Expression evaluation

  describe ：

Enter four arithmetic expressions , Just numbers ,+,-,*,/,（） form , There are no spaces , Ask for its value . Suppose that the result of the operator is an integer ,‘/’ It's also an integer .

Input ：

A string of four arithmetic expressions

Output ：

The value of the expression

The sample input 1:

(6+4)*5

Sample output 1：

50

The sample input 2：

5+9/3-4

Sample output 2：

4

  We can solve this problem recursively , We can use three functions to solve , The first function is to judge with brackets , It is also to judge which numbers are added and multiplied ...... Move to the relative function , The second function is dedicated to dealing with the operator multiplication and division */ Of , The third function is to add the separated expressions together , Is responsible for adding and subtracting +- Of . 

Code ：

#include <iostream>
#include <cstring>
#include <cstdlib>
using namespace std;
int factor_value();
int term_value();
int expression_value();
int factor_value()
{
    int result1=0;
    char c=cin.peek();
    if(c=='(') 
    {
        cin.get();
        result1=expression_value();
        cin.get();
    }
    else 
	{
        while(isdigit(c)) 
		{
            result1=10*result1+c-'0';
            cin.get();
            c=cin.peek();
        } 
    }
    return result1;
}
int term_value() 
{
    int result2=factor_value();
    while(true) 
	{
        char op = cin.peek();
        if(op=='*'||op=='/')
		{
            cin.get();
            int value1=factor_value();
            if(op=='*') 
              result2*=value1;
            else result2/=value1;
        }
        else 
          break;
    }
    return result2;
}
int expression_value()
{
	int result=term_value();
    bool more=true;
    while(more) 
	{
        char op=cin.peek();
        if(op=='+'||op=='-') 
		{
            cin.get();
            int value=term_value();
            if(op=='+') result+=value;
            else result-=value;
        }
        else more=false;
    }
    return result;
}
int main()
{
    cout << expression_value() << endl;
    return 0;
}

There are many unfamiliar functions , Let's see .

cin.penk Watch the first character in a string , Don't take it away （ Meaning in the program ）

  cin.peek() The return value of char Type characters , Its return value is the current character pointed to by the pointer , But it's just observation
The pointer stays at the current position and does not move back ; If the character to be accessed is the end of the file , Then the value of the function is -1

cin.get Take the first character of a string （ Meaning in the program ）

1、cin.get()

Used to extract a character from the specified input stream （ Include white space characters ）,
The return value of the function is the read character . If you encounter the end of the file in the input stream ,
Then the function value returns the end of file flag EOF(End Of File), General with -1 representative EOF

2、 cin.get( A character array , The number of characters n, Stop character ) 
Its function is to read... From the input stream n-1 Characters , Assign to the specified character array ( Or the array pointed to by the character pointer ), If you're reading n-1 The specified terminating character was encountered before characters , End reading ahead of time . If the reading is successful, the function returns true( really ), If you fail ( End of file ) Then the function returns false( false ).

isdigit(“ One cahr Type variable ”) Determine whether this character is a number （ Meaning in the program ）

Determine whether a variable is a number

I would also like to mention here , If you want a detailed interpretation of this program , You can have a look at mine “ Bili, Bili ” Take a look at my first video “ Expression evaluation ”, Thank you for the .

​​​​​​​ Expression evaluation _ Bili, Bili _bilibili from 11 It was said by a child aged C++ Recursive example “ Expression evaluation ”https://www.bilibili.com/video/BV1QR4y1w75s?share_source=copy_web

Little question and answer ;

  Please take a look at these three functions , Translate the three function names into Chinese , The content inside is what I will talk about in the next chapter , Bye .