r - access data frame column using variable

Question

Consider the following code

a = "col1"
b = "col2"
d = data.frame(a=c(1,2,3),b=c(4,5,6))

This code produces the following data frame

  a b
1 1 4
2 2 5
3 3 6

However the desired data frame is

  col1 col2
1 1    4
2 2    5
3 3    6

Further, I'd like to be able to do something like d$a which would then grab d$col1 since a = "col1"

How can I tell R that "a" is a variable and not a name of a column?

Answer 1

After creating your data frame, you need to use ?colnames. For example, you would have:

d = data.frame(a=c(1,2,3), b=c(4,5,6))
colnames(d) <- c("col1", "col2")

You can also name your variables when you create the data frame. For example:

d = data.frame(col1=c(1,2,3), col2=c(4,5,6))

Further, if you have the names of columns stored in variables, as in

a <- "col1"

you can't use $ to select a column via d$a. R will look for a column whose name is a. Instead, you can do either d[[a]] or d[,a].