C++ Round Vanue and Error to First Significant Number of the Error

gistfile1.txt

//============================================================================
// Name        : test.cpp
// Author      : 
// Version     :
// Copyright   : Your copyright notice
// Description : Hello World in C++, Ansi-style
//============================================================================

#include <iostream>
#include <cmath>
#include <utility>

int getFirstDigit(double number){
	// We obtain the first digit of the error (always positive)
	// Like they teached in MEPhI
	number = std::abs(number);
	if (number == 1) return 1;
	if (number > 1){
		while(number >= 10){
			number = number/10;
		}
	}
	else if (number < 1){
		while(number < 1){
			number = number*10;
		}
	}
	return (int)number;
}


std::pair<double, double> roundValueError(const double value, const double error){
	// First we find the decimal point shift
	int decimalPointShift = int(log10(error)); // This will give "-0.6" for 0.0234 error, "3" for
	// For 0 < error < 1 we need to manually shift to the right
	if (error < 1) decimalPointShift--;
	// MEPhI - keep second digit if first is "1"
	if (getFirstDigit(error) == 1) decimalPointShift--;

	// Round error
	double errorRounded = round(error*pow(10, -decimalPointShift));
	double errorReverted = errorRounded*pow(10, decimalPointShift);

	// Do the same operation with value
	double valueRounded = int(value*pow(10, -decimalPointShift));
	double valueReverted = valueRounded*pow(10, decimalPointShift);

	return std::make_pair(valueReverted, errorReverted);
}

int main() {
	double value = 1.432234;
	double error = 0.002234;

	std::pair<double,double> pair = roundValueError(value, error);

	std::cout << "Original: " << value << " +- " << error << std::endl;
	std::cout << "Rounded:  " << pair.first << " +- " << pair.second << std::endl;

	return 0;
}