swift版本

let formatter = NumberFormatter()
        formatter.numberStyle = NumberFormatter.Style(rawValue: UInt(CFNumberFormatterRoundingMode.roundHalfDown.rawValue))!
        let string = formatter.string(from: NSNumber(value: 5))
        print("str = \(string ?? "")")

object-c版本

NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
        formatter.numberStyle = kCFNumberFormatterRoundHalfDown;
        NSString *string = [formatter stringFromNumber:[NSNumber numberWithInt:5.66]];
        NSLog(@"str = %@", string);

这个主要是用NSNumberFormatter这个class提供的格式化方法，好记性不如烂笔头