C language STR function

strlen:

Used to find the length of the string , Start with the first character , To '\0' end ,'\0' Not counting the total length .

Function implementation ：

size_t my_strlen(const char* ptr)
{
	assert(ptr);
	const char* ptx = ptr;
	while (*(++ptx));
	return (size_t)(ptx - ptr);
}

strcmp:

Used to compare two string sizes , Note that size does not refer to string length , It starts with the first character , Compare the size of characters . This function returns a int value , Different compilers , The value returned is different . But the positive and negative are consistent , When the first is greater than the second , Returns a positive value , Less than, it returns a negative value , Equal return 0.

Function implementation ：

int my_strcmp(const char* str1,const char* str2)
{
	assert(str1 && str2);
	while((!(*str1 - *str2)) && ((*(str1++)) * (*(str2++))));
	return (int)(*str1 - *str2);
}

strcpy：

Used to copy strings .

Function implementation ：

char* my_strcpy(char* dest,const char* source)
{
	assert(dest && source);
	char* result = dest;
	while (*(dest++) = *(source++));
	return result;
}

strcat：

Used to append a string to the end of the target string .

Function implementation ：

char* my_strcat(char* a, const char* b)
{
	assert(a && b);
	char* tmp = a;
	while (*(++a));
	while (*(a++) = *(b++));
	*a = '\0';
	return tmp;
}

strstr：

Used to find another string within one string . This in KMP The algorithm is about .

Function implementation ：

const char* my_strstr(const char* a, const char* b)//a Is a long string  b Is a short string 
{
	char* cp = (char*)a;
	char* s1;
	char* s2;
	if (!*b)
		return a;
	while (*cp)
	{
		s1 = cp;
		s2 = (char*)b;
		while (*s1 && *s2 && !(*s1 - *s2))
		{
			s1++;
			s2++;
		}
		if (!*s2)
			return cp;
		cp++;
	}
	return NULL;
}

atoi：

This function is very interesting , It will convert the numeric string of the string into int value .

Function implementation ：

int my_atoi(const char* str)
{
	assert(str);
	int num = 0;
	int result = 0;
	const char* tmp = str;
	while (*str && *str != '.')
	{
		num++;
		str++;
	}
	while (num--)
	{
		result += (*tmp - '0') * (int)pow(10, num);
		tmp++;
	}
	return result;
}

strncpy,strncmp,strncat：

All three functions limit the number of characters , The function is the same .

Function implementation ：

char* my_strncpy(char* dest, const char* sou, size_t num)
{
	assert(dest && sou);
	char* tmp = dest;
	while ((num--) && (*(dest++) = *(sou++)));
	return tmp;
}


char* my_strncat(char* a, const char* b, int n) 
{
	assert(a && b);
	char* tmp = a;
	while (*(++a));
	while ((n--) && (*(a++) = *(b++)));
	*a = '\0';
	return tmp;
}


int my_strncmp(const char* str1, const char* str2,int n)
{
	assert(str1 && str2);
	while ((!(*str1 - *str2)) && ((*(str1++)) * (*(str2++))) && (n--));
	return (int)(*str1 - *str2);
}