how to evaluate arithmetic operation from string in c++

Question

i am struggling with a string related problem in c++.

suppose i have a string s="6*6+4*8+6/6/6-632+81";

My main goal is to do the arithmetic operation from the string. So in the bellow code i am getting correct result when the integer value is a 1 digit integer.

string math[]="1+2"; Result:3;

but when the, string math[]="10+20" Then result is:2;

    #include<iostream>
    #include<string>
    using namespace std;
    int main()
    {

    int A,B,val;
        char math[]="1+2";
        //char math[]="10+20";

        for (int i = 0 ; math[i]!=NULL; i++)
        {
            char ch =math[i];
             if(ch == '+' || ch == '-' || ch == '*' || ch == '/')
             {

                  char ch2=math[i-1];

                 A=(ch2-'0');


                 char ch3=math[i+1];
                 B=(ch3-'0');

                 switch (ch) /* ch is an operator */
                {
                case '*':
                    val = A * B;
                    break;

                case '/':
                    val = A / B;
                    break;

                case '+':
                    val = A + B;
                    break;

                case '-':
                    val = A - B;
                    break;
                }

             }

            if ( math[i] == NULL)
            {
                break;
            }

        }


    cout<<val<<endl;

    }

so I realize the problem is choosing the value of A and B.Here I am not selecting the whole digit. so i want my A to be 10 and B 20.But my programm is selecting A=0 and B=2.So to get the whole digit.like A=10,i made these changes in my code[only for A.As a test].

        if(ch == '+' || ch == '-' || ch == '*' || ch == '/')
        {
         char total;
         int k=i-1;
         cout<<"k="<<k<<endl;
         char p;
         p=math[k];
         for(;k>-1 && isdigit(p)==true;k--)
         {
             char c=math[k];
             cout<<"c"<<c<<endl;
             total=math[i-1];

             total=total+c;

         }

         char ch2=total;
         // char ch2=math[i-1];
        cout<<"ch2:"<<total<<endl;
         A=(ch2-'0');
         cout<<"A="<<A<<endl;

BUT now for A i am getting garbage value.Now how can i solve this.More specifically get the value of A=10 and B=20 and get the correct result for this math expression.

Answer 1

Use std::string and std::stringstream like

#include <iostream>
#include <sstream>
#include <string>
using std::cout;
using std::string;
using std::stringstream;

void func(double &result, char op, double num);

int main() {
    string math = "6*6+4*8+6/6/6-632+81";
    stringstream mathStrm(math);

    double result;
    mathStrm >> result;
    char op;
    double num;
    while (mathStrm >> op >> num) {
        cout << "op: " << op << ", num: " << num << '\n';
        func(result, op, num);
    }
    cout << result << '\n';
}


void func(double &result, char op, double num) {
    switch (op) {
        case '+':
            result += num;
            break;
        case '-':
            result -= num;
            break;
        case '*':
            result *= num;
            break;
        case '/':
            result /= num;
    }
}

Notice that the string is evaluated from left to right without precedence.