我正在重新运行以前使用dplyr包运行的旧代码。它现在停留在我计算一个名为mutated_df的数据框中每个组中特定日期出现的次数的位置，其形式如下：
> mutated_df
Source: local data frame [196,077 x 3]
Groups: Date [138]
Date Value group
(date) (int) (fctr)
1 2016-01-28 84 upper-mid
2 2016-01-28 125 max
3 2016-01-28 78 mid
4 2016-01-28 NA NA
5 2016-01-28 86 upper-mid
6 2016-01-28 NA NA
7 2016-01-28 113 upper
8 2016-01-28 NA NA
9 2016-01-28 95 upper-mid
10 2016-01-29 101 upper
.. ... ... ...
为了得到我的计数：
c <- count(mutated_df, Date, group)
但得到错误：
Error in count(mutated_df, Date, group) : object 'group' not found
mutated_df的结构更详细地说是：
structure(list(Date = structure(c(16895, 16895, 16895, 16895,
16895, 16895, 16895, 16895, 16895, 16895), class = "Date"), Value =
c(89.5999985, 93.1999969, 86, 91.4000015, 91.4000015, 91.4000015, 84.1999969,
84.1999969, 89.5999985, 93.1999969), group = structure(c(4L,
5L, 3L, 5L, 5L, 5L, 3L, 3L, 4L, 5L), .Label = c("lower", "lower-mid",
"mid", "upper-mid", "upper", "max"), class = "factor")), .Names = c("Date",
"Value", "group"), class = c("grouped_df", "tbl_df", "tbl", "data.frame"
), row.names = c(NA, -10L), vars = list(Date), drop = TRUE, indices = list(
0:9), group_sizes = 10L, biggest_group_size = 10L, labels = structure(list(
Date = structure(16895, class = "Date")), class = "data.frame", row.names = c(NA,
-1L), vars = list(Date), drop = TRUE, .Names = "Date"))
为什么这个错误刚出现在曾经运行良好的代码中的任何想法？谢谢！