Monday, January 3, 2011

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
-----------------------------------------

5 comments:

wojteksobala said...

For variables stored in adjacent columns you can use

ds[,names(ds)[grep("var_x",names(ds)):grep("var_y",names(ds))]

Anonymous said...

wojteksobala:

For the adjacent columns it's simpler:
mean(subset(ds, select=cesd1:cesd4), na.rm=TRUE)
but the problem doesn't concern it.

Ken Kleinman said...

I was thinking for the adjacent columns you could just use the column numbers, e.g., ds[, 4:8], but I prefer wojteksobala's approach, which doesn't require you to learn the numeric location of the columns. We'll be using grep() inside the index in next week's entry.

ucfagls said...

You also don't need `seq(1:4)`, `1:4` is sufficient, or `seq(1, 4)`, but not both.

Ken Kleinman said...

Thanks!