Dplyr:如何按组查找第一个非缺失字符串? [英] Dplyr : how to find the first-non missing string by groups?

问题描述

考虑下面的简单例子

group <-c('A','A','A','B','B','B','B')
names<- c(NA,'fred',NA,'josh','josh',NA,NA)
data=data_frame(group,names)

> data
# A tibble: 7 × 2
  group names
  <chr> <chr>
1     A  <NA>
2     A  fred
3     A  <NA>
4     B  josh
5     B  josh
6     B  <NA>
7     B  <NA>

在这里，我想为每个 group 获取 names 中第一个非缺失的名称.我怎样才能做到这一点?下面使用 coalesce 和 first 的解决方案失败.

Here, I would like to get, for each group the first non missing name in names. How can I do that? The solution below using coalesce and first fail.

data %>% group_by(group) %>% mutate(first_non_missing = first(names),
                                    first_non_missing_alt = coalesce(names)) %>% ungroup()

# A tibble: 7 × 4
  group names first_non_missing first_non_missing_alt
  <chr> <chr>             <chr>                 <chr>
1     A  <NA>              <NA>                  <NA>
2     A  fred              <NA>                  fred
3     A  <NA>              <NA>                  <NA>
4     B  josh              josh                  josh
5     B  josh              josh                  josh
6     B  <NA>              josh                  <NA>
7     B  <NA>              josh                  <NA>

确实，对于 A 组，first_non_missing 应该是 fred 所有三个观察值..

Indeed, for group A, first_non_missing should be fred for all three observations..

非常感谢！

推荐答案

Summarise 会给每组一个条目，在这里，使用 which

Summarise will give one entry per group, here, finding the first non-missing using which

data %>%
  group_by(group) %>%
  summarise(first_non_missing = names[which(!is.na(names))[1]])

给予

  group first_non_missing
  <chr>             <chr>
1     A              fred
2     B              josh

如果您仍然需要所有行，请将 summarise 替换为 mutate.

If you still want all of the rows, replace summarise with mutate.

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