Last 2 observations in a grouped sequence-CodePudding

I am working with lab results and have sequential results for 4 patients

set.seed(100)  
df <- data.frame(patient = c(rep("aaa", 5), rep("bbb", 3), rep("ccc", 1), rep("ddd",2)), 
                 seq_visit= c(1,2,3,4,5,1,2,3,1,1,2), 
                 result = c(20,15,17,2,15,13,12,5,10,4,12))

Ultimately, I want to get to a df that has only the last 2 observations for each group and then add column with a "pre" or "post" result so I can do a paired t-test

set.seed(100)  
df2 <- data.frame(patient = c(rep("aaa", 2), rep("bbb", 2), rep("ddd",2)), 
                 seq_visit= c(4,5,2,3,1,2), 
                 result = c(2,15,12,5,4,12),
                 tx = c("pre","post","pre","post","pre","post"))

I really would appreciate a tidy answer

CodePudding user response：

df %>%
  group_by(patient) %>%
  slice_tail(n=2) %>%
  filter(n()==2) %>%
  ungroup()

CodePudding user response：

A base solution

by(df, df$patient, tail, 2) |>
  Filter(f = \(x) nrow(x) > 1) |>
  Reduce(f = rbind) |>
  transform(tx = c("pre", "post"))

#    patient seq_visit result   tx
# 4      aaa         4      2  pre
# 5      aaa         5     15 post
# 7      bbb         2     12  pre
# 8      bbb         3      5 post
# 10     ddd         1      4  pre
# 11     ddd         2     12 post

CodePudding user response：

An option with data.table

library(data.table)
setDT(df)[, if(.N > 1) tail(.SD, 2), patient][,
   tx := rep(c('pre', 'post'), length.out = .N)][]
   patient seq_visit result     tx
    <char>     <num>  <num> <char>
1:     aaa         4      2    pre
2:     aaa         5     15   post
3:     bbb         2     12    pre
4:     bbb         3      5   post
5:     ddd         1      4    pre
6:     ddd         2     12   post

CodePudding user response：

Another data.table option

setDT(df)
df[, if (.N > 1) c(last(.SD, 2), .(tx = c('pre', 'post'))), patient]

#    patient seq_visit result     tx
#     <char>     <num>  <num> <char>
# 1:     aaa         4      2    pre
# 2:     aaa         5     15   post
# 3:     bbb         2     12    pre
# 4:     bbb         3      5   post
# 5:     ddd         1      4    pre
# 6:     ddd         2     12   post