使用R有条件地选择另一个列在组中的最后N个值 [英] Conditionally selecting last N values within a group by another column using R

问题描述

此问题类似于按列但是，我想按组选择最后N个值，其中N取决于相应计数列的值.计数代表特定名称的出现次数.如果count> 3，我只想要最后三个条目，但如果少于3，我只想要最后一个条目.

However, I want to select the last N values by group, with N depending on the value of a corresponding count column. The count represents the number of occurrences of a specific name. If count >3, I only want the last three entries but if it is less than 3, I only want the last entry.

# Sample data
df <- data.frame(Name = c("x","x","x","x","y","y","y","z","z"), Value = c(1,2,3,4,5,6,7,8,9))

# Obtain count for each name
count <- df %>%
  group_by(Name) %>%
  summarise(Count = n_distinct(Value))

# Merge dataframe with count
merge(df, count, by=c("Name"))

# Delete the first entry for x and the first entry for z

# Desired output
data.frame(Name = c("x","x","x","y","y","y","z"), Value = c(2,3,4,5,6,7,9))

推荐答案

在基数R中，先将 df 除以 df $ Name .然后，对于每个子组，检查行数并有条件地提取最后3或最后1行.

In base R, split the df by df$Name first. Then, for each subgroup, check number of rows and extract last 3 or last 1 row conditionally.

do.call(rbind, lapply(split(df, df$Name), function(a)
    a[tail(sequence(NROW(a)), c(3,1)[(NROW(a) < 3) + 1]),]))

或

do.call(rbind, lapply(split(df, df$Name), function(a)
    a[tail(sequence(NROW(a)), ifelse(NROW(a) < 3, 1, 3)),]))
#    Name Value
#x.2    x     2
#x.3    x     3
#x.4    x     4
#y.5    y     5
#y.6    y     6
#y.7    y     7
#z      z     9

对于三个条件值

do.call(rbind, lapply(split(df, df$Name), function(a)
      a[tail(sequence(NROW(a)), ifelse(NROW(a) >= 6, 6, ifelse(NROW(a) >= 3, 3, 1))),]))

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