Truncating a dataframe according to count of vector elements

Question

I have a dataframe df, containing three vectors:

subject  condition  value
01       A          12
01       A          6
01       B          10
01            


        

Answer 1

For each subject/condition pair create a sequence number seq for its rows and then only keep those rows whose sequence number is less than 3.

subset(transform(DF, seq = ave(value, subject, condition, FUN = seq_along)), seq < 3)

giving:

  subject condition value seq
1      01         A    12   1
2      01         A     6   2
3      01         B    10   1
4      01         B     2   2
5      02         A     5   1
6      02         A    11   2
7      02         B     3   1
8      02         B     5   2

Note

The input in reprodudible form is assumed to be:

Lines <- "subject  condition  value
01       A          12
01       A          6
01       B          10
01       B          2
02       A          5
02       A          11
02       B          3
02       B          5
02       B          9"
DF <- read.table(text = Lines, header = TRUE, strip.white = TRUE,
  colClasses = c("character", "character", "numeric"))

Answer 2

Here's a dplyr solution -

df %>%
  group_by(subject, condition) %>%
  filter(row_number() < 3) %>%
  ungroup()

# A tibble: 8 x 3
  subject condition value
  <chr>   <chr>     <dbl>
1 01      A            12
2 01      A             6
3 01      B            10
4 01      B             2
5 02      A             5
6 02      A            11
7 02      B             3
8 02      B             5