Getting subsets of a data frame

4 messages · Liaw, Andy, Brian Ripley, Fernando Saldanha

Original

1

4

Liaw, Andy

Sat, Apr 16, 2005 5:46 AM #

Because a data frame can hold different data types (even matrices) in
different variables, one row of it can not be converted to a vector in
general (where all elements need to be of the same type).

Andy

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Brian Ripley

Sat, Apr 16, 2005 9:17 AM #

Perhaps Fernando will also note that is documented in ?"[.data.frame",
a slightly more appropriate reference than Bill's.

It would be a good idea to read a good account of R's indexing: Bill 
Venables and I know of a couple you will find in the R FAQ.

On Sat, 16 Apr 2005, Liaw, Andy wrote:

Because a data frame can hold different data types (even matrices) in
different variables, one row of it can not be converted to a vector in
general (where all elements need to be of the same type).

Andy

From: Fernando Saldanha

Thanks, it's interesting reading.

I also noticed that

sw[, 1, drop = TRUE] is a vector (coerces to the lowest dimension)

but

sw[1, , drop = TRUE] is a one-row data frame (does not convert it into
a list or vector)

FS


On 4/16/05, Bill.Venables at csiro.au <Bill.Venables at csiro.au> wrote:

You should look at

?"["

and look very carefully at the "drop" argument.  For your example

sw[, 1]

is the first component of the data frame, but

sw[, 1, drop = FALSE]

is a data frame consisting of just the first component, as
mathematically fastidious people would expect.

This is a convention, and like most arbitrary conventions

it can be very

useful most of the time, but some of the time it can be a very nasty
trap.  Caveat emptor.

Bill Venables.

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of

Fernando Saldanha

Sent: Saturday, 16 April 2005 1:07 PM
To: Submissions to R help
Subject: [R] Getting subsets of a data frame

I was reading in the Reference Manual about Extract.data.frame.

There is a list of examples of expressions using [ and [[, with the
outcomes. I was puzzled by the fact that, if sw is a data

frame, then

sw[, 1:3]

is also a data frame,

but

sw[, 1]

is just a vector.

Since R has no scalars, it must be the case that 1 and 1:1

are the same:

1 == 1:1

[1] TRUE

Then why isn't sw[,1] = sw[, 1:1] a data frame?

FS

______________________________________________
R-help at stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
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______________________________________________
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PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

Brian Ripley

Sat, Apr 16, 2005 9:31 AM #

On Sat, 16 Apr 2005, Prof Brian Ripley wrote:

BTW,

sw <- swiss
sw[1,,drop=TRUE] *is* a list (not as claimed, but as documented)
sw[1, ]          is a data frame
sw[, 1]          is a numeric vector.

I should have pointed out that "[.data.frame" is in the See Also of Bill's 
reference.

BTW to Andy: a list is a vector, and Kurt and I recently have been trying 
to correct documentation that means `atomic vector' when it says `vector'.
(Long ago lists in R were pairlists and not vectors.)

[1] TRUE

On Sat, 16 Apr 2005, Liaw, Andy wrote:

Because a data frame can hold different data types (even matrices) in
different variables, one row of it can not be converted to a vector in
general (where all elements need to be of the same type).

Andy

From: Fernando Saldanha

Thanks, it's interesting reading.

I also noticed that

sw[, 1, drop = TRUE] is a vector (coerces to the lowest dimension)

but

sw[1, , drop = TRUE] is a one-row data frame (does not convert it into
a list or vector)

FS


On 4/16/05, Bill.Venables at csiro.au <Bill.Venables at csiro.au> wrote:

You should look at

?"["

and look very carefully at the "drop" argument.  For your example

sw[, 1]

is the first component of the data frame, but

sw[, 1, drop = FALSE]

is a data frame consisting of just the first component, as
mathematically fastidious people would expect.

This is a convention, and like most arbitrary conventions

it can be very

useful most of the time, but some of the time it can be a very nasty
trap.  Caveat emptor.

Bill Venables.

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of

Fernando Saldanha

Sent: Saturday, 16 April 2005 1:07 PM
To: Submissions to R help
Subject: [R] Getting subsets of a data frame

I was reading in the Reference Manual about Extract.data.frame.

There is a list of examples of expressions using [ and [[, with the
outcomes. I was puzzled by the fact that, if sw is a data

frame, then

sw[, 1:3]

is also a data frame,

but

sw[, 1]

is just a vector.

Since R has no scalars, it must be the case that 1 and 1:1

are the same:

1 == 1:1

[1] TRUE

Then why isn't sw[,1] = sw[, 1:1] a data frame?

FS

______________________________________________
R-help at stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html

______________________________________________
R-help at stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html

______________________________________________
R-help at stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide! 
http://www.R-project.org/posting-guide.html

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

Fernando Saldanha

Sat, Apr 16, 2005 10:49 AM #

I am reading as fast as I can! Just started with R five days ago.

I found the following in the documentation:

"Although the default for 'drop' is 'TRUE', the default behaviour when
only one _row_ is left is equivalent to specifying 'drop = FALSE'.  To
drop from a data frame to a list, 'drop = FALSE' has to (sic)
specified explicitly."

I think the exception mentioned in the first sentence is the reason
for my confusion.

I also think the second sentence is wrong and should have 'TRUE'
instead of 'FALSE'.

While it is true that a data frame is a list, it is not a list of
numbers, but rather a list of columns, which, if I understand
correctly, can be either vectors or matrices. So regardless of the
value assigned to 'drop' the returned object is a list.

When I asked "why isn't sw[1, ] a list?" I should have asked instead
"why isn't sw[1, ] a list of vectors?"

I did some experiments with a data frame a, where the columns are
vectors (no matrix columns):

[1] TRUE

[1] TRUE                     (did not sop being a data frame)

[1] TRUE                     (but it is a list)

[1] FALSE                   (no longer a data frame)

[1] TRUE                     (but it is a list)

[1] TRUE                    (still a data frame)

[1] TRUE                    (but it is a list)

I also tried:

$dates.num
[1] 477032400

dates.num
3 477032400  (notice the row name)

$names
[1] "dates.num"

$class
[1] "data.frame"

$row.names
[1] "3"

$names
[1] "dates.num"

FS

On 4/16/05, Prof Brian Ripley <ripley at stats.ox.ac.uk> wrote:

On Sat, 16 Apr 2005, Prof Brian Ripley wrote:

Perhaps Fernando will also note that is documented in ?"[.data.frame",
a slightly more appropriate reference than Bill's.

It would be a good idea to read a good account of R's indexing: Bill Venables
and I know of a couple you will find in the R FAQ.

BTW,

sw <- swiss
sw[1,,drop=TRUE] *is* a list (not as claimed, but as documented)
sw[1, ]          is a data frame
sw[, 1]          is a numeric vector.

I should have pointed out that "[.data.frame" is in the See Also of Bill's
reference.

BTW to Andy: a list is a vector, and Kurt and I recently have been trying
to correct documentation that means `atomic vector' when it says `vector'.
(Long ago lists in R were pairlists and not vectors.)

is.vector(list(a=1))

[1] TRUE

On Sat, 16 Apr 2005, Liaw, Andy wrote:

Because a data frame can hold different data types (even matrices) in
different variables, one row of it can not be converted to a vector in
general (where all elements need to be of the same type).

Andy

From: Fernando Saldanha

Thanks, it's interesting reading.

I also noticed that

sw[, 1, drop = TRUE] is a vector (coerces to the lowest dimension)

but

sw[1, , drop = TRUE] is a one-row data frame (does not convert it into
a list or vector)

FS


On 4/16/05, Bill.Venables at csiro.au <Bill.Venables at csiro.au> wrote:

You should look at

?"["

and look very carefully at the "drop" argument.  For your example

sw[, 1]

is the first component of the data frame, but

sw[, 1, drop = FALSE]

is a data frame consisting of just the first component, as
mathematically fastidious people would expect.

This is a convention, and like most arbitrary conventions

it can be very

useful most of the time, but some of the time it can be a very nasty
trap.  Caveat emptor.

Bill Venables.

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of

Fernando Saldanha

Sent: Saturday, 16 April 2005 1:07 PM
To: Submissions to R help
Subject: [R] Getting subsets of a data frame

I was reading in the Reference Manual about Extract.data.frame.

There is a list of examples of expressions using [ and [[, with the
outcomes. I was puzzled by the fact that, if sw is a data

frame, then

sw[, 1:3]

is also a data frame,

but

sw[, 1]

is just a vector.

Since R has no scalars, it must be the case that 1 and 1:1

are the same:

1 == 1:1

[1] TRUE

Then why isn't sw[,1] = sw[, 1:1] a data frame?

FS

______________________________________________
R-help at stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html

______________________________________________
R-help at stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html

______________________________________________
R-help at stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html