mycopy My_strlen My_strcmp My_strcat My_atoi My_itoa的实现

mycopy函数(4种)

void My_strcpy1(char *dest,const char *src)
{
	int i;
	for( i = 0;src[i]!='\0';i++)
	{
		dest[i]=src[i];
	}
	dest[i]='\0';
}
void My_strcpy2(char *dest,const char *src)
{
	while(*dest++=*src++);
}
char *My_strcpy3(char *dest,const char *src)
{
	char *p=dest;
	while(*dest++=*src++);
	return  p;
}
void My_strcpy4(char *dest,const char *src)
{
	while(*src != '\0')
	{
		*dest=*src;
		dest++;
		src++;
	}
	*dest='\0';
}
char *My_strcnpy(char *dest,const char *src,int n)
{
	int i = 0;
	for( ; i < n && src[i] != '\0';i++)
	{
		dest[i] = src[i];
	}
	dest[i] = '\0';
	return dest;
}

字符串长度My_strlen

int My_strlen(const char *str)
{
	int count = 0;
	while(*str != '\0')
	{
		count++;
		str++;
	}
	return count;
}

字符串比较

int My_strcmp(const char *str1,const char *str2)
{
	assert(str1 != NULL && str2 != NULL);
	int tmp = 0;
	while((tmp = *str1-*str2) == 0 &&  *str2 != '\0')
	{
		str1++;
		str2++;
	}
	return tmp;
}

在字符串尾拷贝

char *My_strcat(char *dest,const char *src)
{
	assert(dest != NULL && src != NULL);
	char *p=dest;
    while(*dest != '\0')
	{
		dest++;//找到连接位置
	}
	//接着复制
	while(*dest++=*src++);
	return p;
}

char *My_strncat(char*dest, const char*src, int n)

{
    assert(dest != NULL && src != NULL);
    char *p = dest;
    while (*dest != '\0')
	{
        dest++;
    }
    while (n -- != 0)
   {
        *dest++ = *src++;
    }
    *dest = '\0';
	return p;
}

字符串变数字   
My_atoi()  "123"   ===>  123   '1'==>1    '1' - '0' = 1
123     1*10+2   12   120+3  12*10+'3'-'0'  123

int My_atoi(const char *str)
{
	assert(str != NULL);
	if(str == NULL)
	{
		return -1;
	}
	while(*str == ' ')
	{
		str++;
	}
	if(*str == '-')
	{
		printf("-");
		str++;
	}

	if(*str == '+')
	{
		str++;
	}
	int num = 0;
	while(isdigit(*str))
	{
		num = num*10+*str-'0';
		str++;
	}
	return num;
}

数字变字符串My_itoa

char *My_itoa(int num,char *str)//str[]  //-123    "123"
{
	//123%10 = 3+'0'== '3'  12 %10    1%10   "321"    "123" 
	
	//1234%10  4
	//num%10+'0'       num/10 = 12
	//char str2[10];//出了函数的作用域  内存被系统回收
	int i = 0;//str的下标
	if(num < 0)
	{
		printf("-");
		num = -num;
	}
	while(num != 0)
	{
		str[i] = num%10+'0';
		num/=10;
		i++;
	}
	str[i] = '\0';
	i--;//从\0向前退一位
	char tmp;
	for(int j = 0;j < i;j++,i--)//逆置
	{
		tmp = str[i];
		str[i] = str[j];
		str[j] = tmp;
	}
	return str;
}