Error in unclass(x) + conform(Vec, x) : non-numeric argument to binary operator

[mt1022]

Describe the bug
Add operation failed when the order of items changed.

To Reproduce

library(sf)
library(spData)
nz_sfc <- st_geometry(nz)
nz_sfc_cent <- st_centroid(nz_sfc)

# works
(nz_sfc - nz_sfc_cent) * 0.5 + nz_sfc_cent

# same (only order changed), but does not work
nz_sfc_cent + (nz_sfc - nz_sfc_cent) * 0.5

output:

> nz_sfc <- st_geometry(nz)
> nz_sfc_cent <- st_centroid(nz_sfc)
> (nz_sfc - nz_sfc_cent) * 0.5 + nz_sfc_cent
Geometry set for 16 features 
Geometry type: MULTIPOLYGON
Dimension:     XY
Bounding box:  xmin: 1147370 ymin: 4836672 xmax: 2060191 ymax: 6130942
CRS:           NA
First 5 geometries:
MULTIPOLYGON (((1710099 6035907, 1707622 603253...
MULTIPOLYGON (((1778687 5917378, 1772497 591766...
MULTIPOLYGON (((1839928 5823696, 1838660 582082...
MULTIPOLYGON (((1992348 5801244, 1993162 579806...
MULTIPOLYGON (((2027669 5713317, 2024943 571452...
> nz_sfc_cent + (nz_sfc - nz_sfc_cent) * 0.5
Error in unclass(x) + conform(Vec, x) : 
  non-numeric argument to binary operator

If reporting a change from previous versions

Please read https://cran.r-project.org/web/packages/sf/news/news.html first.

Additional context
The example is from the online book here: https://r.geocompx.org/geometry-operations

Paste the output of your `sessionInfo()` and `sf::sf_extSoftVersion()`

> sessionInfo()
R version 4.3.2 (2023-10-31)
Platform: aarch64-apple-darwin20 (64-bit)
Running under: macOS Sonoma 14.2.1

Matrix products: default
BLAS:   /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libBLAS.dylib 
LAPACK: /Library/Frameworks/R.framework/Versions/4.3-arm64/Resources/lib/libRlapack.dylib;  LAPACK version 3.11.0

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

time zone: Asia/Shanghai
tzcode source: internal

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] spDataLarge_2.1.1 spData_2.3.0      dplyr_1.1.2       terra_1.7-55      sf_1.0-14        

loaded via a namespace (and not attached):
 [1] compiler_4.3.2     tidyselect_1.2.0   Rcpp_1.0.11        scales_1.2.1       lattice_0.21-9    
 [6] R6_2.5.1           lobstr_1.1.2       sfheaders_0.4.3    generics_0.1.3     classInt_0.4-9    
[11] s2_1.1.4           knitr_1.45         tibble_3.2.1       units_0.8-3        munsell_0.5.0     
[16] DBI_1.1.3          pillar_1.9.0       rlang_1.1.2        utf8_1.2.4         sp_2.0-0          
[21] xfun_0.41          pkgload_1.3.2.1    cli_3.6.1          magrittr_2.0.3     wk_0.8.0          
[26] class_7.3-22       grid_4.3.2         rstudioapi_0.15.0  lifecycle_1.0.4    prettyunits_1.2.0 
[31] vctrs_0.6.4        KernSmooth_2.23-22 proxy_0.4-27       glue_1.6.2         codetools_0.2-19  
[36] fansi_1.0.5        e1071_1.7-13       colorspace_2.1-0   tools_4.3.2        pkgconfig_2.0.3   
> sf::sf_extSoftVersion()
          GEOS           GDAL         proj.4 GDAL_with_GEOS     USE_PROJ_H           PROJ 
      "3.11.0"        "3.5.3"        "9.1.0"         "true"         "true"        "9.1.0"

[edzer]

This is intentional. x + a is meant as shift everything of x with vector a. For that, a must be a vector, which could be encoded as a POINT geometry. How would you shift everything in x when a is a POLYGON? If you want to help, please provide suggestions how to improve the documentation (of ?Ops.sfc).

[mt1022]

Thanks for the explanation! That sounds reasonable. I am a newbie of spatial data analysis and failed to find the document of these operators. Regarding the documentation, I am not sure whether a less technical title like "Arithmetic operators for simple feature geometries" rather than "S3 Ops Group Generic Functions for simple feature geometries" would make is easier for us to understand here: https://r-spatial.github.io/sf/reference/Ops.html