swift NSNumberFormatter()

/**
When greater than or equal to three digits, strictly keep three digits; when
less than three digits, strictly retain the original digits
**/
class func doubleToString3(value:Double)->String{
//Initialize NumberFormatter
let format = NSNumberFormatter()
// Set numberStyle (there are multiple formats)
format.minimumIntegerDigits = 1
format.minimumFractionDigits = 0
format.maximumFractionDigits = 3
//the converted string
let s = format.stringFromNumber(value)
return s!
}