Extracting single row from data.frame without loss of names

Question

I am simply extracting a single row from a data.frame. Consider for example

d=data.frame(a=1:3,b=1:3)
d[1,]    # returns a data.frame
#   a b
# 1 1 1

The output matched my expectation. The result was not as I expected though when dealing with a data.frame that contains a single column.

d=data.frame(a=1:3)
d[1,]   # returns an integer
# [1] 1

Indeed, here, the extracted data is not a data.frame anymore but an integer! To me, it seems a little strange that the same function on the same data type wants to return different data types. One of the issue with this conversion is the loss of the column name.

To solve the issue, I did

extractRow = function(d,index)
{
    if (ncol(d) > 1)
    {
        return(d[index,])
    } else
    {
        d2 = as.data.frame(d[index,])
        names(d2) = names(d)
        return(d2)
    }
}

d=data.frame(a=1:3,b=1:3)
extractRow(d,1)
#   a b    
# 1 1 1

d=data.frame(a=1:3)
extractRow(d,1)
#   a
# 1 1

But it seems unnecessarily cumbersome. Is there a better solution?

This may illustrate the subject: https://stackoverflow.com/questions/69084753/selecting-a-single-column-from-a-tibble-still-returns-a-tibble-instead-of-a-vect/69087492#69087492 — GuedesBF, Sep 09 '21 at 10:05

Tim Biegeleisen · Accepted Answer · 2021-09-09T10:03:12.597

3

Just subset with the drop = FALSE option:

extractRow = function(d, index) {
    return(d[index, , drop=FALSE])
}

edited Sep 09 '21 at 10:03

answered Sep 09 '21 at 09:52

Tim Biegeleisen

502,043
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1

While this works It prints a warning that drop will be ignored (which isn't even true). `return(d[index, , drop=FALSE])` prevents this warning. – Robin Gertenbach Sep 09 '21 at 10:00
@RobinGertenbach Not sure what to make of your comment, as the code in the comment matches my answer already. – Tim Biegeleisen Sep 09 '21 at 10:02
1

My comment uses two commas – Robin Gertenbach Sep 09 '21 at 10:02

score 2 · Answer 2 · answered Sep 09 '21 at 09:53

2

R tries to simplify data.frame cuts by default, the same thing happens with columns:

d[, "a"]

# [1] 1 2 3

Alternatives are:

d[1, , drop = FALSE]
tibble::tibble which has drop = FALSE by default

answered Sep 09 '21 at 09:53

Robin Gertenbach

10,316
3
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score 1 · Answer 3 · answered Sep 09 '21 at 09:51

1

I can't tell you why that happens - it seems weird. One workaround would be to use slice from dplyr (although using a library seems unecessary for such a simple task).

library(dplyr)
slice(d, 1)

  a
1 1

answered Sep 09 '21 at 09:51

user438383

5,716
8
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score 0 · Answer 4 · answered Sep 09 '21 at 10:03

data.frames will simplify to vectors or scallars whith base subsetting [,].

If you want to avoid that, you can use tibbles instead:

> tibble(a=1:2)[1,]

# A tibble: 1 x 1
      a
  <int>
1     1

tibble(a=1:2)[1,] %>% class
[1] "tbl_df"     "tbl"        "data.frame"

Extracting single row from data.frame without loss of names

4 Answers4