替换 dplyr 管道中的重复元素

Question

我想替换组内重复的元素

df <- data.frame(A=c("a", "a", "a", "b", "b", "c"), group = c(1, 1, 2, 2, 2, 3))

我想保留组的第一个元素，同时用 NA 替换其他任何元素。就像是：

df <- df %>% 
     group_by(group) %>%
     mutate(B = first(A))

这不会产生我想要的。我想要的是B <- c(a, NA, a, NA, NA, c)

Answer 1

使用replace有duplicated：

df %>% group_by(group) %>% mutate(B = replace(A, duplicated(A), NA))

# A tibble: 6 x 2
# Groups:   group [3]
#       A group
#  <fctr> <dbl>
#1      a     1
#2     NA     1
#3      a     2
#4      b     2
#5     NA     2
#6      c     3

或者如果只保留第一个元素：

df %>% 
    group_by(group) %>% 
    mutate(B = ifelse(row_number() == 1, as.character(A), NA))

# A tibble: 6 x 2
# Groups:   group [3]
#      A group
#  <chr> <dbl>
#1     a     1
#2  <NA>     1
#3     a     2
#4  <NA>     2
#5  <NA>     2
#6     c     3

或使用replace：

df %>% 
    group_by(group) %>% 
    mutate(B = replace(A, row_number() > 1, NA))

# A tibble: 6 x 2
# Groups:   group [3]
#       A group
#  <fctr> <dbl>
#1      a     1
#2     NA     1
#3      a     2
#4     NA     2
#5     NA     2
#6      c     3