[sword finger offer] - explain the library function ATOI and simulate the realization of ATOI function

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*  Blog column ：【 The finger of the sword offer】*

Introduction to this article ：C In library functions atoi Function details and simulation implementation ！

Get to know the author ： Aspire to be a great programmer , Xiaobai, who is currently a sophomore in College .

Inspirational terms ： The tediousness of programming , Let's study together and become interesting ！

            

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Catalog

Preface

atoi Function function introduction

atoi Function handling special cases

Pass null pointer

  An empty string

Space  

character string “ + ” And " - "

  A cross-border visit

Non numeric string

  Simulation Implementation atoi function

Implementation conversion is illegal 0 And legal 0

Null pointer

An empty string

Space

character string " + " And " - "

Nonnumeric character

A cross-border visit

Realization atoi Function entire source code

Conclusion

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Preface

      Let's first introduce , This is together 《 The finger of the sword offer 》 Interview questions .

      atoi function , What is it ？ I think that's a lot of people's question , When I first saw this function , I'm also confused b Of , But after the detailed explanation of the teacher and his own simulation , It can be said that this function has been groped for about , good , Next, let's introduce atoi Function and how to simulate the implementation atoi function .

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atoi Function function introduction

Let's see what's in the Library atoi Functional ！

  We can see the library function atoi Its function is to convert a string into an integer .

  And the header file it uses , yes <stdlib.h>.

Let's look at a piece of code , See how it converts strings to integers .

#include<stdio.h>
#include<stdlib.h>
int main()
{
	char arr[20] = "123456";
	int ret = atoi(arr);
	printf("%d\n", ret);// With %d In the form of printing 
	return 0;
}

Running results ：

This shows that it really converts strings into integers .

atoi Function handling special cases

Pass null pointer

The program will hang up , like this ！

  An empty string

#include<stdio.h>
#include<stdlib.h>
int main()
{
	char arr[] = "";
	int ret = atoi(arr);
	printf("%d\n", ret);// With %d In the form of printing 
	return 0;
}

Running results ：

We can see that when transmitting an empty string , It just returns one 0, Now let's assume this 0 It's an illegal 0.

Space  

#include<stdio.h>
#include<stdlib.h>
int main()
{
	char arr[] = "  ";// return 0  Suppose this is a legal 0
	char arr_1[20] = "  123456";
	int ret = atoi(arr);
	int ret_1 = atoi(arr_1);
	printf("%d\n", ret);// With %d In the form of printing 
	printf("%d\n", ret_1);
	return 0;
}

Running results ：

  We can see two situations , One is pure transmit space , It's back to 0, Suppose this is a legal 0, When there is a space in front of the string , It will skip spaces , Directly convert the following string to integer .

character string “ + ” And " - "

#include<stdio.h>
#include<stdlib.h>
int main()
{
	char arr[] = "-12345";// return 0  Suppose this is a legal 0
	int ret = atoi(arr);
	printf("%d\n", ret);// With %d In the form of printing 
	return 0;
}

Running results ：

We can see that it will put the string “+  -” The contents of are converted into integers and printed .

  A cross-border visit

The so-called cross-border access is to give a large string , When this string is converted , More than the C According to the language INT_MAX Value or INT_MIN The value of , What does the compiler do ！

#include<stdio.h>
#include<stdlib.h>
int main()
{
	char arr[] = "+1234511111111111111";// Returns a large number 
	int ret = atoi(arr);
	printf("%d\n", ret);// With %d In the form of printing 
	return 0;
}

Running results ：

  You can see , When crossing the line , It converts a large number , When we simulate the implementation , It can be converted into illegal 0.

Non numeric string

#include<stdio.h>
#include<stdlib.h>
int main()
{
	char arr[] = "-1234abc";
	char arr_1[20] = "abcd";// return 0  Suppose this is a legal 0
	int ret = atoi(arr);
	int ret_1 = atoi(arr_1);
	printf("%d\n", ret);// With %d In the form of printing 
	printf("%d\n", ret_1);
	return 0;
}

Running results ：

We can see that there are two situations , When it is all non numeric strings , Suppose a legal 0, But when there are other numeric strings , Will not print non numeric strings directly .

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  Simulation Implementation atoi function

Implementation conversion is illegal 0 And legal 0

We can enumerate constants , Indicates the current status , The default conversion is illegal 0.

enum State
{
    VAILD,
    INVAILD
}Sta=INVAILD;// Creating variables defaults to illegal

Null pointer

You can add assert() Function assertion , Prevention is null pointer .

assert(str);

An empty string

If you encounter an empty string （ namely ‘\0’ when ）, We go straight back 0, At this time 0 It's an illegal 0.

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

Space

To judge the blank space, we can use isspace() This function , If it is a space, return 1, Skip spaces directly .（ A reference header file is required #include<ctype.h>）

while (isspace(*str))
    {
        str++;
    }

character string " + " And " - "

We can define one flag Record which string is currently encountered , Skip the string again " + " perhaps " - ".

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

Nonnumeric character

It can be used isdigit To judge , It's a numeric character , return 1, Enter the code and continue to execute , Otherwise directly return ret;(ret For the return value ).

if (isdigit(*str))
        {
            ret = ret * 10 + flag * (*str - '0');// Subtract character 0, That's the number 0
        }

A cross-border visit

You can see that this string is converted into an integer , Is it greater than INT_MAX Or less than INT_MIN, Illegal return 0.

long long ret = 0;
    while (*str != '\0')
    {
        if (isdigit(*str))
        {
            ret = ret * 10 + flag * (*str - '0');// Subtract character 0, That's the number 0
            if (ret > INT_MAX || ret < INT_MIN)
            {
                return 0;
            }
        }
        else
        {
            return (int)ret;// Force type to int（ The return value of the function is int）
        }
        str++;
    }

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Realization atoi Function entire source code

#include<stdio.h>
#include<assert.h>
#include<ctype.h>
#include<stdlib.h>
enum State
{
	VAILD,
	INVAILD
}Sta=INVAILD;// Creating variables defaults to illegal 
int my_atoi(const char* str)
{
	assert(str);
	if (*str == '\0')
	{
		return 0;
	}
	while (isspace(*str))
	{
		str++;
	}
	int flag = 1;
	if (*str == '+')
	{
		flag = 1;
		str++;
	}
	else if (*str == '-')
	{
		flag = -1;
		str++;
	}
	long long ret = 0;
	while (*str != '\0')
	{
		if (isdigit(*str))
		{
			ret = ret * 10 + flag * (*str - '0');// Subtract character 0, That's the number 0
			if (ret > INT_MAX || ret < INT_MIN)
			{
				return 0;
			}
		}
		else
		{
			return (int)ret;// Force type to int（ The return value of the function is int）
		}
		str++;
	}
	if (*str == '\0')
	{
		Sta = VAILD; // The normal conversion is over , To the end of  \0
	}
	return (int)ret;


}
int main()
{
	char arr[20] = "-1234";
	int ret = my_atoi(arr);
	if (Sta == VAILD)
	{
		printf(" Legal conversion :%d\n", ret);
	}
	else if (Sta == INVAILD)
	{
		printf(" Illegal conversion :%d\n", ret);
	}
	return 0;
}

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Conclusion

Simulation Implementation atoi Function will be introduced and simulated , Feel that if you learn a little , You can give the blogger triplicate ！！！