[R] Subscript Error

[z2.0]

json_dir is a list of JSON lists mapping lat/long route points between
locations using CloudMade's API.
post_url is the URL of the HTTP request

    for (n in json_dir) {
        i = i + 1
            if (typeof(json_dir[[i]]) != "NULL") {
                if (i == 1) {
                    dat_add <- ldply(json_dir[[i]], function(x)
t(data.frame(x)), .progress = "text")
                    names(dat_add) <- c("lat", "lon")
                    json_path <- list(dat_add)
                } else {
                    dat_add <- ldply(json_dir[[i]], function(x)
t(data.frame(x)), .progress = "text")
                    names(dat_add) <- c("lat", "lon")
                    json_path <- c(json_path, list(dat_add))
                }

                p = p + geom_path(aes(lon, lat), data = json_path[[i]])
        }
        print(paste("Processed ", i, " of ", as.character(length(json_dir)),
" in route set.", sep = ""))
    }

This runs until i = 101 and then errors out with, 
"Error in json_path[[i]] : subscript out of bounds"

typeof(json_dir[[101]]) = "list", so it's not that the first if-block is
somehow resetting json_path in an errant fashion.

Do lists have a default, built-in limit on no. of elements? Each element I'm
passing contains hundreds or thousands of lat/long pairs, so it's also
possible I'm hitting some upper bound on per-object memory, if that exists,
but Googling around leads me to think that's not the case.

I think I've fucked something up in my logic, but I'm not sure what.


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