Dollar sign before a variable

by

To access a column, use:

my_data[ , cond]

or

my_data[[cond]]

The ith row can be accessed with:

my_data[i, ]

Combine both to obtain the desired value:

my_data[i, cond]

or 

my_data[[cond]][i]

More Related Contents:

  1. Aggregate / summarize multiple variables per group (e.g. sum, mean)
  2. Dynamically select data frame columns using $ and a character value
  3. Sort (order) data frame rows by multiple columns
  4. The difference between bracket [ ] and double bracket [[ ]] for accessing the elements of a list or dataframe
  5. Numbering rows within groups in a data frame
  6. Simultaneously merge multiple data.frames in a list
  7. Calculate the mean by group
  8. Select the row with the maximum value in each group
  9. Filter data.frame rows by a logical condition
  10. Count number of rows within each group
  11. Remove rows with all or some NAs (missing values) in data.frame
  12. Drop unused factor levels in a subsetted data frame
  13. Split data.frame based on levels of a factor into new data.frames
  14. Combine a list of data frames into one data frame by row
  15. Subset data frame based on number of rows per group
  16. Pass a data.frame column name to a function
  17. Drop data frame columns by name
  18. How to split a data frame?
  19. Why is it not advisable to use attach() in R, and what should I use instead?
  20. Create an empty data.frame
  21. How to plot all the columns of a data frame in R
  22. Sample random rows in dataframe
  23. Extracting specific columns from a data frame
  24. Subset / filter rows in a data frame based on a condition in a column
  25. How does one reorder columns in a data frame?
  26. Rename multiple columns by names
  27. Convert a dataframe to a vector (by rows)
  28. Should I use a data.frame or a matrix?
  29. Re-ordering factor levels in data frame [duplicate]
  30. Convert list to data frame while keeping list-element names

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