使用dplyr從數據幀中的中間值

Question

計算中值似乎爲a bit of an achilles heel for R（即，no data.frame method）。使用dplyr從數據框中獲取組中值所需的最少打字量是多少？

my_data <- structure(list(group = c("Group 1", "Group 1", "Group 1", "Group 1", 
"Group 1", "Group 1", "Group 1", "Group 1", "Group 1", "Group 1", 
"Group 1", "Group 1", "Group 1", "Group 1", "Group 1", "Group 2", 
"Group 2", "Group 2", "Group 2", "Group 2", "Group 2", "Group 2", 
"Group 2", "Group 2", "Group 2", "Group 2", "Group 2", "Group 2", 
"Group 2", "Group 2"), value = c("5", "3", "6", "8", "10", "13", 
"1", "4", "18", "4", "7", "9", "14", "15", "17", "7", "3", "9", 
"10", "33", "15", "18", "6", "20", "30", NA, NA, NA, NA, NA)), .Names = c("group", 
"value"), class = c("tbl_df", "data.frame"), row.names = c(NA, 
-30L)) 

library(dplyr) 

# groups 1 & 2 
my_data_groups_1_and_2 <- my_data[my_data$group %in% c("Group 1", "Group 2"), ] 

# compute medians per group 
medians <- my_data_groups_1_and_2 %>% 
    group_by(group) %>% 
    summarize(the_medians = median(value, na.rm = TRUE)) 

其中給出：

Error in summarise_impl(.data, dots) : 
    STRING_ELT() can only be applied to a 'character vector', not a 'double' 
In addition: Warning message: 
In mean.default(sort(x, partial = half + 0L:1L)[half + 0L:1L]) : 
    argument is not numeric or logical: returning NA 

什麼是最省力的解決方法嗎？

Answer 1

正如評論說ivyleavedtoadflax，錯誤是通過提供一個非數字或非邏輯參數median造成的，因爲你的value列是character型（可以告訴你，他們不是numeric通過觀察這些數字被引用）。這裏有兩種簡單的方法來解決這個問題：

my_data %>% 
    filter(group %in% c("Group 1", "Group 2")) %>% 
    group_by(group) %>% 
    summarize(the_medians = median(as.numeric(value), na.rm = TRUE)) 

或者

my_data %>% 
    filter(group %in% c("Group 1", "Group 2")) %>% 
    mutate(value = as.numeric(value)) %>% 
    group_by(group) %>% 
    summarize(the_medians = median(value, na.rm = TRUE)) 

要檢查在自己的數據列的type結構，你可以方便地使用

str(my_data) 
#Classes ‘tbl_df’ and 'data.frame': 30 obs. of 2 variables: 
# $ group: chr "Group 1" "Group 1" "Group 1" "Group 1" ... 
# $ value: chr "5" "3" "6" "8" ...