iOS 保留有效小数

- (NSString*)formatString2:(NSString *)str{
    NSString *formatStr = [NSString stringWithFormat:@"%f",str.floatValue*100];
    NSNumber * nsNumber = @(formatStr.floatValue);
    NSString * outNumber = [NSString stringWithFormat:@"%@",nsNumber];
    formatStr = [NSString stringWithFormat:@"%@%%",outNumber];
    return formatStr;
}