Select character variables that have all missing values

Question

I have a SAS dataset with around 3,000 variables, and I would like to get rid of the character variables for which all values are missing. I know how to do this for numeric

Answer 1

Rob and cmjohns, thank you SO MUCH for your help. Based on your solutions and an idea I had over the weekend, here is what I came up with:

%macro removeEmptyCols(origDset, outDset);
    * get the number of obs in the original dset;
    %let dsid  = %sysfunc(open(&origDset));
    %let origN = %sysfunc(attrn(&dsid, nlobs));
    %let rc    = %sysfunc(close(&dsid));

    proc transpose data= &origDset out= transpDset;
        var _all_;
    run;

    data transpDset;
        set transpDset;
        * proc transpose converted all old vars to character,
          so the . from old numeric vars no longer means 'missing';
        array oldVar_ _character_;
        do over oldVar_;
            if strip(oldVar_) = "." then oldVar_ = "";
        end;

        * each row from the old dset is now a column with varname starting with 'col';
        numMiss = cmiss(of col:);

        numCols = &origN;
    run;

    proc sql noprint;
      select _NAME_ into: varsToKeep separated by ' '
      from transpDset
      where numMiss < numCols;
    quit;

    data &outDset;
        set &origDset (keep = &varsToKeep);
    run;
%mend removeEmptyCols;

I will try all 3 ways and report back on which one is fastest...

P.S. added 23 Dec 2010 for future reference: SGF Paper 048-2010: Dropping Automatically Variables with Only Missing Values

Answer 2

This is very simple method useful for all variables

proc freq data=class nlevels ;
ods output nlevels=levels(where=(nmisslevels>0 and nnonmisslevels=0));
run;

proc sql noprint;
select TABLEVAR into :_MISSINGVARS separated by ' ' from levels;
quit;
data want;
set class (keep=&_MISSINGVARS);
run;

Answer 3

I created a macro that will check for empty character columns and either remove them from the original or create a new data set with the empty columns removed. It takes two optional arguments: The name of the data set (default is the most recently created data set), and a suffix to name the new copy (set suffix to nothing to edit the original).

It uses proc freq with the levels option and a custom format to determine the empty character columns. proc sql is then used to create a list of the columns to be removed and store them in a macro variable.

Here is the macro:

%macro delemptycol(ds=_last_, suffix=_noempty);

option nonotes;
proc format;
  value $charmiss
    ' '= ' '
    other='1';
run;
%if "&ds"="_last_" %then %let ds=&syslast.;

ods select nlevels;
ods output nlevels=nlev;
proc freq data=&ds.(keep=_character_) levels ;
  format _character_ $charmiss.;
run;
ods output close;

/* create macro var with list of cols to remove */
%local emptycols;
proc sql noprint;
  select tablevar into: emptycols separated by ' '
  from nlev
  where NNonMissLevels=0;
quit;

%if &emptycols.=  %then %do;
  %put DELEMPTYCOL: No empty character columns were found in data set &ds.;
  %end;
%else %do;
  %put DELEMPTYCOL: The following empty character columns were found in data set &ds. : &emptycols.;
  %put DELEMPTYCOL: Data set &ds.&suffix created with empty columns removed;
  data &ds.&suffix. ;
    set &ds(drop=&emptycols);
  run;
%end;
options notes;

%mend;

Examples usage:

/* create some fake data: Here char5 will be empty */
data chardata(drop= j randnum);
length char1-char5 $8.;
array chars(5) char1-char5;
  do i=1 to 100;
    call missing(of char:);
    randnum=floor(10*ranuni(i));
    do j=2 to 5;
      if (j-1)<randnum<=(j+1) then chars(j-1)="FOO";
    end;
    output;
  end;
run;

%delemptycol();  /* uses default _last_ for the data and "_noempty" as the suffix */
%delemptycol(ds=chardata, suffix=); /* removes the empty columns from the original */

Answer 4

There's probably a simpler way but this is what I came up with.

Cheers Rob

EDIT: Note that this works for both character and numeric variables.

**
** TEST DATASET
*;
data x;
  col1 = "a"; col2 = ""; col3 = "c"; output;
  col1 = "" ; col2 = ""; col3 = "c"; output;
  col1 = "a"; col2 = ""; col3 = "" ; output;
run;

**
** GET A LIST OF VARIABLE NAMES
*;
proc sql noprint;
  select name into :varlist separated by " " 
  from sashelp.vcolumn
  where upcase(libname) eq "WORK" 
    and upcase(memname) eq "X";
quit;

%put &varlist;


**
** USE A MACRO TO CREATE A DATASTEP.  FOR EACH COLUMN THE 
** THE DATASTEP WILL CREATE A NEW COLUMN WITH THE SAME NAME
** BUT PREFIXED WITH "DELETE_".  IF THERE IS AT LEAST 1 
** NON-MISSING VALUE FOR THE COLUMN THEN THE "DELETE" COLUMN
** WILL FINISH WITH A VALUE OF 0, ELSE 1.  WE WILL ONLY
** KEEP THE COLUMNS CALLED "DELETE_" AND OUTPUT ONLY A SINGLE
** OBSERVATION TO THE FINAL DATASET.
*;
%macro find_unused_cols(iDs=);
  %local cnt;

    data vars_to_delete;
      set &iDs end=eof;

      %let cnt = 1;
      %let varname = %scan(&varlist, &cnt);
      %do %while ("&varname" ne "");
        retain delete_&varname;
        delete_&varname = min(delete_&varname, missing(&varname));
        drop &varname;
        %let cnt = %eval(&cnt + 1);
        %let varname = %scan(&varlist, &cnt);
      %end;

      if eof then do;
        output;
      end;

    run;

%mend;
%find_unused_cols(iDs=x);

**
** GET A LIST OF VARIABLE NAMES FROM THE NEW DATASET
** THAT WE WANT TO DELETE AND STORE TO A MACRO VAR.
*;
proc transpose data=vars_to_delete out=vars_to_delete;
run;

proc sql noprint;
  select substr(_name_,8) into :vars_to_delete separated by " " 
  from vars_to_delete
  where col1;
quit;

%put &vars_to_delete;


**
** CREATE A NEW DATASET CONTAINING JUST THOSE VARS
** THAT WE WANT TO KEEP
*;
data new_x;
  set x;
  drop &vars_to_delete;
run;