c++ - 如何读取可能以字符串或数字开头的行？

Question

我必须从这种格式的文件中读取输入:

12 + 56
9 divided by -3
45 minus 15
Min of 2 and 1
Max of 3 and 5
34 plus 33

而且我必须以这种格式输出到另一个文件:

12 + 56 is 68
9 divided by -3 is -3
45 minus 15 is 30
Min of 2 and 1 is 1
Max of 3 and 5 is 5
34 plus 33 is 67

我以这种方式接近它，但卡在了一个点上。我已将每一行读入一个字符串，但我该如何访问字符串中的操作数，因为它们位于不同字符串中的不同位置？

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>

using namespace std;

int compute(string s) {
    //Not sure how to do this
    return 0;
}

int main() {
    ifstream in;
    ofstream out;
    string s1 = "Input.txt";
    string s2 = "CalculationResults.txt";
    string s;
    int operand1, operand2, result;

    in.open(s1); 
    out.open(s2);

    if (in) {
        while (getline(in, s)) {
            result = compute(s);
            out << s << " is " << result << endl;
        }
        in.close();
        out.close();
    } else {
        cout << "\nCouldn't find the input file\n\n";
    }

    system("pause");
    return 0;
}

Answer 1

我认为最简单的做法是读取一个字符串直到第一个空格，将其与您的命令进行比较，如果不匹配，则将其转换为数字:

if (in1 >> firstthing) {
    out << firstthing << " ";

    if (firstthing == "Min") {
        result = compute_min(in1, out); //reads in " of " and then the numbers. Also writes.

    } else if (firstthing == "Max") {
        result = compute_max(in1, out); //reads in " of " and then the numbers. Also writes.

    } else {
        int first_operand = atoi(firstthing.c_str());
        //reads in an operator, then the second operand. Also writes.
        result = compute_expression(in1, out, first_operand);
    }
    out << " is " << result << endl;
}

各种函数体比较简单:

int compute_min(std::istream& in1, std::ofstream& out) {
   std::string ofstring;
   int first_operand;
   std::string andstring;
   int second_operand;

   in1 >> ofstring >> first_operand >> andstring >> second_operand;
   out << ofstring << first_operand << andstring << second_operand;

   return std::min(first_operand, second_operand);
}

int compute_expression(std::istream& in1, std::ofstream& out, int first_operand) {
   std::string operation;
   int second_operand;

   in1 >> operation >> second_operand;
   out << operation << second_operand;

   if (operation=="+" || operation=="plus")
       return first_operand + second_operand;
   else if 
       //more

   else
       throw std::runtime_error("Invalid operation "+operation);
}