[RESOLVED] how to format double in custom format

[DaveDavis]

I have c# code to format double:

string str= "12.123456";
            string strFormat = "G";
            strOutput = Convert.ToDouble(str.Replace(",", "")).ToString(strFormat); //12.123456            
            str = "-12.1";
            strFormat = "'正'0.00;'负'0.00;零"; 
            strOutput = Convert.ToDouble(str.Replace(",", "")).ToString(strFormat); //负12.10
            str = "12345.6789";
            strFormat = "#,##0.00"; 
            strOutput = Convert.ToDouble(str.Replace(",", "")).ToString(strFormat); //12,345.68

how to write a general function with custom format string as C#'s ToString(format)?

ChatGPT told me I can use #include <System.SysUtils.hpp> (from Delphi/C++Builder library), but I don't know how to find the hpp file.

[si_the_geek]

The way you've phrased things isn't totally clear, are you aiming for something (almost) equivalent to {double}.ToString(strFormat) in C++ ?

[OptionBase1]

ChatGPT told me I can use #include <System.SysUtils.hpp> (from Delphi/C++Builder library), but I don't know how to find the hpp file.

Did you ask ChatGPT where you could find the file?

If you aren't using Borland/Abracadabra C++ Builder, then that's a non-starter suggestion. I have BCB C++ 3.0 handy, and I can tell you that the SysUtils.hpp file has a large import-chain of several other Borland specific .hpp and .h files.

[DaveDavis]

The way you've phrased things isn't totally clear, are you aiming for something (almost) equivalent to {double}.ToString(strFormat) in C++ ?

Is it impossible for C#'s ToString(strFormat) Or VB.NET's Strings.Format in c++?

I can do simple formatting.
C#: strFormatDouble = ToDouble(str.Replace(",", "")).ToString("##,##0." + new string('0', 2));

string CHAR_DECIMAL = ".";
string CHAR_THOUSAND = ",";
strFormatDouble = ToDouble(str.Replace(CHAR_THOUSAND, "")).ToString("##" + CHAR_DECIMAL + "##0." + new string('0', decimalLength));


c++:

_bstr_t FormatDecimalString(std::string str, int decimalLength)
{
double d = std::stod(str);

std:stringstream oss;
oss << std::fixed << std::setprecision(decimalLength) << d;
std::string strFormatDouble = oss.str();

size_t pos = strFormatDouble .find(CHAR_DECIMAL);
if (pos != std::string::npos) {
int digits = pos - (strFormatDouble [0] == '-' ? 1 : 0);
if (digits >= 4) {
std::string strThousand(1, CHAR_THOUSAND);
for (int i = 3; i < digits; i += 3) {
strFormatDouble .insert(pos - i, strThousand);
}
}
}

return strFormatDouble .c_str();
}

[DaveDavis]

Did you ask ChatGPT where you could find the file?

If you aren't using Borland/Abracadabra C++ Builder, then that's a non-starter suggestion. I have BCB C++ 3.0 handy, and I can tell you that the SysUtils.hpp file has a large import-chain of several other Borland specific .hpp and .h files.

I like FormatFloat in System::Sysutils. If the real world has similar thing in c++ cpp, please tell me. I don't want (know how) to create such complex wheel.

[jmcilhinney]

Is it impossible for C#'s ToString(strFormat)

It's not C#'s anything. That's a standard method, available to any and all .NET languages. If you're writing managed C++ (is it C++/CLI they call it?) then you can use it too. If you're not then you can't but I'm certain that C++ supports composite formatting, a la String.Format in .NET. It's so long since I used C++ I can;t recall how though. That's what you should be looking for, in that case.

[DaveDavis]

It's not C#'s anything. That's a standard method, available to any and all .NET languages. If you're writing managed C++ (is it C++/CLI they call it?) then you can use it too. If you're not then you can't but I'm certain that C++ supports composite formatting, a la String.Format in .NET. It's so long since I used C++ I can;t recall how though. That's what you should be looking for, in that case.

Thank you Sir. Nice to get your input.
In VB.NET, I use Strings.Format. In C#, I am using double.ToString(myformat) to do formatting. But I have no idea how to do same thing in vc++ ATL.
I can't write meaningful reply because I am very new in c++. I am using vc++ ATL and I am in draft stage of conversion from one of my WinForms component in vb.NET and c# .

[jmcilhinney]

In VB.NET, I use Strings.Format. In C#, I am using double.ToString(myformat) to do formatting.

Well that's silly for a start. That Strings.Format is basically a holdover from VB6 and should only be used in upgraded VB6 code that already uses it. The "proper" .NET equivalent is String.Format, which can be used in VB, C# and any other .NET language. They are both for composite formatting though, so if you're just formatting a single value then you should call its ToString method that, again, can be used in VB, C# and any other .NET language.

I think that what you probably want in C++ is the sprintf function.

[2kaud]

In C++20 there is std::format:
https://en.cppreference.com/w/cpp/utility/format/format
https://en.cppreference.com/w/cpp/ut...rmat/formatter

[DaveDavis]

In C++20 there is std::format:
https://en.cppreference.com/w/cpp/utility/format/format
https://en.cppreference.com/w/cpp/ut...rmat/formatter

oh, it is very nice. My project is using c++ 14, no wonder I didn't see it. Thank you Sir.

[2kaud]

My project is using c++ 14

I suggest you update to at least C++20 (C++23 will be released shortly - it's currently in final draft). There's been very many major and important additions to C++ since the C++14 standard was issued (there's now a new standard every 3 years).

[wqweto]

For most flexibility and speed (esp. multi-threaded) you can use this: https://github.com/fmtlib/fmt

cheers,
</wqw>