Example 8.19: Referencing lists of variables

In section 1.11.4 (p. 50), we discuss referring to lists of variables in a data set. In SAS, this can be done for variable stored in adjacent columns with the "var_x -- var_y" syntax and for variables with sequentially enumerated suffixes with the "var_n1 - var_n2" syntax. We state in the above referenced section that R has no straightforward equivalent ability to reference a list of variables by name, though to reference by location is trvial. Wayne Richter (of the NY State Department of Environmental Conservation) pointed out a reference from Muenchen's excellent text that makes this task relatively straightforward to undertake in R for variables with sequential numerical suffixes.

R

Here we demonstrate this by displaying the means of the cesd1, cesd2, cesd3, and cesd4 variables measuring depressive symptoms at each of the followup time points for the HELP study.

ds = read.csv("http://www.math.smith.edu/r/data/help.csv")
mean(ds[, paste('cesd', seq(1:4), sep = '')], na.rm=TRUE)

which generates the output:

   cesd1    cesd2    cesd3    cesd4 
22.71545 23.58373 22.06855 20.14286 

This approach selects a set of variables by generating a character vector of variable names using the paste() function (section 1.4.5) and the seq() function (section 1.11.3). Then the mean() function is applied to the selected columns.

SAS

This task is straightforward in SAS, using the - syntax (section 1.11.4) in the var statement in proc means.

proc means data=ds maxdec=2 n mean;
   var cesd1 - cesd4;
run;

The MEANS Procedure

Variable    Label       N            Mean
-----------------------------------------
CESD1       1 cesd    246           22.72
CESD2       2 cesd    209           23.58
CESD3       3 cesd    248           22.07
CESD4       4 cesd    266           20.14
-----------------------------------------