Need help with rounding function for big number

[icarpenter]

Hi,
I'm trying to round a number in a C++ program to the tenthousandths place. Here's what I have, but it only rounds to the thousandth place then stops:

float Roundtenthousandth( float tenthousandth ) 
{ 
return static_cast<float>(floor( tenthousandth * 10000 + 0.5 ) / 10000 ); 
}

Thanks for your help, Iris