Github user shivaram commented on a diff in the pull request: https://github.com/apache/spark/pull/8276#discussion_r37465795 --- Diff: R/pkg/R/DataFrame.R --- @@ -628,18 +628,49 @@ setMethod("dim", setMethod("collect", signature(x = "DataFrame"), function(x, stringsAsFactors = FALSE) { - # listCols is a list of raw vectors, one per column - listCols <- callJStatic("org.apache.spark.sql.api.r.SQLUtils", "dfToCols", x@sdf) - cols <- lapply(listCols, function(col) { - objRaw <- rawConnection(col) - numRows <- readInt(objRaw) - col <- readCol(objRaw, numRows) - close(objRaw) - col - }) - names(cols) <- columns(x) - do.call(cbind.data.frame, list(cols, stringsAsFactors = stringsAsFactors)) - }) + names <- columns(x) + ncol <- length(names) + if (ncol <= 0) { + # empty data.frame with 0 columns and 0 rows + data.frame() + } else { + # listCols is a list of columns + listCols <- callJStatic("org.apache.spark.sql.api.r.SQLUtils", "dfToCols", x@sdf) + stopifnot(length(listCols) == ncol) + + # An empty data.frame with 0 columns and number of rows as collected + nrow <- length(listCols[[1]]) + if (nrow <= 0) { + df <- data.frame() + } else { + df <- data.frame(row.names = c(1 : nrow)) + } + + # Append columns one by one + for (colIndex in 1 : ncol) { + # Note: appending a column of list type into a data.frame so that + # data of complex type can be held. But getting a cell from a column + # of list type returns a list instead of a vector. So for columns of + # non-complex type, append them as vector. + col <- listCols[[colIndex]] + if (length(col) <= 0) { + df[[names[colIndex]]] <- col --- End diff -- this is just to handle empty columns ? Can't we just return from line 644 if one of the columns is empty ? Or do we want to handle cases where some columns are empty and some are not etc.
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