Friday, April 8, 2011

Determining whether a numeric value has at most two decimal places

I was working on a .NET website and wanted to write some C# code to validate that a value input by the user was numeric and had at most two decimal places. This is useful, for example, when validating that the input represents a dollar amount.

I first tried this, which didn't work:
try
{
// must be numeric value
double d = double.Parse(s);
// max of two decimal places
if (100 * d != (int)(100 * d)) // max of two decimal places
return false;
return true;
}
catch
{
return false;
}

The above is unreliable because, since d is a floating-point number, 100 * d isn't always exactly equal to (int)(100 * d), even when d has two or fewer decimal places. For example, 100 * 1.23 might evaluate to, say, 122.9999999.

This post on StackOverflow offers several solutions, but none of them looked right for my purpose. Instead, I came up with this:
try
{
// must be numeric value
double d = double.Parse(s);
// max of two decimal places
if (s.IndexOf(".") >= 0)
{
if (s.Length > s.IndexOf(".") + 3)
return false;
}
return true;

catch
{
return false;
}

The same thing could be accomplished using a regular expression, if you prefer.

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