Unit Conversion Program v2.0

//Unit conversion Program to metres
#include "../../std_lib_facilities.h"

double metre_conversion(double input, string unit, string conversion_type);
void error_prompt(string error_type);
void vector_print_metres(vector<double> stored_values);
bool check_vector_for_unit(vector<string> accepted_units, string input_unit);
void print_new_smallest_value(double input_value, string input_unit, double smallest_value, string smallest_unit);
void print_new_largest_value(double input_value, string input_unit, double largest_value, string largest_unit);

int main()
{
    vector<double> stored_values;
    vector<string> accepted_units{"m", "in", "ft", "cm"};
    double input_value, smallest_value, largest_value;
    string input_unit = "", largest_unit, smallest_unit;

    //First assignment to smallest and largest value/unit until true
    while (1)
    {
        cin >> input_value >> input_unit;
        if (input_value < 0)
        {
            error_prompt("value");
            continue;
        }
        if (check_vector_for_unit(accepted_units, input_unit))
        {
            error_prompt("unit");
            continue;
        }
        else
        {
            smallest_value = input_value;
            largest_value = input_value;
            smallest_unit = input_unit;
            largest_unit = input_unit;
            stored_values.push_back(input_value);
            break;
        }
    }

    //Reads inputs until a non-integer is entered
    while (cin >> input_value >> input_unit)
    {
        //Use vector member function to check if unit matches our assigned accepted_units
        if (check_vector_for_unit(accepted_units, input_unit))
        {
            error_prompt("unit");
            continue;
        }
        //Ensure that input value isn't negative
        if (input_value < 0)
        {
            error_prompt("value");
            continue;
        }

        //Input is new smallest number
        if (metre_conversion(input_value, input_unit, "convert_to_m") < metre_conversion(smallest_value, smallest_unit, "convert_to_m"))
        {
            print_new_smallest_value(input_value, input_unit, smallest_value, smallest_unit);
            //Assigns new Smallest value and prompts user
            smallest_value = input_value;
            smallest_unit = input_unit;
        }

        //Input is new Largest number
        else if (metre_conversion(input_value, input_unit, "convert_to_m") > metre_conversion(largest_value, largest_unit, "convert_to_m"))
        {
            print_new_largest_value(input_value, input_unit, largest_value, largest_unit);
            //Assigns new largest value and prompts user
            largest_value = input_value;
            largest_unit = input_unit;
        }
        else
        {
            cout << input_value << input_unit << " :: " << "Has been stored.\n";
        }

        stored_values.push_back(metre_conversion(input_value, input_unit, "convert_to_m"));
    }
    //Inputs have been terminated by user

    //Vector adds sum of all stored values in metres
    double stored_value_sum = 0;
    for (double i : stored_values) stored_value_sum += i;

    //Prints the smallest, largest, input count, inputs sum and all stored values in metres
    cout << "========== PROGRAM ANALYTICS ==========\n"
        << "Final Smallest Input: " << smallest_value << smallest_unit << '\n'
        << "Final Largest Input: " << largest_value << largest_unit << '\n'
        << "Amount of inputs: " << stored_values.size() - 1 << '\n'
        << "Total Sum of inputs (in Metres): " << stored_value_sum << "m\n";
    vector_print_metres(stored_values);
    return 0;
}

double metre_conversion(double input, string unit, string conversion_type)
{
    double centimetre_ratio = 100;
    double inch_ratio = 39.37;
    double foot_ratio = 3.28;

    //Conversion from original unit to metres
    if (conversion_type == "convert_to_m")
    {
        if (unit == "cm")
            return input / centimetre_ratio;
        else if (unit == "in")
            return input / inch_ratio;
        else if (unit == "ft")
            return input / foot_ratio;
        else if (unit == "m")
            return input;
        else
            error_prompt("unit");
            return input;
    }
    //Conversion from metres to original unit
    else if (conversion_type == "convert_from_m")
    {
        if (unit == "cm")
            return input * centimetre_ratio;
        else if (unit == "in")
            return input * inch_ratio;
        else if (unit == "ft")
            return input * foot_ratio;
        else if (unit == "m")
            return input;
        else
            error_prompt("unit");
        return input;
    }
    //If input conversion string is invalid return original input and propmt.
    else
    {
        error_prompt("unit");
        return input;
    }
}

void vector_print_metres(vector<double> stored_values)
{
    //Sort and print all stored values in metres
    sort(stored_values);

    for (int i = 0; i < stored_values.size() - 1; ++i)
    {
        cout << stored_values[i] << ", ";
    }

    cout << stored_values[stored_values.size()] << '\n';
}

void error_prompt(string error_type)
{
    if(error_type == "value")
        cout << "Invalid Value, Please Revise\n";
    if (error_type == "unit")
        cout << "Invalid Unit Type, Please Revise\n";
}

void print_new_smallest_value(double input_value, string input_unit, double smallest_value, string smallest_unit)
{
    cout << "Input: " << input_value << input_unit << '\n'
         << "Previous Smallest: " << smallest_value << smallest_unit << '\n'
         << "New smallest value: " << input_value << input_unit << '\n';
}
void print_new_largest_value(double input_value, string input_unit, double largest_value, string largest_unit)
{
    cout << "Input: " << input_value << input_unit << '\n'
        << "Previous Smallest: " << largest_value << largest_unit << '\n'
        << "New smallest value: " << input_value << input_unit << '\n';
}

void program_termination();

bool check_vector_for_unit(vector<string> accepted_units, string input_unit)
{
    if (find(accepted_units.begin(), accepted_units.end(), input_unit) == accepted_units.end())
        return true;
    else
        return false;
}