manimal - 6 months ago 28

R Question

This question relates generally to a previous SO question concerning the creation of minor tick marks on a

`ggplot2`

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

Answer

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))
```