Variable precision in numeric format string

[mdeaver2003]

I'm trying to output a double using a precision that
varies, governed by the value of a precision variable.
In C I can do it like this:

double pi = 3.14159;
int prec = 4;

printf( "%.*f", prec, pi ); // 3.1416

The * in "%.*f" says to use the value of the next
argument (must be an int) for the precision. Is there
a numeric format in C# that can do this? The best
workaround I've come up with is:

double pi = 3.14159;
int prec = 4;

StringBuilder sb = new StringBuilder( "#0." );
sb.Append( new string( '0', prec ) ); // sb = "#0.0000"

Console.Write( pi.ToString( sb.ToString() ) ); // 3.1416

Thanks,
Mason Deaver
General Dynamics C4 Systems, VertexRSI

[Kevin Spencer]

Here are 3 example methods:

// using ToString, IndexOf, and Substring
public static string FormatDoubleA(double d, int precision)
{
string precise = pi.ToString();
return precise.Substring(0, piPrecise.IndexOf('.') + prec);
}

// Using NumberFormatInfo
public static string FormatDoubleB(double d, int precision)
{
System.Globalization.NumberFormatInfo nfi =
new System.Globalization.CultureInfo( "en-US",
false ).NumberFormat;
nfi.NumberDecimalDigits = precision;
return d.ToString("F", nfi);
}

// Using Standard Numeric format string
public static string FormatDoubleC(double d, int precision)
{
return d.ToString("F" + precision.ToString());
}

I'm not sure which of these three is the most efficient, but I suspect it is
the third. I would suspect that the second is the least efficient.

--
HTH,

Kevin Spencer
Microsoft MVP
..Net Developer
You can lead a fish to a bicycle,
but you can't make it stink.

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I'm trying to output a double using a precision that
varies, governed by the value of a precision variable.
In C I can do it like this:

double pi = 3.14159;
int prec = 4;

printf( "%.*f", prec, pi ); // 3.1416

The * in "%.*f" says to use the value of the next
argument (must be an int) for the precision. Is there
a numeric format in C# that can do this? The best
workaround I've come up with is:

double pi = 3.14159;
int prec = 4;

StringBuilder sb = new StringBuilder( "#0." );
sb.Append( new string( '0', prec ) ); // sb = "#0.0000"

Console.Write( pi.ToString( sb.ToString() ) ); // 3.1416

Thanks,
Mason Deaver
General Dynamics C4 Systems, VertexRSI

[mdeaver2003]

Thanks, Kevin. Your third method will work fine for me.

By the way, method 1's return statement needs to be written
like this to avoid an off-by-one-digit error:

return precise.Substring(0, precise.IndexOf('.') + prec + 1);

This method won't round up when needed, e.g., using my
original precision value you'll get 3.1415 instead of 3.1416.
Methods 2 and 3 do round correctly.

Mason Deaver
General Dynamics C4 Systems, VertexRSI