Insert blanks into a vector for, e.g., minor tick labels in R

Question

This question relates generally to a previous SO question concerning the creation of minor tick marks on a ggplot2 axis and specifically to a comment in the answer to that question suggesting a function to insert blanks into a sequence may prove useful.

As I frequently add minor tick marks to similar plots, I took a stab at creating such a function (see my answer below).

Answer 1

The following function allows the user to request that every n^th element nth of a vector x either be (1) replaced with a empty character placeholder (empty = TRUE; default) or (2) omitted from the vector (empty = FALSE). Additionally, it provides the option of requesting the inverse (inverse = TRUE; not default) of the operation. The functionality is illustrated with some examples below.

First, the function:

every_nth <- function(x, nth, empty = TRUE, inverse = FALSE) 
  {
  if (!inverse) {
    if(empty) {
      x[1:nth == 1] <- ""
      x
      } else {
        x[1:nth != 1]
        }
    } else {
      if(empty) {
        x[1:nth != 1] <- ""
        x
        } else {
          x[1:nth == 1]
        }
    }
}

Some examples of replacing or omitting vector elements:

numvec <- 0:20
charvec <- LETTERS

## Replace every 3rd element with an empty character
every_nth(numvec, 3) # conversion to character vector

[1] ""   "1"  "2"  ""   "4"  "5"  ""   "7"  "8"  ""   "10" "11" ""   "13"
[15] "14" ""   "16" "17" ""   "19" "20"

every_nth(charvec, 3)
[1] ""  "B" "C" ""  "E" "F" ""  "H" "I" ""  "K" "L" ""  "N" "O" ""  "Q"
[18] "R" ""  "T" "U" ""  "W" "X" ""  "Z"

## Omit (drop) every 3rd element
every_nth(numvec, 3, empty = FALSE) # vector mode is preserved
[1]  1  2  4  5  7  8 10 11 13 14 16 17 19 20

every_nth(charvec, 3, empty = FALSE)
[1] "B" "C" "E" "F" "H" "I" "K" "L" "N" "O" "Q" "R" "T" "U" "W" "X" "Z"

However, for the creation of minor ticks, it is preferred to return the inverse of this operation using the inverse = TRUE option:

## Retain every 3rd element, replacing all others with an empty character
every_nth(numvec, 3, inverse = TRUE) # conversion to character vector
[1] "0"  ""   ""   "3"  ""   ""   "6"  ""   ""   "9"  ""   ""   "12" ""  
[15] ""   "15" ""   ""   "18" ""   ""

every_nth(charvec, 3, inverse = TRUE)
[1] "A" ""  ""  "D" ""  ""  "G" ""  ""  "J" ""  ""  "M" ""  ""  "P" "" 
[18] ""  "S" ""  ""  "V" ""  ""  "Y" ""

## Retain every 3rd element, omitting (dropping) all other elements
every_nth(numvec, 3, empty = FALSE, inverse = TRUE) # vector mode is preserved
[1]  0  3  6  9 12 15 18

every_nth(charvec, 3, empty = FALSE, inverse = TRUE)
[1] "A" "D" "G" "J" "M" "P" "S" "V" "Y"

To illustrate the function's use in the creation of minor ticks:

library(ggplot2)
df <- data.frame(x = rnorm(1000), y = rnorm(1000))

## ggplot2 default axis labelling
p <- ggplot(df, aes(x, y)) + geom_point() + theme_bw()
p

## Add minor ticks to axes
custom_breaks <- seq(-3, 3, 0.25)
p + 
  scale_x_continuous(breaks = custom_breaks,
                     labels = every_nth(custom_breaks, 4, inverse = TRUE)) + 
  scale_y_continuous(breaks = custom_breaks,
                     labels = every_nth(custom_breaks, 2, inverse = TRUE))