How to extract the first n rows per group?

by

yep, just use .SD and index it as needed.

  DT[, .SD[1:2], by=date]

           date age   name
  1: 2000-01-01   3 Andrew
  2: 2000-01-01   4    Ben
  3: 2000-01-02   6   Adam
  4: 2000-01-02   7    Bob

Edited as per @eddi’s suggestion.

@eddi’s suggestion is spot on:

Use this instead, for speed: 

  DT[DT[, .I[1:2], by = date]$V1]

  # using a slightly larger data set
  > microbenchmark(SDstyle=DT[, .SD[1:2], by=date], IStyle=DT[DT[, .I[1:2], by = date]$V1], times=200L)
  Unit: milliseconds
      expr       min        lq    median        uq      max neval
   SDstyle 13.567070 16.224797 22.170302 24.239881 88.26719   200
    IStyle  1.675185  2.018773  2.168818  2.269292 11.31072   200

More Related Contents:

  1. Understanding exactly when a data.table is a reference to (vs a copy of) another data.table
  2. Extract row corresponding to minimum value of a variable by group
  3. Group by multiple columns and sum other multiple columns
  4. Left join using data.table
  5. How can one work fully generically in data.table in R with column names in variables
  6. Faster version of combn
  7. Pass column name in data.table using variable [duplicate]
  8. What does .SD stand for in data.table in R
  9. Apply multiple functions to multiple columns in data.table
  10. Convert column classes in data.table
  11. Using lists inside data.table columns
  12. Perform a semi-join with data.table
  13. Find the intersection of overlapping ranges in two tables using data.table function foverlaps
  14. Adding new columns to a data.table by-reference within a function not always working
  15. Referring to data.table columns by names saved in variables
  16. Calculate multiple aggregations on several variables using lapply(.SD, …)
  17. Merge rows in a dataframe where the rows are disjoint and contain NAs
  18. Calculate row-wise maximum
  19. Efficient row-wise operations on a data.table
  20. Replace logical values (TRUE / FALSE) with numeric (1 / 0)
  21. from data table, randomly select one row per group
  22. How to change type of target column when doing := by group in a data.table in R?
  23. How to add a index by set of data when using rbindlist?
  24. Dynamically add column names to data.table when aggregating
  25. Insert a row in a data.table
  26. rolling joins data.table in R
  27. Efficiently merging two data frames on a non-trivial criteria
  28. assigning by reference into loaded package datasets
  29. Index unique values in data.table
  30. calculating sum of previous 3 rows in R data.table (by grid-square)

Leave a Comment