Nearest-neighbour distances of a point pattern by year

Question

I wish to calculate the nearest-neighbour distances (NNDs) of a point pattern by year with spatstat but get an error, although I cannot see any difference with the example provided in the documentation . What's the right way to do it?

library(spatstat)

#Build point pattern
pp <- structure(list(window = structure(list(type = "rectangle", xrange = c(952727.038715708, 
969663.326063713), yrange = c(1928725.27732809, 1943334.46685032
), units = structure(list(singular = "unit", plural = "units", 
    multiplier = 1), class = "unitname")), class = "owin"), n = 6L, 
    x = c(959238.044802669, 968582.344942023, 960218.207335433, 
    969663.326063713, 964665.091671559, 952727.038715708), y = c(1941763.77812852, 
    1934201.29833662, 1943334.46685032, 1939411.6354699, 1928725.27732809, 
    1931868.51667007), markformat = "vector", marks = c("1994", 
    "1994", "2005", "2005", "2005", "2005")), class = "ppp")

#Calculate NNDs
nndist(pp, by=marks(pp))

#Error in split.ppp(X %mark% idX, f = by, un = FALSE) : 
#  f must be a factor, a logical vector, a tessellation, a window, an image, or the name of a column of marks

Answer 1

The error message gives you a hint:

> nndist(pp, by=marks(pp))
Error in split.ppp(X %mark% idX, f = by, un = FALSE) :
  f must be a factor, a logical vector, a tessellation, a window, an image, or the name of a column of marks

It indicates that marks should be a factor, logical vector, or a tessellation.

Upon inspection, it turns out, that pp$marks is a character:

> str(pp$marks)
 chr [1:6] "1994" "1994" "2005" "2005" "2005" "2005"

Transforming it into a factor fixes the issue:

pp$marks <- as.factor(pp$marks)
nndist(pp, by=marks(pp))
          1994      2005
[1,] 12021.108  1851.427
[2,] 12021.108  5321.291
[3,]  1851.427 10227.359
[4,]  5321.291 10227.359
[5,]  6732.880 11797.483
[6,] 11845.227 12344.920