Return a list containing the index of multiple column names [duplicate]

Question

Suppose I have the following data frame:

df <- data.frame(A = c(1, 2, 3), B = c("a", "b", "c"), C = c(4, 5, 6))

  A B C
1 1 a 4
2 2 b 5
3 3 c 6

If I wanted to know the position of a column e.g. column B, then I can use:

which(names(df)=="B")

Or

grep("B", names(df))

In both cases, I get 2, but what if I wanted to know the positions of columns A and C at the same time? That is, I want to enter a vector of column names, and get a vector of their positions. So, if I entered "A", "C", the result should be 1 3.

The two above examples I've used don't seem to work when entering a vector of column names instead of a single one.

I know I can do this with loops, but is there a method that achieves better performance?

Answer 1

No *apply/loop needed. You need match. See the doc at ?match. For instance:

match(c("A","C"),names(df))
#[1] 1 3

Other *apply/loops solutions are way worse performance-wise.

Answer 2

consider sapply() as internally a for loop; which iterates through the list of columnnames and then applies grep/which

sapply(vector.of.columns, function(x) which(names(df) == x))