[R] Beginner's query - segmentation fault

[Uwe Ligges]

Laura Quinn wrote:

> thanks, have used
> 
> temp [temp==0]<- NA

Please use
   is.na(temp[temp==0]) <- TRUE


> and this seems to have worked, though it won't let me access individual
> columns (ie temp$t1 etc) 

No! temp$t1 is a list element or column of a data.frame, but not a 
column of a matrix. *PLEASE*, read manuals, help pages, or books on R 
how to use index / extract elements.

Please read my previous answer on how to access individual columns.


> to work on - is there any real advantage in using
> a matrix, or would i be better advised to deal with dataframes? (I have
> double checked and temp is currently a matrix).

Working on matrices is supposed to be faster. But matrices have the 
restriction of one data type for all columns (e.g. numeric).

Uwe Ligges


> On Tue, 7 Oct 2003, Prof Brian Ripley wrote:
> 
> 
>>On Tue, 7 Oct 2003, Laura Quinn wrote:
>>
>>
>>>I am dealing with a huge matrix in R (20 columns, 54000 rows) and have
>>>lots of missing values within the dataset which are currently displayed as
>>>the value "-999.00" I am trying to create a new matrix (or change the
>>>existing one) to display these values as "NA" so that I can then perform
>>>the necessary analysis on the columns within the matrix.
>>>
>>>The matrix name is temp and the column names are t1 to t20 inclusive.
>>>
>>>I have tried the following command:
>>>
>>>temp$t1[temp$t1 == -999.00] <- NA
>>>
>>>and it returns a segmentation fault, can someone tell me what I am doing
>>>wrong?
>>
>>Well, R should not segfault, so there is bug here somewhere.  However, I
>>don't think what you have described can actually work. Is temp really a
>>matrix?  If so temp$t1 will return NULL, and you should get an error
>>message.
>>
>>
>>If temp is a matrix
>>
>>temp[temp == -999.00] <- NA
>>
>>will do what you want.
>>
>>
>>If as is more likely temp is a data frame with all columns numeric,
>>there are several ways to do this, e.g.
>>
>>temp[] <- lapply(temp, function(x) ifelse(x == -999, NA, x))
>>
>>temp[as.matrix(temp) == -999] <- NA  # only in recent versions of R
>>
>>as well as explicit looping over columns.
>>
>>--
>>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
>>
>>
> 
> 
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