r - 如何找到满足条件的组的第一个元素

Question

structure(list(group = c(17L, 17L, 17L, 18L, 18L, 18L, 18L, 19L, 
19L, 19L, 20L, 20L, 20L, 21L, 21L, 22L, 23L, 24L, 25L, 25L, 25L, 
26L, 27L, 27L, 27L, 28L), var = c(74L, 49L, 1L, 74L, 1L, 49L, 
61L, 49L, 1L, 5L, 5L, 1L, 44L, 44L, 12L, 13L, 5L, 5L, 1L, 1L, 
4L, 4L, 1L, 1L, 1L, 49L), first = c(0, 0, 1, 0, 1, 0, 0, 0, 1, 
0, 0, 1, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0)), .Names = c("group", 
"var", "first"), class = c("tbl_df", "tbl", "data.frame"), row.names = c(NA, 
-26L))

使用前两列的数据，我想创建第三列(称为 first )，其中 first == 1仅当 var == 1第一次在一个小组中。换句话说，我想标记 group 中的第一个元素那个满足var == 1 .我怎么能在 dplyr 中做到这一点?当然group_by应该使用，但接下来呢？

Answer 1

library(dplyr)

df$first = NULL

df %>%
  group_by(group) %>%
  mutate(first = as.numeric(row_number() == min(row_number()[var == 1]))) %>%
  ungroup()

# # A tibble: 26 x 3
#   group   var first
#   <int> <int> <dbl>
# 1    17    74     0
# 2    17    49     0
# 3    17     1     1
# 4    18    74     0
# 5    18     1     1
# 6    18    49     0
# 7    18    61     0
# 8    19    49     0
# 9    19     1     1
# 10   19     5     0
# # ... with 16 more rows

这个想法是标记最小行号，其中 var = 1，在每个组内。

这将返回一些警告，因为在某些组中没有 var = 1 例。

另一种选择是这样的:

library(dplyr)

df$first = NULL

# create row id
df$id = seq_along(df$group)

df %>%
  filter(var == 1) %>%                         # keep cases where var = 1
  distinct(group, .keep_all = T) %>%           # keep distinct cases based on group
  mutate(first = 1) %>%                        # create first column
  right_join(df, by=c("id","group","var")) %>% # join back original dataset
  mutate(first = coalesce(first, 0)) %>%       # replace NAs with 0
  select(-id)                                  # remove row id

# # A tibble: 26 x 3
#   group   var first
#   <int> <int> <dbl>
# 1    17    74     0
# 2    17    49     0
# 3    17     1     1
# 4    18    74     0
# 5    18     1     1
# 6    18    49     0
# 7    18    61     0
# 8    19    49     0
# 9    19     1     1
#10    19     5     0
# # ... with 16 more rows