Reading shapefiles using for loop: Error in readOGR() : length(dsn) == 1L is not TRUE

Question

I'm using a for loop to read in multiple shapefiles within a directory and manipulate them individually. Presently I cannot do this because the for loop results in the error: "Error in readOGR(shp_paste) : length(dsn) == 1L is not TRUE". Not a single shapefile is read in; this error occurs immediately. I do not know what this means, and can only find a similar post with this error for writeOGR (link here). When I try to load the shapefiles individually into R Studio, I'm successful in doing so (though I haven't tried individually for all of them, there are too many). What could be causing this error? It's difficult to go through the files in the directory individually to identify potential problems because there are over 10,000. Below is my code:

    library(rgdal)
    input_path<- "/Volumes/Directory/data_folder/"
    files<- list.files(input_path, pattern = "[.]shp$")

    for(f in files){
    ifile<- list.files(input_path, f)
    shp_paste<- paste(input_path, ifile, sep = "")
    shp1<- readOGR(shape_paste)
    print("success!")
    }

Running this code produces the error above.

Answer 1

Seems like you are passing a list to the read function instead of a single file

When you call

shp_paste<- paste(input_path, ifile, sep = "")

the ifile object can be a list of files, so shp_paste has a length greater than 1.

You might change this by changing the code to

shp_paste<- paste(input_path, f, sep = "")

since you are iterating over files. Or you could just list the full path in the first place:

files<- list.files(input_path, pattern = "[.]shp$", full.names=T)

So this code should work:

    library(rgdal)
    input_path <- "/Volumes/Directory/data_folder/"
    files <- list.files(input_path, pattern = "[.]shp$", full.names=T)

    for (f in files){
    shp1 <- readOGR(f)
    print("success!")
    }

Answer 2

Use full.names=TRUE to get the full path to the shapefiles, and then use lapply to loop over the shapefiles and read them all into a list:

> files<- list.files(input_path, pattern = "[.]shp$",full.names=TRUE)
> allShapes = lapply(files, readOGR)

> plot(allShapes[[1]]) # to plot the first one.

or loop over them:

> for(shp in files){
    sp = readOGR(shp)
    message("REad ",nrow(sp))
  }