11.字符函数和字符串函数

11.1求字符串长度

strlen

Get the length of a string.
size_t strlen( const char *string );

typedef unsigned int size_t;
strlen 返回值为 unsigned int
strlen返回字符串中 \0 前面出现的字符个数
参数指向的字符串必须要以\0 结束

int main() {
	char arr[] = "hello ruarua";
	printf("%u\n", strlen(arr));
	return 0;
}

strlen模拟

//方案1.计数器
size_t my_strlen(const char* str){
	int count = 0;
	while (*str != '\0') {
		count++;
		str++;
	}
	return count;
}
//方案2.指针-指针
size_t my_strlen(const char* str) {
	char* end = str;
	while (*end != '\0')
		end++;
	return end - str;
}
//方案3.递归
size_t my_strlen(const char* str) {
	if (*str == 0)
		return 0;
	else
		return 1 + my_strlen(str + 1);
}

int main() {
	int sz = my_strlen("ruarua");
	printf("%u\n", sz);//%u unsigned int
	return 0;
}

11.2长度不受限制的字符串函数

strcpy

Copy a string.
char *strcpy( char *strDestination, const char *strSource );

源字符串必须以\0结束，\0会拷贝到目标空间。
目标空间必须足够大，以确保能存放源字符串。
目标空间必须可修改。

int main() {
	char arr1[20] = { 0 };
	char arr2[] = "hello ruarua";
	//char arr2[] = {'r','u','a'};//没有'\0' err
	strcpy(arr1, arr2);
	printf("%s\n", arr1);
	return 0;
}

模拟strcpy

#include <assert.h>
char* my_strcpy(char* dest, const char* src) {
	assert(dest != NULL);
	assert(src != NULL);
	char* ret = dest;
	while (*src != '\0') {
		*dest = *src;
		dest++;
		src++;
	}
	*dest = *src;//copy '\0'
	return ret;
}
//简化-->
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[20] = { 0 };
	char arr2[] = "hello ruarua";
	my_strcpy(arr1, arr2);
	printf("%s\n", arr1);
	return 0;
}