Re-computing the levels of factor

Problem

You want to do re-compute the levels of a factor. This is useful when a factor contains levels that aren’t actually present in the data. This can happen during data import, or when you remove some rows.

Solution

For a single factor object:

  1. # Create a factor with an extra level (gamma)
  2. x <- factor(c("alpha","beta","alpha"), levels=c("alpha","beta","gamma"))
  3. x
  4. #> [1] alpha beta  alpha
  5. #> Levels: alpha beta gamma
  7. # Remove the extra level
  8. x <- factor(x)
  9. x
  10. #> [1] alpha beta  alpha
  11. #> Levels: alpha beta

After importing data, you may have a data frame with a mix of factors and other kinds vectors, and want to re-compute the levels of all the factors. You can use the droplevels() function to do this.

  1. # Create a data frame with some factors (with extra levels)
  2. df <- data.frame(
  3.     x = factor(c("alpha","beta","alpha"), levels=c("alpha","beta","gamma")),
  4.     y = c(5,8,2),
  5.     z = factor(c("red","green","green"), levels=c("red","green","blue"))
  6. )
  8. # Display the factors (with extra levels)
  9. df$x
  10. #> [1] alpha beta  alpha
  11. #> Levels: alpha beta gamma
  12. df$z
  13. #> [1] red   green green
  14. #> Levels: red green blue
  17. # Drop the extra levels
  18. df <- droplevels(df)
  20. # Show the factors again, now without extra levels
  21. df$x
  22. #> [1] alpha beta  alpha
  23. #> Levels: alpha beta
  24. df$z
  25. #> [1] red   green green
  26. #> Levels: red green

See also

To re-compute the levels of all the factor columns in a data frame, see ../Re-computing_the_levels_of_all_factor_columns_in_a_data_frame.