字符串处理类

strlen

简单介绍

size_t strlen ( const char * str );

Geting string length.

参数：str，返回值：字符串长度

字符串以\0作为结束标志，返回\0之前的字符个数。返回值是size_t（无符号）。

模拟实现

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//方法1：计时器
int My_strlen1(char* ps) {
	int count = 0;
	while (*ps) {
		count++;
		ps++;
	}
	return count;
}
//方法2：指针相减
int My_strlen2(char* ps) {
	char* start = ps;
	while (*ps) {
		ps++;
	}
	return ps - start;
}
//方法3：递归
int My_strlen3(char* ps) {
	if (*ps == '\0')
		return 0;
	else
		return 1+My_strlen3(ps+1);
}

strcpy

简单介绍

char * strcpy ( char * destination, const char * source );

Copies the C string pointed by source into the array pointed by destination, including the terminating null character (and stopping at that point).

以\0结束，且将\0拷贝到目标空间

模拟实现

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char* My_strcpy(char* des, char* src) {
	char* rst = des;
	while (*src) {
		*des = *src;
		des++;
		src++;
	}
	*des = *src;
	return rst;
}
//改进
char* My_strcpy1(char* des, char* src) {
	char* rst = des;
	while (*des++ = *src++) {
		;
	}
	return rst;
}

strcat

简单介绍

char * strcat ( char * destination, const char * source );

Concatenate strings.

模拟实现

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char* My_strcat(char* des, char* src) {
	char* ret = des;
	//找到des的'\0'
	while (*des) {
		des++;
	}
	//字符串拷贝
	while (*des++ = *src++) {
		;
	}
	return ret;
}

strcmp

简单介绍

int strcmp ( const char * str1, const char * str2 );

Compare two strings.

模拟实现

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

strstr

简单介绍

const char * strstr ( const char * str1, const char * str2 );

模拟实现

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const char* My_strstr(const char* str1, const char* str2) {
	const char* ps1 = str1;
	const char* ps2 = str2;
	const char* cur = str1;
	while (*ps1) {
		ps1 = cur;
		while (*ps1 == *ps2 && *ps1 && *ps2) {
			ps1++;
			ps2++;
		}
		if (*ps2 == '\0') {
			return cur;
		}
		cur++;
		ps2 = str2;
	}
	return NULL;
}
//*ps1和*ps2不相等时，要重新找ps2在ps1中的位置，ps2复原，ps1继续找下一位（也就是cur的下一位）。

内存操作类

memcpy

简单介绍

void * memcpy ( void * destination, const void * source, size_t num );

Copy block of memory.

模拟实现

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void* My_memcpy(void* des, void* src, size_t count) {
	void* ret = des;
	while (count--) {
		*(char*)des = *(char*)src;
		des = (char*)des + 1;
		src = (char*)src + 1;
	}
	return ret;
}

memmove

简单介绍

void * memmove ( void * destination, const void * source, size_t num );

Move block of memory,allowing the destination and source to overlap.

模拟实现

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void* My_memmove(void* des, void* src, size_t count) {
	if (des < src) {
		//从前向后拷贝
		while (count--) {
			*(char*)des = *(char*)src;
			des = (char*)des + 1;
			src = (char*)src + 1;
		}
		
	}
	else {
		//从后向前拷贝
		des = (char*)des + count - 1;
		src = (char*)src + count - 1;
		while (count--) {
			*(char*)des = *(char*)src;
			des = (char*)des - 1;
			src = (char*)src - 1;
	}
}