as.data.frame but keep row names?

[dereckmezquita]

May I propose a simple way to convert a data.table to a data.frame but keep and move row names? I often have a column named "sample" or some identifier for my data.

I have myself often using data.tables but having to convert them to data.frames or matrices in order to interact and work with other base R functions/code.

I often do this:

dt # a data.table with "rownames" in the sample column
df <- (function(df) {
    df <- as.data.frame(df)
    rownames(df) <- df$sample
    df$sample <- NULL
    return(df)
})(dt)

I took some time to study the source code for data.table and found I can modify the method to work as such:

as.data.frame.data.table = function(x, row.names = NULL, ...) {
    ans = copy(x)
    setattr(ans,"row.names",.set_row_names(nrow(x)))   # since R 2.4.0, data.frames can have non-character row names
    setattr(ans,"class","data.frame")
    setattr(ans,"sorted",NULL)  # remove so if you convert to df, do something, and convert back, it is not sorted
    setattr(ans,"index",NULL)  #4889 #5042
    setattr(ans,".internal.selfref",NULL)
    # leave tl intact, no harm,

    if(!is.null(row.names)) {
        rownames(ans) =  ans[, row.names]
        ans[, row.names] = NULL
    }

    ans
}

test <- data.table::data.table(iris)
test$sample <- paste("sample", 1:nrow(test), sep = "_")

test2 <- as.data.frame(test, row.names = "sample")

Would this be welcome to the data.table package? I would love to contribute and submit a pull request to data.table!

---

Edit: this is now a pull request and a bug fix; see PR #5320 allows a user to specify row.names argument as a character vector in as.data.frame.data.table S3 method. Code for as.data.frame.data.table is now simplified and uses setDF on a deep copy of the input. Thanks to @ben-schwen for guiding my contribution.

[ben-schwen]

Ty for the report. At the current status as.data.frame.data.table is just silently dropping the row.names argument from as.data.frame(x, row.names = NULL, optional = FALSE, ...).

This is also inconsistent to setDF(x, rownames = NULL) which allows to set rownames.