[SOLVED] Store float with exactly 2 decimal places in C++

Issue

I would like to take a decimal or non-decimal value and store it as a string with exactly 2 decimal places in C++. I want to do this to show it as a monetary value, so it is always $10.50 or $10.00 rather than $10.5 or $10.

I don’t just want to print this, I want to store it, so I don’t believe setprecision will work here. I’m doing this in a Qt application, so if there is a way to do it using Qt I can use that as well.

For example:

int cents = 1000;
std::string dollars; //should get value from cents formatted to 10.00

UPDATE:
It seems I don’t have the vocabulary yet as I am just beginning to learn C++ to articulate what I am trying to do. Here is what I want to do using Python:

str_money = '$ {:.2f}'.format(num)

In this example, num can be a decimal or not (10 or 10.5 for example) and str_money is a variable that is assigned the value of num as a decimal with exactly 2 numbers after the decimal (in this example, str_money would become 10.00 or 10.50). I want it to store this in a string variable, and I don’t need it to store the ‘$’ with the value.

Can I do this in C++?

Solution

Your decision to store monetary amounts as integer number of cents is a wise one, because floating-point data types (such as float or double) are generally deemed unsuitable for dealing with money.

Also, you were almost there by finding std::setprecision. However, it needs to be combined with std::fixed to have the expected effect (because std::setprecision means different things depending on which format option is used: the default, scientific or fixed).

Finally, to store the formatting result in an std::string instead of directly printing it to the console, you can use a string-based output stream std::ostringstream. Here is an example:

#include <iomanip>
#include <iostream>
#include <sstream>
#include <string>

std::string cents_to_dollars_string(const int cents)
{
    static constexpr double cents_per_dollar{ 100. };
    static constexpr int decimal_places{ 2 };

    std::ostringstream oss;
    oss << std::fixed << std::setprecision(decimal_places) << cents / cents_per_dollar;
    return oss.str();
}

int main()
{
    const int balance_in_cents{ -420 };
    const std::string balance_in_dollars{ cents_to_dollars_string(balance_in_cents) };
    std::cout << "Your balance is " << balance_in_dollars << '\n';
}

Here, we first define the function cents_to_dollars_string, which takes the amount in cents as an int and returns an std::string containing the formatted amount of dollars. Then, in main we call this function to convert an amount (in cents) stored in an int variable balance_in_cents to a string and store it into an std::string variable balance_in_dollars. Finally, we print the balance_in_dollars variable to the console.

Answered By – heap underrun

Answer Checked By – Jay B. (BugsFixing Admin)