[R-sig-Geo] Help with as.numeric(rownames(over(SpatialPoints(set1, set2, returnList= TRUE)[[1]]))

[Alan Briggs]

Hello.

Below is a fully repeatable R-Script that I'm having trouble with.
Generally, here's what I'm trying to do:

1) Randomly generate a tile.list()
2) Randomly generate a new point
3) Identify which polygon in the tile.list the new randomly generated point
is in

This works fine MOST of the time. However, occasionally, I get an error
returned:

Error in `[<-.data.frame`(`*tmp*`, list, 3, value = numeric(0)) :
>   replacement has length zero


While troubleshooting, I realized I get numeric(0) returned for certain
sets of new random points when I run the command
as.numeric(rownames(over(SpatialPoints(walker[i,1:2]),vpl,returnList=
TRUE)[[1]])). I thought maybe this was a boundary issue, but the points
don't lie on the edge, nor are they the centroid.

Any help you can provide would be greatly appreciated!

Regards,

Alan

R-Script Below:

### Help Question for  r-sig-geo ###
> rm(list = ls())
> library(deldir)
> library(sp)
> library(plyr)
> side_length = 100
> ## Create random SET of XY coordinates (size = 100x100)
> set.seed(11)
> df = data.frame(matrix(sample(1:100,16,replace=TRUE),nrow=8))
> ## Convert df to SPatialPointsDataFrame
> spdf <- SpatialPointsDataFrame(df,df)
> ## deldir() function creates tesselation (voronoi) plot
> z <- deldir(df,plotit=TRUE,wl='te')
> ## tile.list() creates a list of data for tiles
> zz <- tile.list(deldir(df,plotit=TRUE,wl='te'))
> ## Voronoi Polygons Function
> voronoipolygons = function(layer) {
>   require(deldir)
>   crds = layer at coords
>   z = deldir(crds[,1], crds[,2])
>   w = tile.list(z)
>   polys = vector(mode='list', length=length(w))
>   require(sp)
>   for (i in seq(along=polys)) {
>     pcrds = cbind(w[[i]]$x, w[[i]]$y)
>     pcrds = rbind(pcrds, pcrds[1,])
>     polys[[i]] = Polygons(list(Polygon(pcrds)), ID=as.character(i))
>   }
>   SP = SpatialPolygons(polys)
>   voronoi = SpatialPolygonsDataFrame(SP, data=data.frame(x=crds[,1],
>             y=crds[,2], row.names=sapply(slot(SP, 'polygons'),
>             function(x) slot(x, 'ID'))))
> }
> ## Generate SpatialPolygonsDataFrame to use as input for over() function
> vpl <- voronoipolygons(spdf)
> ## Random Walk Function generates North, South East or West movement
> ## with transit from across screen (like PacMan, going out one side,
> ## coming back on the other side) to prevent getting stuck in corner
> random_walk <- function(step_quantity, step_length, plot = FALSE){
>   require(ggplot2)
>
>   walker <- data.frame(matrix(c(0,0), nrow = step_quantity, ncol = 3,
> byrow = T))
>   names(walker)[1]<-paste("x")
>   names(walker)[2]<-paste("y")
>   names(walker)[3]<-paste("which")
>
>   ## Seed random initial starting point
>   walker[1,1:2] <- matrix(sample(1:100,2,replace=TRUE),nrow=1)
>   walker[1,3] <-
> as.numeric(rownames(over(SpatialPoints(walker[1,1:2]),vpl,returnList=
> TRUE)[[1]]))
>
>   where_to <- as.numeric()
>
>   for(i in 2:step_quantity){
>     where_to <- as.numeric()
>     where_to <- walker[i-1,1:2]
>     which_next <- sample(c("bb","dd","ff","hh"),1)
>
>     if (which_next == "bb") {
>       if (walker[i-1,2] == side_length) {where_to[1,2] <- 0
>       } else {where_to[1,2] <- walker[i-1,2]+step_length}
>     }
>
>     else if (which_next == "dd") {
>       if (walker[i-1,1] == 0 ) {where_to[1,1] <- side_length
>       } else {where_to[1,1] <- walker[i-1,1]-step_length}
>     }
>
>     else if (which_next == "ff") {
>       if (walker[i-1,1] == side_length) {where_to[1,1] <- 0
>       } else {where_to[1,1] <- walker[i-1,1]+step_length}
>     }
>     else {
>       if (walker[i-1,2] == 0) {where_to[1,2] <- side_length
>       } else {where_to[1,2] <- walker[i-1,2]-step_length}
>     }
>
>     walker[i,1:2] <- where_to
>   }
>
>   walker[i,3] <- as.numeric(rownames(over(SpatialPoints(walker[i,1:2]),
>                                           vpl,returnList= TRUE)[[1]]))
>
>
>   if(plot){
>   require(ggplot2)
>   p <- ggplot(walker, aes(x = x, y = y))
>   p <- p + geom_path()
>   print(p)
>   }
>
>   return(walker)
> }
> try(transits <- random_walk(5000,1),silent=F)

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