How do you delete a column by name in data table

Question

To get rid of a column named  foo  in a data frame  I can do   df  lt - df -grep  foo   colnames df     However  once df is converted to a data table object  there is no way to just remove a column   Example   df  lt - data frame id   1 100  foo   rnorm 100   df2  lt - df -grep  foo   colnames df      works df3  lt - data table df  df3 -grep  foo   colnames df3       But once it is converted to a data table object  this no longer works

User · Answer

For a data table  assigning the column to NULL removes it   DT  c  col1    col1    col2    col2     lt - NULL    ---- Notice the extra comma if DT is a data table       which is the equivalent of   DT col1  lt - NULL DT col2  lt - NULL DT col3  lt - NULL DT col4  lt - NULL   The equivalent for a data frame is   DF c  col1    col1    col2    col2     lt - NULL                ---- Notice the missing comma if DF is a data frame   Q  Why is there a comma in the version for data table  and no comma in the version for data frame   A  As data frames are stored as a list of columns  you can skip the comma  You could also add it in  however then you will need to assign them to a list of NULLs  DF   c  col1    col2    col3     lt - list NULL

User · Answer

I simply do it in the data frame kind of way   DT col   NULL   Works fast and as far as I could see doesn t cause any problems   UPDATE  not the best method if your DT is very large  as using the   lt - operator will lead to object copying  So better use   DT   col  NULL

User · Answer

You can also use set for this  which avoids the overhead of   data table in loops   dt  lt - data table  a letters  b LETTERS  c seq 26   d letters  e letters   set  dt  j c 1L 3L 5L   value NULL    gt  dt 1 5     b d 1  A a 2  B b 3  C c 4  D d 5  E e   If you want to do it by column name  which colnames dt   in  c  a   c   e    should work for j

User · Answer

Here is a way when you want to set a   of columns to NULL given their column names a function for your usage     deleteColsFromDataTable  lt - function  train  toDeleteColNames            for  myNm in toDeleteColNames          train  lt - train    myNm   NULL          return  train

User · Answer

DT  c  NULL    remove column c

User · Answer

Very simple option in case you have many individual columns to delete in a data table and you want to avoid typing in all column names  careadviced  dt  lt - dt   -c 1 4 6 17 83 104     This will remove columns based on column number instead   It s obviously not as efficient because it bypasses data table advantages but if you re working with less than say 500 000 rows it works fine

User · Answer

Any of the following will remove column foo from the data table df3     Method 1  and preferred as it takes 0 00s even on a 20GB data table  df3  foo  NULL   df3   c  foo   bar    NULL     remove two columns  myVar    foo  df3    myVar   NULL      lookup myVar contents    Method 2a -- A safe idiom for excluding  possibly multiple    columns matching a regex df3   grep   foo    colnames df3    NULL     Method 2b -- An alternative to 2a  also  safe  in the sense described below df3   which grepl   foo    colnames df3     NULL    data table also supports the following syntax      Method 3  could then assign to df3   df3     foo       though if you were actually wanting to remove column  foo  from df3  as opposed to just printing a view of df3 minus column  foo   you d really want to use Method 1 instead    Do note that if you use a method relying on grep   or grepl    you need to set pattern   foo   rather than  foo   if you don t want columns with names like  fool  and  buffoon   i e  those containing foo as a substring  to also be matched and removed    Less safe options  fine for interactive use   The next two idioms will also work -- if df3 contains a column matching  foo  -- but will fail in a probably-unexpected way if it does not  If  for instance  you use any of them to search for the non-existent column  bar   you ll end up with a zero-row data table   As a consequence  they are really best suited for interactive use where one might  e g   want to display a data table minus any columns with names containing the substring  foo   For programming purposes  or if you are wanting to actually remove the column s  from df3 rather than from a copy of it   Methods 1  2a  and 2b are really the best options     Method 4  df3    SD   SDcols    patterns   foo         Lastly there are approaches using with FALSE  though data table is gradually moving away from using this argument so it s now discouraged where you can avoid it  showing here so you know the option exists in case you really do need it     Method 5a  like Method 3  df3     foo   with FALSE     Method 5b  like Method 4  df3    grep   foo    names df3    with FALSE    Method 5b  another like Method 4  df3    grepl   foo    names df3    with FALSE

User · Answer

Suppose your dt has columns col1  col2  col3  col4  col5  coln   To delete a subset of them   vx  lt - as character bquote c col1  col2  col3  coln    -1  DT   paste0 vx   NULL

[r] How do you delete a column by name in data.table?

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