r sf package centroid within polygon - r

Question

I need to add labels to polygons and I normally use the centroid, however the centroid does not fall inside the polygon. I found this question Calculate Centroid WITHIN / INSIDE a SpatialPolygon but I'm using the sf package.
Below is a toy data
rm(list = ls(all = TRUE)) #start with empty workspace
library(sf)
library(tidyverse)
library(ggrepel)
pol <- st_polygon(list(rbind(c(144, 655),c(115, 666)
,c(97, 660),c(86, 640)
,c(83, 610),c(97, 583)
,c(154, 578),c(140, 560)
,c(72, 566),c(59, 600)
,c(65, 634),c(86, 678)
,c(145, 678),c(144, 655)))) %>%
st_sfc()
a = data.frame(NAME = "A")
st_geometry(a) = pol
a <- a %>%
mutate(lon = map_dbl(geometry, ~st_centroid(.x)[[1]]),
lat = map_dbl(geometry, ~st_centroid(.x)[[2]]))
ggplot() +
geom_sf(data = a, fill = "orange") +
geom_label_repel(data = a, aes(x = lon, y = lat, label = NAME))
which results in the following

Answer 1

The simple answer is to replace st_centroid with st_point_on_surface. This won't return the true centroid in cases where the centroid is inside the polygon.
a2 <- a %>%
mutate(lon = map_dbl(geometry, ~st_point_on_surface(.x)[[1]]),
lat = map_dbl(geometry, ~st_point_on_surface(.x)[[2]]))
ggplot() +
ggplot2::geom_sf(data = a2, fill = "orange") +
geom_label_repel(data = a2, aes(x = lon, y = lat, label = NAME))
Alternatively
If the polygon has a centroid that is inside the polygon, use that, otherwise, find a point within the polygon.
st_centroid_within_poly <- function (poly) {
# check if centroid is in polygon
centroid <- poly %>% st_centroid()
in_poly <- st_within(centroid, poly, sparse = F)[[1]]
# if it is, return that centroid
if (in_poly) return(centroid)
# if not, calculate a point on the surface and return that
centroid_in_poly <- st_point_on_surface(poly)
return(centroid_in_poly)
}
a3 <- a %>%
mutate(lon = map_dbl(geometry, ~st_centroid_within_poly(.x)[[1]]),
lat = map_dbl(geometry, ~st_centroid_within_poly(.x)[[2]]))
ggplot() +
ggplot2::geom_sf(data = a3, fill = "orange") +
geom_label_repel(data = a3, aes(x = lon, y = lat, label = NAME))
The function above st_centroid_within_polygon is adapted from the question you reference for the sf package. A more thorough review of how st_point_on_surface works can be found here.

Answer 2

Expanding on Mitch's answer because the st_centroid_within_poly function provided above only works on single polygons.
To use on multiple polygons, use:
st_centroid_within_poly <- function (poly) {
# check if centroid is in polygon
ctrd <- st_centroid(poly, of_largest_polygon = TRUE)
in_poly <- diag(st_within(ctrd, poly, sparse = F))
# replace geometries that are not within polygon with st_point_on_surface()
st_geometry(ctrd[!in_poly,]) <- st_geometry(st_point_on_surface(poly[!in_poly,]))
ctrd
}