Question

Given a table of the form:

  a b c
X1 0 1 0
X2 1 0 0
X3 1 0 0

In order to pull a column from the table:

col.1 <- table$a

Assume you have a variable:

col.name <- 'a'

col.1 <- table$col.name

Why doesn’t this work? Is there a way to make this work?

Answer 1

It doesn’t work because the “$” operator does not evaluate its argument. What you need is to use “[” or “[[“. (It’s not a good idea to call your table. “table”. It confuses users who use the table function.)

table <- structure(c(0, 1, 1, 1, 0, 0, 0, 0, 0), .Dim = c(3L, 3L), .Dimnames = list(
    c("X1", "X2", "X3"), c("a", "b", "c")), class = "table")
table[ , col.name]
# X1 X2 X3 
#  0  1  1
table[ , "a"]
# X1 X2 X3 
#  0  1  1  
table
table[["a"]]
Error in table[["a"]] : subscript out of bounds