C language implements various character manipulation functions

content

strlen function (to find the length of a string)

counter method

recursive method

pointer-pointer method

strcpy function (string copy)

strcat function (string append)

strcmp function (string comparison)

strstr function (find the first occurrence of a substring in a long string)

strncpy function (copy n characters)

strncat function (append n characters)

---

strlen function (to find the length of a string)

counter method

#include <stdio.h>
#include <assert.h>
int my_strlen(const char* str)
{
	assert(str);
	int count = 0;
	while (*str++)
	{
		count++;
	}
	return count;
}

recursive method

#include <stdio.h>
#include <assert.h>
int my_strlen(const char* str)
{
	assert(str);
	if (*str)
	{
		return 1 + my_strlen(str + 1);
	}
	else
	{
		return 0;
	}
}

pointer-pointer method

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

int my_strlen(const char* str)
{
	assert(str);
	char* tmp = str;
	while (*++tmp)
	{
		;
	}
	return tmp - str;
}

Note: The subtraction of two pointers is the number of elements in the middle of the two pointers.

strcpy function (string copy)

#include <stdio.h>
#include <assert.h>
//strcpy函数返回的是目标空间的起始地址
//strcpy函数的返回类型的设置是为了实现链式访问
char* my_strcpy(char* dest, const char* str)
{
	assert(dest && str);
	char* ret = dest;
	//记录起始地址
	while (*dest++ = *str++)
	{
		;
	}
	return ret;
}

int main()
{
	char arr1[20] = { 0 };
	char arr2[] = "hello world";

	printf("%s\n", my_strcpy(arr1, arr2));
	return 0;
}

strcat function (string append)

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

char* my_strcat(char* dest, const char* str)
{
	assert(dest && str);
	char* ret = dest;//记录起始地址
    //找目标字符串中的'\0'
	while (*dest)
	{
		dest++;
	}
	//拷贝
	while (*dest++ = *str++)
	{
		;
	}
	return ret;
}

int main()
{
	char arr1[20] = "hello ";
	char arr2[] = "world";

	printf("%s\n", my_strcat(arr1, arr2));
	return 0;
}

strcmp function (string comparison)

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

int my_strcmp(const char* dest, const char* str)
{
	assert(dest && str);
	while (*dest == *str)
	{
		if (*dest == '\0')
		{
			return 0;//相等
		}
		dest++;
		str++;
	}
	//不相等
	return *dest - *str;
}

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

strstr function (find the first occurrence of a substring in a long string)

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

char* my_strstr(const char* dest, const char* str)
{
	assert(dest && str);
	const char* s1 = dest;
	const char* s2 = str;

	const char* cur = dest;
	while (*cur)
	{
		s1 = cur;
		s2 = str;

		while (*s1 && *s2 && *s1 == *s2)
		{
			s1++;
			s2++;
		}
		if (*s2 == '\0')
		{
			return (char*)cur;
		}
		cur++;
	}
	return NULL;
}

int main()
{
	char arr1[] = "abheabcdg";
	char arr2[] = "bcd";
	char* ret = my_strstr(arr1, arr2);

	printf("%s\n", ret);
	return 0;
}

strncpy function (copy n characters)

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

char* my_strncpy(char* dest, const char* str, int ret)
{
	assert(dest && str);
	char* tmp = dest;
	int i = 0;
	for (i = 0; i < ret; i++)
	{
		*dest++ = *str++;
	}
	return tmp;
}

int main()
{
	char arr1[10] = "helloxxxx";
	char arr2[7] = "world";
	printf("%s\n", my_strncpy(arr1, arr2, 5));
	return 0;
}

strncat function (append n characters)

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

char* my_strncat(char* dest, const char* str, int n)
{
	assert(dest && str);
	char* ret = dest;//记录起始地址
	//找目标字符串中的'\0'
	while (*dest)
	{
		dest++;
	}
	//拷贝
	int i = 0;
	for (i = 0; i < n; i++)
	{
		*dest++ = *str++;
	}
	//字符串追加最后要多加一个'\0'
	*dest = '\0';
	return ret;
}
int main()
{
	char arr1[20] = "hello\0xxxxxxxx";
	char arr2[7] = "world";
	printf("%s\n", my_strncat(arr1, arr2, 5));
	return 0;
}