R：提取一列中的唯一值，并按另一列中的值分组 [英] R: Extract unique values in one column grouped by values in another column

问题描述

我原以为可以用data.table解决这个问题，但是看起来有点困难。我的数据框如下所示：

I had thought I could solve this with data.table but looks like it is a bit more difficult. My dataframe looks like this:

userID1   A
userID1   A
userID1   B
userID2   A
userID2   A

输出应该看起来像这样：

The output is supposed to look like this:

userID1   A
userID1   B
userID2   A

基本上为每个唯一的用户和项目出现创建一行。大多数示例都是关于计数唯一元素，而不是实际提取这些元素。有建议吗？

Basically create a row for each unique user and item occurence. Most of the examples are about counting unique elements but not about actually extracting these. Any suggestions?

推荐答案

这是一个dplyr解决方案，应获得这些结果

This is a dplyr solution that should get those results

library(dplyr)

df <- data.frame(user = c(rep("userID1",3), rep("userID2",2)), 
                 group = c("A","A","B","A","A"), 
                 stringsAsFactors = FALSE)


df <- df %>% 
      group_by(user, group) %>% 
      filter(row_number() == 1)

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