r - Selecting multiple odd or even columns/rows for dataframe

Question

Is there a way in R to select many non-consecutive i.e. odd or even rows/columns?

I'm plotting the loadings for my Principal Components Analysis. I have 84 rows of data ordered like this: x_1 y_1 x_2..... x_42 y_42

And at the moment I am creating the dataframes for the x and y loadings figures like this:

data.pc = princomp(as.matrix(data))

x.loadings <- data.frame(x=data.pc$loadings[c(1, 3, 5, 7, 9, 11, 13 ,15, 17, 19, 
21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41), 1])

yloadings <- data.frame(y=data.pc$loadings[c(2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 
22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42), 1])

Surely there's an easier way to do this?

Answer 1

When logical vectors are used for indexing, they are recycled so this gets you odd columns or odd rows

 calld[ c(TRUE,FALSE), ]  # rows
 calld[ , c(TRUE,FALSE) ] #columns

Even rows or columns:

 calld[ !c(TRUE,FALSE), ]  # rows
 calld[ , !c(TRUE,FALSE) ] #columns

Every third column:

  calld[ , c(TRUE,FALSE, FALSE) ]   #columns 1,4,7 , ....

A recent commenter claims this no longer works. I'm not finding that in R 4.0.4 running in Ubuntu:

> d <- data.frame(as.list(1:10))  # simple example construction
> d
  X1L X2L X3L X4L X5L X6L X7L X8L X9L X10L
1   1   2   3   4   5   6   7   8   9   10
> d[, c(TRUE,FALSE)]
  X1L X3L X5L X7L X9L
1   1   3   5   7   9
> d[, c(TRUE,FALSE,FALSE)]  # example: # of columns not exact multiple of length of logical vector
  X1L X4L X7L X10L
1   1   4   7   10