converting a monetary value from string to long double

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I'm writing a program that inputs the amount of money and outputs it preceded by dollar sign and divided by commas into groups of three digits
I'll enter the money as string and call function called mstold()and returns an equivalent number as long double so my problem is how can I get the a number long double equivalent to string and this is my code : :)

C#

long double mstold(string money)
{
    long double amnt;
    for(int i=0;i<money.length();i++)
    {
        amnt=money[i]*10+1;
    }
    return amnt;
}
int _tmain(int argc, _TCHAR* argv[])
{
    string money;
    cout<<"Enter the amount of money you want to convert to long double";
    cin>>money;
    cout<<"\n\namount in long double :  "<<mstold(money)<<endl;
return 0;
}

Thank You .

[update]
Thanks a lot to OriginalGriff and Alain Rist for repling to my question your answers really help.
[/update]

Posted 23-Feb-11 7:12am

goodtogo

Updated 23-Feb-11 21:23pm

CPallini

v2

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Comments

Richard MacCutchan 23-Feb-11 13:20pm

Do not ever use double or float for monetary values unless you are happy to get incorrect results in your calculations.

Sergey Alexandrovich Kryukov 23-Feb-11 14:19pm

Richard, I strongly disagree. Right approach is not using integer values until the final result. This is very known effect of accumulation of rounding errors in some situations.
--SA

CPallini 23-Feb-11 15:25pm

?

Sergey Alexandrovich Kryukov 23-Feb-11 19:29pm

Yes, yes. What, you look at money from (some) accountant's stand point? If you looked from the economist stand point, you would understand that money is floating point by its nature. Surprise?
--SA

CPallini 24-Feb-11 3:24am

I don't think so. :-)

Sergey Alexandrovich Kryukov 24-Feb-11 3:34am

Your only argument is integer cents. Cents are not money, only the artifact representing money, assignations. This is misconception.
--SA

CPallini 24-Feb-11 3:40am

How do you know my only argument? Please note even matter is discrete, at its very nature.
:-)

Sergey Alexandrovich Kryukov 24-Feb-11 3:57am

And floating point is discrete, it is not a real number.
You don't count length or mass in discrete units. This is all about relationships between model and real word.
--SA

CPallini 24-Feb-11 4:17am

Maybe you don't count in discrete units.

Sergey Alexandrovich Kryukov 24-Feb-11 13:01pm

If you measure length, what is the discrete value? Number of Plank lengths? Do you know what is it? Do you know that a length (and mass) depend on reference point? By definition, most things are measured in real numbers. In computer, there are no real numbers at all, but floating point type model them with some accuracy.
Try to offer scientists or engineer to measure in discrete units and see what happens. As too money, it depends what phenomena you're calculating. Finance is not just transactions with integer number of cents.
--SA

CPallini 24-Feb-11 15:37pm

Length discrete value is its discrete value (dependes on the approximation you choose). Mass actually doesn't need a reference point. By definition discrete quantities are measured by natural numbers (even ocean molecules, if you like: real numbers are just, sometimes, a convenient approximation). Well I'm supposed to be a physicist too and, of course, I don't dislike discrete units. OP question looks more related to "transactions with discrete quantities" than "finance statistics & creativity" :-)

Sergey Alexandrovich Kryukov 24-Feb-11 19:19pm

Stop, stop. I means you should be familiar with special relativity. Mass depends on the relative speed, do you know? So it depends on coordinate system... This makes the masses a continuum (probably I did not express myself clearly).
You started talking about matter, not I did. If you're a physicist, you should understand the problem if measurement...

--SA

CPallini 25-Feb-11 3:20am

That's a misconcept. Mass doesn't really change.

Sergey Alexandrovich Kryukov 25-Feb-11 19:01pm

Mass in not "changed", it depends on observer. The point is: not everything in matter is discrete.
Wait, I said mass depends on relative speed, you say not. Do you think Relative Theory of Relativity is misconception? Please answer.
Look... argument becomes ridiculous.
--SA

CPallini 26-Feb-11 10:01am

Unique mass is the rest mass. I think quantum field thery (that includes special relativity) is the 'state of the art' if you don't consider gravity.
Yes the discussion is just becoming boring...

Sergey Alexandrovich Kryukov 26-Feb-11 13:37pm

This does not mean not using floating point... relativistic mass is still used. It was just the example of unavoidable floating point.
Did I already say that? (about boring). Let's finish here.
Thank you.
--SA

Richard MacCutchan 24-Feb-11 4:29am

Sorry, but you are totally wrong; the inherent precision errors of floating point variables make them useless for financial values. Money may well be 'floating point' in decimal terms but convert that to IEEE floating values and you start to lose the accuracy required in financial transactions.

Sergey Alexandrovich Kryukov 24-Feb-11 12:56pm

Transaction is not all financial operation. If you calculate average cost of anything, it is integer? By the nature of it, it is not. Transaction, yes, you wire money.
--SA

Richard MacCutchan 25-Feb-11 4:45am

Sorry I don't understand; the original question was about monetary values, and there are plenty of resources on the internet that explain why you should not use floating point variables in such circumstances.

Sergey Alexandrovich Kryukov 25-Feb-11 19:02pm

I think I'll find emough which says otherwise. Do you want that argument? A bit boring, to my taste...
--SA

Richard MacCutchan 26-Feb-11 4:56am

No thanks; you go ahead and do it however you want. I'll just make sure I don't bank with any organisation that follows your ideas.

Sergey Alexandrovich Kryukov 26-Feb-11 13:20pm

I will, thank you for the discussion (this is not my idea, I know many samples). I'm not talking about bank transactions, they are all integer.
--SA

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OriginalGriff · Accepted Answer · 2011-02-23T08:22:00

You do realise that a string is a sequence of characters, don't you?
And that a string of characters such as "123" needs to be handled properly in order to convert it to a number?
Your code

C#

for(int i=0;i<money.length();i++)
{
    amnt=money[i]*10+1;
}

Looks at each character in the string: '1' then '2' then '3'. But these are not the same as the numbers 1, 2, and 3: they are characters. What that means is that they have a character value which is translated to a display device as a visible digit, not the numeric value that you see. Here[^] is a table showing a system of character values against display values. If you try to treat them as numbers they way you are doing, you will get huge errors in your numbers.

Instead, use the built in functions which convert strings to numbers: atoi, atol, atod etc.

amnt = atod(money);

instead of your for loop. (There are other ways, but I'll leave them for your tutor!)

Alain Rist · Accepted Answer · 2011-02-23T10:21:00

Hi,
The Standard C++ Library provides the necessary toolset, not very sexy, but works:

C++

#include <locale>     // for locale, money_get, use_facet
#include <sstream>    // for istringstream, string
#include <iterator>   // for istreambuf_iterator
long double mstold(const std::string& amount)
{
    using std::ios;
    using std::use_facet;
    using std::money_get;
    using std::istreambuf_iterator;

    const std::locale loc("us");
    std::istringstream iss(amount);
    iss.imbue (loc);
    ios::iostate state = ios::goodbit;
    long double value = {0};

    use_facet<money_get<char> >(loc).get(
        iss, istreambuf_iterator<char>(), false, iss, state, value);

    if ((state & ios::failbit) == ios::failbit)
    {
        // Handle error
    }

    return value / 100;
}

[Edited for code style and error handling]
cheers,
AR

converting a monetary value from string to long double

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converting a monetary value from string to long double

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