0')
		{
    
    
			return cur;//找到子串
		}
		cur++;
	}
	return NULL;//找不到子串
}

int main()
{
    
    
	char *p1 = "abcdef";
	char *p2 = "def";
	char* ret = my_strstr(p1, p2);
	if (ret == NULL)
	{
    
    
		printf("子串不存在\n");
	}
	else
	{
    
    
		printf("%s\n", ret);
	}
	return 0;
}

模拟实现memcpy（内存拷贝）

#include <stdio.h>
#include <assert.h>

void* my_memcpy(void* dest, const void* src, size_t num)
{
    
    
	void* ret = dest;
	assert(dest != NULL);
	assert(src != NULL);
	while (num--)
	{
    
    
		*(char*)dest = *(char*)src;
		++(char*)dest;
		++(char*)src;
	}
	return ret;
}

int main()
{
    
    
	int arr1[] = {
    
     1, 2, 3, 4, 5 };
	int arr2[5] = {
    
     0 };
	int i = 0;
	my_memcpy(arr2, arr1, sizeof(arr1));
	for (i = 0; i < 5; i++)
	{
    
    
		printf("%d ", arr2[i]);
	}
	return 0;
}

模拟实现memmove（内存移动）

#include <stdio.h>
#include <assert.h>

void* my_memmove(void* dest, const void* src, size_t num)
{
    
    
	void * ret = dest;
	assert(dest != NULL);
	assert(src != NULL);
	if (dest <= src || (char *)dest >= ((char *)src + num))
	{
    
    
		while (num--)
		{
    
    
			*(char *)dest = *(char *)src;
			dest = (char *)dest + 1;
			src = (char *)src + 1;
		}
	}
	else 
	{
    
    
		dest = (char *)dest + num - 1;
		src = (char *)src + num - 1;
		while (num--)
		{
    
    
			*(char *)dest = *(char *)src;
			dest = (char *)dest - 1;
			src = (char *)src - 1;
		}
	}
	return ret;
}

int  main()
{
    
    
	int arr[] = {
    
     1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
	int i = 0;
	my_memmove(arr + 2, arr, 20);
	for (i = 0; i < 10; i++)
	{
    
    
		printf("%d ", arr[i]);
	}
	return 0;
}