[sword finger offer] analog implementation ATOI

One 、 Usage mode

int atoi( const char *string );

atoi What we achieve is String reshaping The operation of ：

int main()
{
    
	char arr1[20] = "123456";
	char arr2[20] = "-123456";
	char arr3[20] = "12abc3456";// Non numeric stop conversion encountered 
	int ret1 = atoi(arr1);
	int ret2 = atoi(arr2);
	int ret3 = atoi(arr3);
	printf("%d %d %d", ret1, ret2, ret3);
	return 0;
}

Now let's simulate the implementation ：

Two 、 Simulation Implementation

2.1 Regular conversion

Suppose you want to put the string "123456" Convert to plastic ：

	int ret = 0;
	while (*str)
	{
    
		ret = ret * 10 + *str - '0';
		str++;
	}

2.2 Special situations to deal with

1️⃣ Null pointer
2️⃣ An empty string
3️⃣ Space
4️⃣ The sign
5️⃣ Transboundary
6️⃣ Nonnumeric character

---

An empty string ：

When we pass an empty string , What should we return ？
If you return 0, That passes characters 0 What to do when ？
So we should judge whether it is legal .

We can create an enumeration variable to judge

enum status
{
    
	VALID,// 0
	INVALID// 1
}sta = INVALID;

sta Global variable , Assume first that it is illegal , If the legitimate , Change later .

    if (*str == '\0')
	{
    
		return 0;// illegal 0
	}

---

Space ：
If you encounter spaces in Start of string Will automatically skip , If it is in the middle of the string, it is 6️⃣ In this case , Just skip .

    if (isspace(*str))// Judge whether it is a character '0'
	{
    
		str++;
	}

---

The sign ：

	int flag = 1;
	if (*str == '+')
	{
    
		str++;
	}
	else if(*str == '-')
	{
    
		flag = -1;
		str++;
	}

---

Nonnumeric character ：

	while (*str)
	{
    
		if (isdigit(*str))// Determine whether it is a numeric character 
		{
    
			ret = ret * 10 + *str - '0';
		}
		else
		{
    
			return ret;
		}
		str++;
	}

---

Cross border judgment ：
To judge whether to cross the line is to see ret Is it greater than INT_MAX, For a negative number, it is less than INT_MIN, it is to be noted that int Is never more than INT_MAX, Should use the long long

if (isdigit(*str))// Determine whether it is a numeric character 
		{
    
			ret = ret * 10 + flag * (*str - '0');
			if (ret > INT_MAX || ret < INT_MIN)
			{
    
				return 0;
			}
		}
		else
		{
    
			return (int)ret;
		}

---

Full code ：

enum status
{
    
	VALID,// 0
	INVALID// 1
}sta = INVALID;

int my_atoi(const char* str)
{
    
	int flag = 1;
	assert(str);
	if (*str == '\0')
	{
    
		return 0;// illegal 0
	}

	if (isspace(*str))// Judge whether it is a character '0'
	{
    
		str++;
	}

	if (*str == '+')
	{
    
		str++;
	}
	else if(*str == '-')
	{
    
		flag = -1;
		str++;
	}

	long long ret = 0;
	while (*str)
	{
    
		if (isdigit(*str))// Determine whether it is a numeric character 
		{
    
			ret = ret * 10 + flag * (*str - '0');
			if (ret > INT_MAX || ret < INT_MIN)
			{
    
				return 0;
			}
		}
		else
		{
    
			return (int)ret;
		}
		str++;
	}
	if (*str == '0')
	{
    
		sta = VALID;
	}
	return (int)ret;
}

int main()
{
    
	char arr[200] = "123abc45";
	int ret = my_atoi(arr);
	if (sta == INVALID)
	{
    
		printf(" Illegal return : %d", ret);
	}
	else if (sta == VALID)
	{
    
		printf("%d", ret);
	}
	return 0;
}

---

---