Ios 如何在函数中使用数字格式化程序？

大家好，我是swift的新手，当我在UILabel上设置文本以显示小数点到2位时，我需要使用数字格式化程序，但是我遇到了这个错误。。。“NumberFormatter”类型的值没有成员“String”，有人能告诉我如何正确执行吗

func theseAretheFunctions() {


        let formatter = NumberFormatter()
        formatter.numberStyle = .decimal
        formatter.minimumFractionDigits = 2
        formatter.maximumFractionDigits = 2
        if reactionVolumeInt == 50{


        bufferVolumeLabel.text = String(Double(numberOfSamples)! * 10)
        dNTPsVolumeLabel.text = numberOfSamples
        forwardPrimerLabel.text = String(Double(numberOfSamples)! * 2.5)
        reversePrimerLabel.text = String(Double(numberOfSamples)! * 2.5)
        dnaPolymeraseLabel.text = String(Double(numberOfSamples)! * 0.5)
        }
        else if reactionVolumeInt == 20{
            bufferVolumeLabel.text = String(Double(numberOfSamples)! * 4)
            dNTPsVolumeLabel.text = formatter.String(Double(numberOfSamples)! * 0.4)// This is where i get the error.
            forwardPrimerLabel.text = String(Double(numberOfSamples)! * 1)
            reversePrimerLabel.text = String(Double(numberOfSamples)! * 1)
            dnaPolymeraseLabel.text = String(Double(numberOfSamples)! * 0.2)



        }

改变

dNTPsVolumeLabel.text = formatter.string(from:Double(numberOfSamples)! * 0.4)