从C ++中的一行字符串中提取数字

Question

I am making a natural language calculator in C++. The user will input a line of string for calculation. The program will extract the numbers and the operation and apply it accordingly. Following is part of my code

#include<iostream>
#include<string>
#include<sstream>
using namespace std;

int main()
{
    string inp;
    float n1,n2;
    string s1,s2;

    cout<<"Enter your string"<<endl;
    getline(cin,inp);

    stringstream ss;
    ss.str(inp);

    ss>>s1>>n1>>s2>>n2;
}

The program will run successfully if the user enters in correct format ie Add 2 and 3, Subtract 8 from 12. But the problem is in two cases

1. If the user enters in some other format like "7 plus 6".
2. Even in the correct format but only one number "square root of 25".

Is there a solution which can extract the floats regardless of the position or number of floats?

Thanks

Answer 1

If what you want to do is literally extract the float s, you can take advantage of the fact that std::stof can additionally return where it leaves off, which you can use to determine if the entire "word" is a float (eg for "6c") and catch the invalid_argument for words that are definitely not floats (eg for "plus"):

std::vector<float> getFloats(const std::string& s) {
    std::istringstream iss(s);
    std::string word;
    std::vector<float> result;

    size_t pos = 0;
    while (iss >> word) {
        try {
            float f = std::stof(word, &pos);
            if (pos == word.size()) {
                result.push_back(f);
            }   
        }   
        catch (std::invalid_argument const& ) { 
            // no part of word is a float
            continue;
        }   
    }   

    return result;
}

With that, getFloats("7 plus 6") yields {7, 6} and getFloats("square root of 25") yields {25} .