Dynamically select data frame columns using and a character value

Question

I have a vector of different column names and I want to be able to loop over each of them to extract that column from a data frame  For example  consider the data set mtcars and some variable names stored in a character vector cols  When I try to select a variable from mtcars using a dynamic subset of cols  nether of these work  cols  lt - c  mpg    cyl    am   col  lt - cols 1  col    1   mpg   mtcars col   NULL mtcars cols 1    NULL   how can I get these to return the same values as  mtcars mpg   Furthermore how can I loop over all the columns in cols to get the values in some sort of loop   for x in seq along cols        value  lt - mtcars  order mtcars cols x

User · Answer

if you want to select column with specific name then just do

A=mtcars[,which(conames(mtcars)==cols[1])]
#and then
colnames(mtcars)[A]=cols[1]

you can run it in loop as well reverse way to add dynamic name eg if A is data frame and xyz is column to be named as x then I do like this

A$tmp=xyz
colnames(A)[colnames(A)=="tmp"]=x

again this can also be added in loop

User · Answer

Another solution is to use  get    gt  cols  lt - c  cyl    am    gt  get cols 1   mtcars    1  6 6 4 6 8 6 8 4 4 6 6 8 8 8 8 8 8 4 4 4 4 8 8 8 8 4 4 4 8 6 8 4

User · Answer

too late   but I guess I have the answer -  Here s my sample study df dataframe -      gt study df    study   sample       collection dt other column    1 DS-111 ES768098 2019-01-21 04 00 30          lt NA gt     2 DS-111 ES768099 2018-12-20 08 00 30   some value    3 DS-111 ES768100                 lt NA gt    some value   And then -   gt     Selecting Columns in an Given order  gt     Create ColNames vector as per your Preference  gt    gt  selectCols  lt - c  study   collection dt   sample    gt    gt     Select data from Study df with help of selection vector  gt  selectCols   gt   select  data study df       study       collection dt   sample 1 DS-111 2019-01-21 04 00 30 ES768098 2 DS-111 2018-12-20 08 00 30 ES768099 3 DS-111                 lt NA gt  ES768100  gt

User · Answer

If I understand correctly  you have a vector containing variable names and would like loop through each name and sort your data frame by them  If so  this example should illustrate a solution for you  The primary issue in yours  the full example isn t complete so I m not sure what else you may be missing  is that it should be order Q1 R1000  parameter X    instead of order Q1 R1000 parameter X    since parameter is an external object that contains a variable name opposed to a direct column of your data frame  which when the   would be appropriate    set seed 1  dat  lt - data frame var1 round rnorm 10                       var2 round rnorm 10                       var3 round rnorm 10    param  lt - paste0  var  1 3  dat     var1 var2 var3  1    -1    2    1  2     0    0    1  3    -1   -1    0  4     2   -2   -2  5     0    1    1  6    -1    0    0  7     0    0    0  8     1    1   -1  9     1    1    0  10    0    1    0  for p in rev param       dat  lt - dat order dat  p        dat     var1 var2 var3  3    -1   -1    0  6    -1    0    0  1    -1    2    1  7     0    0    0  2     0    0    1  10    0    1    0  5     0    1    1  8     1    1   -1  9     1    1    0  4     2   -2   -2

User · Answer

You can t do that kind of subsetting with    In the source code  R src main subset c  it states        The   subset operator       We need to be sure to only evaluate the first argument       The second will be a symbol that needs to be matched  not evaluated         Second argument  What   You have to realise that    like everything else in R   including for instance           etc  is a function  that takes arguments and is evaluated  df V1 could be rewritten as      df   V1    or indeed      df    V1     But         df   paste0  V1          for instance will never work  nor will anything else that must first be evaluated in the second argument  You may only pass a string which is never evaluated     Instead use    or    if you want to extract only a single column as a vector     For example   var  lt -  mpg   Doesn t work mtcars var  These both work  but note that what they return is different   the first is a vector  the second is a data frame mtcars  var   mtcars var    You can perform the ordering without loops  using do call to construct the call to order  Here is a reproducible example below      set seed for reproducibility set seed 123  df  lt - data frame  col1   sample 5 10 repl T    col2   sample 5 10 repl T    col3   sample 5 10 repl T        We want to sort by  col3  then by  col1  sort list  lt - c  col3   col1       Use  do call  to call order  Seccond argument in do call is a list of arguments    to pass to the first argument  in this case  order      Since  a data frame is really a list  we just subset the data frame    according to the columns we want to sort in  in that order df  do call  order   df    match  sort list   names df                 col1 col2 col3 10    3    5    1 9     3    2    2 7     3    2    3 8     5    1    3 6     1    5    4 3     3    4    4 2     4    3    4 5     5    1    4 1     2    5    5 4     5    3    5

User · Answer

Had similar problem due to some CSV files that had various names for the same column  This was the solution   I wrote a function to return the first valid column name in a list  then used that       Return the string name of the first name in names that is a column name in tbl   else null ChooseCorrectColumnName  lt - function tbl  names    for n in names        if  n  in  colnames tbl             return n          return null     then     cptcodefieldname   ChooseCorrectColumnName file  c  CPT    CPT Code    icdcodefieldname   ChooseCorrectColumnName file  c  ICD 10 CM Code    ICD10 Code     if  is null cptcodefieldname     is null icdcodefieldname             print  Bad file column name        Here we use the hash table implementation where    we have a string key and list value so we need actual strings    not Factors file cptcodefieldname    as character file cptcodefieldname   file icdcodefieldname    as character file icdcodefieldname   for  i in 1 length file cptcodefieldname          cpt valid icds file cptcodefieldname  i    lt  lt - unique c cpt valid icds  file cptcodefieldname  i     file icdcodefieldname  i

User · Answer

Using dplyr provides an easy syntax for sorting the data frames  library dplyr  mtcars   gt   arrange gear  desc mpg     It might be useful to use the NSE version as shown here to allow dynamically building the sort list  sort list  lt - c  gear    desc mpg    mtcars   gt   arrange   dots   sort list

User · Answer

mtcars do call order  mtcars cols

[r] Dynamically select data frame columns using $ and a character value

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