我试图总结出现在"中间" position s的f频率数据，即，在第一个和最后一个position之间。我完成此任务的方法是过滤这些数据，执行summarise，然后将新数据与过滤它们的 Dataframe 重新连接。这对于训练数据很有效：

library(tidyverse)
df %>%
  group_by(rowid) %>%
  # summarize frequencies for middle postions:
  filter(position != first(position) & position != last(position)) %>%
  # summarise:
  summarize(across(position),
            middle_position = mean(f, na.rm = TRUE),
            word = str_c(word, collapse=" ")
            ) %>%
  left_join(df, ., by = c("rowid", "position"))

但是，应用到实际数据时，我得到以下错误消息：

Error in `left_join()`:
! Join columns must be present in data.
✖ Problem with `position`.
Run `rlang::last_error()` to see where the error occurred.
> rlang::last_error()
<error/rlang_error>
Error in `left_join()`:
! Join columns must be present in data.
✖ Problem with `position`.
---
Backtrace:
 1. ... %>% left_join(bnc_X, ., by = c("rowid", "position"))
 3. dplyr:::left_join.data.frame(bnc_X, ., by = c("rowid", "position"))
Run `rlang::last_trace()` to see the full context.
> rlang::last_trace()
<error/rlang_error>
Error in `left_join()`:
! Join columns must be present in data.
✖ Problem with `position`.
---
Backtrace:
    ▆
 1. ├─... %>% left_join(bnc_X, ., by = c("rowid", "position"))
 2. ├─dplyr::left_join(bnc_X, ., by = c("rowid", "position"))
 3. └─dplyr:::left_join.data.frame(bnc_X, ., by = c("rowid", "position"))
 4.   └─dplyr:::join_mutate(...)
 5.     └─dplyr:::join_cols(...)
 6.       └─dplyr:::standardise_join_by(...)
 7.         └─dplyr:::check_join_vars(by$x, x_names, error_call = error_call)
 8.           └─rlang::abort(bullets, call = error_call)

主要的问题似乎是变量position-为什么它不能被识别？我已经花了几个小时试图解决这个问题，但不能，并将感谢帮助!
数据：

df <- data.frame(
  size = c(3,3,3,
              3,3,3,
              4,4,4,4,
              5,5,5,5,5,
              3,3,3),
  rowid = c(1,1,1,2,2,2,3,3,3,3,4,4,4,4,4,5,5,5),
  turn = c(rep("How are you?",3),
           rep("I'm fine.",3),
           rep("How's the weather?",4),
           rep("It's really very cold.",5),
           rep("I love you",3)),
  word = c("how","are","you",
           "i","'m","fine",
           "how","'s","the","weather",
           "it","'s","really", "very","cold",
           "i","love","you"),
  f = c(400,300,250,
        600,555,1,
        400,500,700,20,
        390,500,177,200,35,
        600,199,400),
  position = c(1,2,3,
               1,2,3,
               1,2,3,4,
               1,2,3,4,5,
               1,2,3)
)