I have a project to write a program that receives a polynomial string from the user up to the 5th power (ex. x^3 6x^2 9x 24) and prints out all the real and imaginary roots. The coefficients should be stored in a dynamic array.

The problem is getting these coefficients from the string. One of the coefficients can be a 0 (ex. 2x^2-18) so I can't store the coefficients from left to right by using an increment, because in this case a=2, b=-18, and c has no value, which is wrong.

Another problem is if the coefficient is 1, because in this case nothing will be written beside the x for the program to read (ex. x^2-x 14). Another problem is if the user adds a space, several, or none (ex. x ^3 4x^ 2- 12 x 1 3).

I have been thinking of pseudocode for a long time now, but nothing is coming to mind. I thought of detecting numbers from left to right and reading numbers and stopping at x, but the first and second problems occur. I thought of finding each x and then checking the numbers before it, but the second problem occurs, and also I don't know how big the number the user inputs.

CodePudding user response：

Here is another Regex that you can use to get your coefficients after deleting whitespace characters:

(\d*)(x?\^?)(\d*)

It uses groups (indicated by the brackets). Every match has 3 groups:

1. Your coefficient
2. x^n, x or nothing
3. The exponent

If (1) is null (e.g. does not exist), it means your coefficient is 1.

If (2) and (3) are null, you have the last single number without x.

If only (3) is null, you have a single x without ^n.

You can try some examples on online regex sites like this one, where you can see the results on the right.

There are many tutorials online how to use Regex with C .

CodePudding user response：

You should normalize your input string, for example, remove all space then parse coefficients.

Let see my example. Please change it for your case.

#include <iostream>
#include <regex>
#include <iterator>
#include <string>
#include <vector>
#include <algorithm>

int main(int argc, char *argv[]) {

    std::string input {argv[1]};
    input.erase(remove_if(input.begin(), input.end(), isspace), input.end());
    std::cout << input  << std::endl;
    std::vector<int> coeffs;
    std::regex poly_regex(R"(\s*\ ?\-?\s*\d*\s*x*\^*\s*\d*)");
    auto coeff_begin = std::sregex_iterator(input.begin(), input.end(), poly_regex);
    auto coeff_end = std::sregex_iterator();
    for (std::sregex_iterator i = coeff_begin; i != coeff_end;   i) {
        std::smatch match = *i;
        std::string match_str = match.str();
        // std::cout << " " << match_str << "\n";
        std::size_t plus_pos = match_str.find(' ');
        std::size_t minus_pos = match_str.find('-');
        std::size_t x_pos = match_str.find('x');
        if (x_pos == std::string::npos) {
            std::cout << match_str.substr(plus_pos   1) << std::endl;            
        } else if (x_pos == 0) {
            std::cout << 1 << std::endl;
        } else if (minus_pos != std::string::npos) {
            if (x_pos - minus_pos == 1) std::cout << -1 << std::endl;
            else std::cout << match_str.substr(minus_pos, x_pos - minus_pos) << std::endl;            
        }
        else {
            std::cout << match_str.substr(plus_pos   1, x_pos - plus_pos - 1) << std::endl;            
        }
    }
    for (auto i: coeffs) std::cout << i << " ";
    return 0;
}