Fix and test xyzmatrix<- for 0 row dataframes

* closes #433
* @alexanderbates this came up because some of your neuros have 0 row connector data.frames

R/xform.R

@@ -327,6 +327,16 @@ xyzmatrix.mesh3d<-function(x, ...){
 
 #' @export
 `xyzmatrix<-.default`<-function(x, value){
+  # count number of elements in matrices/data.frames and vectors
+  nelems <- function(y) {
+    dy=dim(y)
+    if(is.null(dy)) length(y) else prod(dy)
+  }
+
+  # short circuit if x and value have no elements
+  if(isTRUE(nrow(x)==0 && nelems(value)==0))
+    return(x)
+
   xyzn=c("X","Y","Z")
   if(ncol(x)==3) {
     x[,]=value

tests/testthat/test-xform.R

@@ -185,6 +185,18 @@ test_that("can replace xyz coords of a matrix",{
   expect_equal(mx3, mx3.saved)
 })
 
+test_that("can replace xyz coords of a data.frame",{
+  # we just need to handle some edge cases with 0 row data here
+
+  df <- df2 <- data.frame(x=numeric(), y=numeric(), z=numeric())
+  expect_silent(xyzmatrix(df) <- xyzmatrix(df)+1)
+  # check no change
+  expect_equal(df, df2)
+  # nb generates both error and warning
+  expect_warning(expect_error(xyzmatrix(df) <- 1:3))
+})
+
+
 test_that("can extract xyz coords from a neuronlist",{
   xyz12=rbind(xyzmatrix(kcs20[[1]]),xyzmatrix(kcs20[[2]]))
   expect_is(xyzmatrix(kcs20[1:2]),'matrix')