[R] convert list to numeric

[Don MacQueen]

An example will help show the difference between single vs. double brackets.

## xl is  a list with three elements
>  xl <- list( a=1:3, b=2:7, c=c('X','Y'))
>  xl
$a
[1] 1 2 3

$b
[1] 2 3 4 5 6 7

$c
[1] "X" "Y"

>

## with single brackets, we get a subset. A subset of a list is still a list
>  xl[1]
$a
[1] 1 2 3

## extract the first element of xl, as itself, whatever it is
>  xl[[1]]
[1] 1 2 3

## with single brackets, we get a subset. In the next example the subset
## consists of the first and third elements of xl, i.e., a list 
having two elements
>  xl[c(1,3)]
$a
[1] 1 2 3

$c
[1] "X" "Y"


## Similarly for data frames, and note how the formatting is 
different when the object
## is printed to the screen

>  xd <- data.frame( a=1:3, b=2:4, c=c('X','Y','Z'))
>  xd
   a b c
1 1 2 X
2 2 3 Y
3 3 4 Z
>  class(xd)
[1] "data.frame"

>  xd[2]
   b
1 2
2 3
3 4
>  class(xd[2])
[1] "data.frame"

>  xd[[2]]
[1] 2 3 4
>  class(xd[[2]])
[1] "integer"



At 6:22 AM -0800 11/2/09, dadrivr wrote:
>Great, that works very well.  What is the purpose of double brackets vs
>single ones?  I will remember next time to include a subset of the data, so
>that readers can run the script.  Thanks again for your help!
>
>
>Benilton Carvalho wrote:
>>
>>  it appears that what you really want is to use:
>>
>>  task[[i]]
>>
>>  instead of task[i]
>>
>>  b
>>
>>  On Nov 1, 2009, at 11:04 PM, dadrivr wrote:
>>
>>>
>>>  I would like to preface this by saying that I am new to R, so I 
>>>  would ask
>>>  that you be patient and thorough, so that I'm not completely 
>>>  clueless.  I am
>>>  trying to convert a list to numeric so that I can perform 
>>>  computations on it
>>>  (specifically mean-center the variable), but I am running into 
>>>  problems.  I
>>>  have imported the data set into "task" (data frame).  The data frame 
>>>  is made
>>>  of factors with variable names in the first row.  I am running a 
>>>  loop to set
>>>  a variable equal to a column in the data frame.  Here is an example 
>>>  of my
>>>  problem:
>>>
>>>  for (i in 1:dim(task)[2]){
>>>  predictor.loop <- c(task[i])
>>>  predictor.loop.mc <- predictor.loop - mean(predictor.loop, na.rm=T)
>>>  }
>>>
>>>  I get the following error:
>>>  Error in predictor.loop - mean(predictor.loop, na.rm = T) :
>>>   non-numeric argument to binary operator
>>>  In addition: Warning message:
>>>  In mean.default(predictor.loop, na.rm = T) :
>>>   argument is not numeric or logical: returning NA
>>>
>>>  The column is entirely made up of numerical data, except for the 
>>>  header,
>>>  which is a string.  My problem is that I receive an error because the
>>>  predictor.loop variable is not numerical, so I need to find a way to 
>>>  convert
>>>  it.  I tried using:
>>>  predictor.loop <- c(as.numeric(task[i]))
>>>  But I get the following error: "Error: (list) object cannot be 
>>>  coerced to
>>>  type 'double'"
>>>
>>>  If I call the variable, I can assign it to a numerical list (e.g., 
>>>  predictor
>>>  loop <- task$variablename), but since I am assigning the variable in 
>>>  a loop,
>  >> I have to find another way as the variable name would have to change
>  >> in each
>  >> loop iteration.  Any help would be greatly appreciated.  Thanks!
>  >> --
>  >> View this message in context:
>  >> http://*old.nabble.com/convert-list-to-numeric-tp26155039p26155039.html
>>>  Sent from the R help mailing list archive at Nabble.com.
>>>
>>>  ______________________________________________
>>>  R-help at r-project.org mailing list
>>>  https://*stat.ethz.ch/mailman/listinfo/r-help
>>>  PLEASE do read the posting guide
>>>  http://*www.*R-project.org/posting-guide.html
>>>  and provide commented, minimal, self-contained, reproducible code.
>>
>>  ______________________________________________
>>  R-help at r-project.org mailing list
>>  https://*stat.ethz.ch/mailman/listinfo/r-help
>>  PLEASE do read the posting guide
>>  http://*www.*R-project.org/posting-guide.html
>>  and provide commented, minimal, self-contained, reproducible code.
>>
>>
>
>--
>View this message in context: 
>http://*old.nabble.com/convert-list-to-numeric-tp26155039p26157105.html
>Sent from the R help mailing list archive at Nabble.com.
>
>______________________________________________
>R-help at r-project.org mailing list
>https://*stat.ethz.ch/mailman/listinfo/r-help
>PLEASE do read the posting guide http://*www.*R-project.org/posting-guide.html
>and provide commented, minimal, self-contained, reproducible code.


-- 
--------------------------------------
Don MacQueen
Environmental Protection Department
Lawrence Livermore National Laboratory
Livermore, CA, USA
925-423-1062