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Robin Hankin

2005-Dec-02 08:50 UTC

[R] extracting rows of a dataframe

Hi

look at the following session, in which I have a dataframe,
and I want to extract the second row, without the first column.

Everything works as expected until the last line, where I set
the names of x to NULL, and get a non-desired object (I
want c(4,3).).


Three questions:

(1)  why is as.vector(a[2,-1]) not a vector?

(2) How come setting names to NULL gives me bad weirdness?

(3) Can I structure my dataframe in a better way, so that
this problem does not occur?




 > a <- data.frame(male=c(T,T,F),mass=c(1,4,3),height=c(4,3,2))
 > a
    male mass height
1  TRUE    1      4
2  TRUE    4      3
3 FALSE    3      2
 > x <- as.vector(a[2,-1])
 > x
   mass height
2    4      3
 > names(x) <- NULL
 > x
   structure("4", class = "AsIs") structure("3",
class = "AsIs")
2                              4                              3
 >

--
Robin Hankin
Uncertainty Analyst
National Oceanography Centre, Southampton
European Way, Southampton SO14 3ZH, UK
  tel  023-8059-7743

Berwin A Turlach

2005-Dec-02 09:14 UTC

head link

[R] extracting rows of a dataframe

>>>>> "RH" == Robin Hankin <r.hankin at
noc.soton.ac.uk> writes:
    RH> Three questions:

    RH> (1)  why is as.vector(a[2,-1]) not a vector?
Did you read the help file of as.vector() ?
> a <- data.frame(male=c(T,T,F),mass=c(1,4,3),height=c(4,3,2))
> a   male mass height
1  TRUE    1      4
2  TRUE    4      3
3 FALSE    3      2> x <- as.vector(a[2,-1])
> mode(x)
[1] "list"> str(x)`data.frame':	1 obs. of  2 variables:
 $ mass  : num 4
 $ height: num 3

Clearly you want:
> x <- as.vector(a[2,-1], mode="numeric")
> str(x)
 num [1:2] 4 3> x[1] 4 3


    RH> (2) How come setting names to NULL gives me bad
weirdness?> names(x) <- NULL
> str(x)`data.frame':	1 obs. of  2 variables:
 $ : num 4
 $ : num 3> x  structure("4", class = "AsIs") structure("3",
class = "AsIs")
2                              4                              3

With your "names(x)<-NULL" command you are wiping out the names of
the
variables in your data frame.  Looking at print.data.frame(), the
answer to the so-called weirdness can presumably be found in
format.data.frame(). 

    RH> (3) Can I structure my dataframe in a better way, so that
    RH> this problem does not occur?
Not really, it's more a question of the appropriate use of
as.vector(). :-))

Cheers,

        Berwin

========================== Full address ===========================Berwin A
Turlach                      Tel.: +61 (8) 6488 3338 (secr)
School of Mathematics and Statistics        +61 (8) 6488 3383 (self)      
The University of Western Australia   FAX : +61 (8) 6488 1028
35 Stirling Highway                   
Crawley WA 6009                e-mail: berwin at maths.uwa.edu.au
Australia                        http://www.maths.uwa.edu.au/~berwin

Petr Pikal

2005-Dec-02 09:25 UTC

head link

[R] extracting rows of a dataframe

Hi

or maybe 

 x <- as.numeric(a[2,-1])

can suit to your needs.

Petr




On 2 Dec 2005 at 17:14, Berwin A Turlach wrote:

From:           	Berwin A Turlach <berwin at maths.uwa.edu.au>
Date sent:      	Fri, 2 Dec 2005 17:14:42 +0800
To:             	Robin Hankin <r.hankin at noc.soton.ac.uk>
Copies to:      	RHelp <r-help at stat.math.ethz.ch>
Subject:        	Re: [R] extracting rows of a dataframe
Send reply to:  	berwin at maths.uwa.edu.au
	<mailto:r-help-request at stat.math.ethz.ch?subject=unsubscribe>
	<mailto:r-help-request at stat.math.ethz.ch?subject=subscribe>
> >>>>> "RH" == Robin Hankin <r.hankin at
noc.soton.ac.uk> writes:
> 
>     RH> Three questions:
> 
>     RH> (1)  why is as.vector(a[2,-1]) not a vector?
> Did you read the help file of as.vector() ?
> 
> > a <- data.frame(male=c(T,T,F),mass=c(1,4,3),height=c(4,3,2))
> > a
>    male mass height
> 1  TRUE    1      4
> 2  TRUE    4      3
> 3 FALSE    3      2
> > x <- as.vector(a[2,-1])
> > mode(x)
> [1] "list"
> > str(x)
> `data.frame':	1 obs. of  2 variables:
>  $ mass  : num 4
>  $ height: num 3
> 
> Clearly you want:
> 
> > x <- as.vector(a[2,-1], mode="numeric")
> > str(x)
>  num [1:2] 4 3
> > x
> [1] 4 3
> 
> 
>     RH> (2) How come setting names to NULL gives me bad weirdness? >
> names(x) <- NULL > str(x) `data.frame':	1 obs. of  2 variables:
>  $ : num 4
>  $ : num 3
> > x
>   structure("4", class = "AsIs")
structure("3", class = "AsIs")
> 2                              4                              3
> 
> With your "names(x)<-NULL" command you are wiping out the
names of the
> variables in your data frame.  Looking at print.data.frame(), the
> answer to the so-called weirdness can presumably be found in
> format.data.frame(). 
> 
>     RH> (3) Can I structure my dataframe in a better way, so that RH>
>     this problem does not occur?
> Not really, it's more a question of the appropriate use of
> as.vector(). :-))
> 
> Cheers,
> 
>         Berwin
> 
> ========================== Full address ===========================>
Berwin A Turlach                      Tel.: +61 (8) 6488 3338 (secr)
> School of Mathematics and Statistics        +61 (8) 6488 3383 (self)  
>    The University of Western Australia   FAX : +61 (8) 6488 1028 35
> Stirling Highway                   Crawley WA 6009               
> e-mail: berwin at maths.uwa.edu.au Australia                       
> http://www.maths.uwa.edu.au/~berwin
> 
> ______________________________________________
> 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
Petr Pikal
petr.pikal at precheza.cz

Patrick Burns

2005-Dec-02 11:11 UTC

head link

[R] extracting rows of a dataframe

The key things here are:

1) a data frame is a list with some attributes (unlike a
matrix which is (generally) an atomic vector with attributes).

2) 'as.vector' removes attributes.

Patrick Burns
patrick at burns-stat.com
+44 (0)20 8525 0696
http://www.burns-stat.com
(home of S Poetry and "A Guide for the Unwilling S User")

Robin Hankin wrote:
>Hi
>
>look at the following session, in which I have a dataframe,
>and I want to extract the second row, without the first column.
>
>Everything works as expected until the last line, where I set
>the names of x to NULL, and get a non-desired object (I
>want c(4,3).).
>
>
>Three questions:
>
>(1)  why is as.vector(a[2,-1]) not a vector?
>
>(2) How come setting names to NULL gives me bad weirdness?
>
>(3) Can I structure my dataframe in a better way, so that
>this problem does not occur?
>
>
>
>
> > a <- data.frame(male=c(T,T,F),mass=c(1,4,3),height=c(4,3,2))
> > a
>    male mass height
>1  TRUE    1      4
>2  TRUE    4      3
>3 FALSE    3      2
> > x <- as.vector(a[2,-1])
> > x
>   mass height
>2    4      3
> > names(x) <- NULL
> > x
>   structure("4", class = "AsIs")
structure("3", class = "AsIs")
>2                              4                              3
> >
>
>--
>Robin Hankin
>Uncertainty Analyst
>National Oceanography Centre, Southampton
>European Way, Southampton SO14 3ZH, UK
>  tel  023-8059-7743
>
>______________________________________________
>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
>
>
>
>  
>

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