invalid_to_na: Set invalid pupil measurements to missing
In WillemSleegers/eyepatch:

invalid_to_na sets invalid pupil measurements to missing (NA).

1	invalid_to_na(pupil, condition)

`pupil`	A numeric vector containing pupil data.
`condition`	A condition to determine which values should be set to NA.

# Example 1: Artificial vector
# Set the value '4' to missing in a vector from '1' to '10':
invalid_to_na(1:10, 1:10 == 4)

# Example 2: Artificial data
# Load the "dplyr" package:
library(dplyr)

# Create some artificial data:
data <- tibble(
  pupil_left = c(3.15, 3.14, 3.13, -1, -1, 3.11),
  pupil_right = c(2.92, 2.89, 2.93, -1, -1, 2.97),
  validity_left = c(0, 0, 0, 4, 4, 0),
  validity_right = c(0, 0, 0, 4, 4, 0)
)
data

# Set all pupil sizes with a value of -1 to NA:
mutate(data,
  pupil_left = invalid_to_na(pupil_left, pupil_left == -1),
  pupil_right = invalid_to_na(pupil_right, pupil_right == -1)
)

# Or, set pupil sizes to NA based on values in the validity columns:
mutate(data,
  pupil_left = invalid_to_na(pupil_left, validity_left == 4),
  pupil_right = invalid_to_na(pupil_right, validity_right == 4)
)

# Example 3: Realistic data
# Inspect data:
trial1
count(trial1, validity_left, validity_right)
slice(trial1, 100:115)

# Set invalid pupil observations of the left eye to NA:
trial1 <- mutate(trial1, pupil_left = invalid_to_na(pupil_left,
  validity_left == 4))

# Inspect the data frame again to see whether it worked:
slice(trial1, 100:115)