String function and memory function (2)

Today, let's learn about the implementation of string functions and memory functions .

Catalog

（ One ） String function

（1）strlen

（2）strcpy

（3）strcat

（4）strcmp

（5）strncpy

（6）strncat

（ Two ） Memory function

（1）memcpy

（2）memmove

---

（ One ） String function

（1）strlen

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <assert.h>
size_t my_strlen(const char* pc)
{
	assert(pc);
	int count = 0;
	while ((*pc) != '\0')
	{
		count++;
		pc++;
	}
	return count;
}
int main()
{
	char arr[] = "l love you 3000!";
	int ret = my_strlen(arr);
	printf("%d\n", ret);
	return 0;
}

（2）strcpy

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <assert.h>
char* my_strcpy(char* str1, const char* str2)
{
	char* dest = str1;
	assert(str1 != NULL);
	assert(str2 != NULL);
	while (*dest++ = *str2++)
	{
		;
	}
	return str1;
}
int main()
{
	char arr1[] = "xxxxxxxxxxxxxxxxxxxxxxxxxx";
	char arr2[] = "I love you 3000!";
	printf("%s\n", my_strcpy(arr1, arr2));
	return 0;
}

（3）strcat

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

char* my_strcat(char* dest, const char* source)
{
	assert(dest && source);
	char* ret = dest;
	while (*dest != '\0')
	{
		dest++;
	}
	while (*dest++ = *source++)
	{
		;
	}
	return ret;
	
}
int main()
{
	char arr[20] = "Hello ";
	my_strcat(arr, "world");
	puts(arr);
	return 0;
}

（4）strcmp

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <assert.h>
int my_strcmp(const char* ch1, const char* ch2)
{
	assert(ch1 && ch2);
	while (*ch1 == *ch2)
	{
		if (*ch1 == 0)
			return 0;
		ch1++;
		ch2++;
	}
	return (*ch1 - *ch2);

}

int main()
{
	char arr1[] = "abcd";
	char arr2[] = "abcde";
	int ret = my_strcmp(arr1, arr2);
	if (ret > 0)
		printf(">\n");
	else if (ret == 0)
		printf("=\n");
	else
		printf("<\n");
	return 0;
}

（5）strncpy

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <string.h>
#include <assert.h>
char* my_strncpy(char* destination, const char* source, size_t num)
{
	assert(destination && source);
	char* ret = destination;
	while (num--)
	{
		*destination = *source;
		destination++;
		source++;
	}
	return ret;
}
int main()
{
	char arr1[] = "abcdef";
	char arr2[10] = "xxxxxxxxx";
	my_strncpy(arr2, arr1, 5);
	puts(arr2);
	return 0;
}

（6）strncat

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <string.h>
#include <assert.h>
char* my_strncat(char* destination, const char* source, size_t num)
{
	assert(destination && source);
	char* ret = destination;
	while (*destination != 0)
	{
		destination++;
	}
	while (num--)
	{
		*destination = *source;
		destination++;
		source++;
	}
	*++destination = '\0';
	return ret;
}

int main()
{
	char arr1[10] = "abc";
	char arr2[] = "defgh";
	my_strncat(arr1, arr2, 4);
	puts(arr1);
	return 0;
}

（ Two ） Memory function

（1）memcpy

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <assert.h>
void* my_memcpy(void* dest, const void* src, size_t count)
{
	assert(dest && src);
	void* ret = dest;
	while (count--)
	{
		*(char*)dest = *(char*)src;
		dest = (char*)dest + 1;
		src = (char*)src + 1;
	}
	return ret;
}
int main()
{
	int arr1[] = { 1,2,3,4,5,6,7,8,9,10 };
	int arr2[10] = { 0 };
	my_memcpy(arr2, arr1, 40);
	printf("arr2:");
	for (int i = 0; i < 10; i++)
	{
		printf("%d ", arr2[i]);
	}
	return 0;
}

（2）memmove

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include<assert.h>
void* my_memcpy(void* dest, const void* src, size_t count)
{
	assert(dest && src);
	void* ret = dest;
	if (dest < src)
	{
		while (count--)
		{
			*(char*)dest = *(char*)src;
			dest = (char*)dest + 1;
			src = (char*)src + 1;
		}
	}
	else
	{
		while (count--)
		{
			*((char*)dest - count) = *((char*)src - count);
		}
	}
	return ret;
}
int main()
{
	int arr[10] = { 1,2,3,4,5,6,7,8,9,10 };
	memcpy(arr, arr + 2, 20);
	for (int i = 0; i < 10; i++)
	{
		printf("%d ", arr[i]);
	}

	return 0;
}