递归返回语句无法正确返回变量

我很难正确返回变量。

我打印要返回返回语句上方的变量，看起来还不错。一旦我尝试将值返回并将其打印到控制台上，而不是打印-Nan(ind(。我不明白为什么会发生这种情况。

我使用Visual Studio在C 中编程。我正在使用此库将字符串解析为表达式：http：//www.partow.net/programming/exprtk/index.html

这是打印结果的功能和语句：

#include "stdafx.h"
#include "exprtk.hpp"
#include <iostream>
#include <string>
#include <algorithm>
#include <cmath>
typedef double T; // numeric type (float, double, mpfr etc...)
typedef exprtk::expression<T>     expression_t;
typedef exprtk::parser<T>             parser_t;
expression_t expression;
parser_t parser;
bool closeEnough(std::string value1, std::string value2, double levelOfSimilarity) {
    if (abs( std::stod(value1) ) - abs (std::stod(value2) ) > levelOfSimilarity) {
        return false;
    }
    else {
        return true;
    }
}
std::string replaceChars2Strings(std::string string, const std::string& start, const std::string& end) {
    size_t init_pos = 0;
    while ((init_pos = string.find(start, init_pos)) != std::string::npos) {
        string.replace(init_pos, start.length(), end);
    }
    return string;
}
double FofX(std::string function, std::string value) {
    std::string newfunction = replaceChars2Strings(function, std::string("x"), value);

    if (!parser.compile(newfunction, expression))
    {
        printf("Something went wrong when the expression was being parsed");
    }
    T result = expression.value();
    return result;
}
double DofFofX(std::string function, std::string value) {
    std::string SDplus = replaceChars2Strings(function, std::string("x"), "(" + value + "+" + "0.00001" + ")");
    std::string SDminus = replaceChars2Strings(function, std::string("x"), "(" + value + "-" + "0.00001" + ")");
    if (!parser.compile(SDplus, expression))
    {
        printf("Something went wrong when Dplus was being parsed");
    }
    T Dplus = expression.value();
    if (!parser.compile(SDminus, expression))
    {
        printf("Something went wrong when Dminus was being parsed");
    }
    T Dminus = expression.value();
    return (Dplus - Dminus) / 0.00002;
}
double newton(std::string function, std::string guess) {
    double guess2;
    //std::cout << "guess:" << guess << std::endl;
    //in here () are taken off so that the compiler can calculate the value of guess 2 easier
    guess2 = std::stod(guess.substr(1, guess.size() - 2)) - FofX(function, guess) / DofFofX(function, guess);
    //std::cout << "guess 2:" << guess2 << std::endl;
    //take the () off of guess before we give it away
    if (closeEnough(guess.substr(1, guess.size() - 2), std::to_string(guess2), 0.001)) {
        std::cout << "final guess  = " << guess2 << std::endl;
        return guess2;
    }
    else {
        //put the () back on before we give it away so that the parser can read things as multiplication right
        newton(function, "(" + std::to_string(guess2) + ")");
    }
}
int main()
{
    std::string function = "x*x";
    //remember to put () around guess
    std::string guess = "(5)";
    double answer = newton(function, guess);
    return 0;
}

当此程序运行时，它会打印以下内容：

final guess  = 0.0006105
solution = -nan(ind)

有人对我打印最终猜测与打印解决方案之间正在发生的事情有一个想法吗？