在R中使用选择功能错误 [英] Error using select function in R

问题描述

我想获得用户最常播放的歌曲。在csv文件中我想要的三个字段是userId，songId和playCount，但是select函数给出一个错误：

I want to get the song that user's play most frequently. The three fields I want in the csv file are userId,songId and playCount but the select function is giving an error:

write.csv(group_by(mydata,userId) %.%
summarise(one=max(playCount)) %.%
select(userId,songId,playCount), file="FavouriteSongs.csv")

Error in eval(expr, envir, enclos) : object 'songId' not found

数据的一个例子看起来像这样

An example of the data looks like this

userId      songId            playCount
A           568r              85
A           711g              18
C           34n               18
E           454j              65
D           663a              72
B           35d               84
A           34c               72
A           982s              65
E           433f              11
A           565t              7

提前感谢

推荐答案

在你的链中d序列 dplyr 操作，总结调用将产生两列：分组变量和摘要函数的结果。

In your chained sequence of dplyr operations, the summarise call will produce two columns: the grouping variable and the result of the summary function.

df %.%
  group_by(userId) %.%
  summarise(
    one = max(playCount))

# Source: local data frame [5 x 2]
# 
#   userId one
# 1      A  85
# 2      B  84
# 3      C  18
# 4      D  72
# 5      E  65

当您尝试选择从生成的数据框中的songID变量总结找不到songID变量。

When you then try to select the songID variable from the data frame generated by summarise, the songID variable is not found.

df %.%
  group_by(userId) %.%
  summarise(
    one = max(playCount)) %.%
  select(userId, songId, playCount)
# Error in eval(expr, envir, enclos) : object 'songId' not found 

在这种情况下，更合适的 dplyr 函数是过滤。在这里，我们选择条件 playCount == max（playCount）中的行 c 每个组

A more suitable dplyr function in this case is filter. Here we select rows where the condition playCount == max(playCount) is TRUE within each group.

df %.%
  group_by(userId) %.%
  filter(
    playCount == max(playCount))

# Source: local data frame [5 x 3]
# Groups: userId
# 
#   userId songId playCount
# 1      A   568r        85
# 2      C    34n        18
# 3      E   454j        65
# 4      D   663a        72
# 5      B    35d        84

你会发现几个很好的 dplyr example here 。

You find several nice dplyr examples here.

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