[R] How to apply a function on each column of a matrix

[(Ted Harding)]

 On 24-Jan-10 00:27:37, Jim Lemon wrote:
> On 01/24/2010 11:11 AM, anna wrote:
>>
>> Here is the last code that I wrote but it would give me the
>> same problem:
>> I have the matrix mat with n columns mat.1, mat.2 ...mat.n
>>
>> #To be able to use lapply I convert it to a data.frame:
>> mat<- data.frame(mat)
>>
>> lapply(mat, function, argument of function)
>>
>> It works but I still get for all elements the function applied
>> for the last element. The elements of my results are all the
>> same I don't understand I did exactly as shown on this website:
>> http://www.ats.ucla.edu/stat/r/library/advanced_function_r.htm#lapply
> 
> Hi anna,
> If you could post your matrix "mat" (or something else that
> produces the problem you describe if "mat" is private or too big)
> and the commands you used, someone will probably figure out what
> is going wrong.
> 
> Jim

I agree with Jim's comments. It may help to formulate your reply
to considet the following simple case of applying a function "on
each column of a matrix":

  X2 <- function(x){ x^2 }
  M <- matrix(c(1,2,3,4,5,6,7,8,9),nrow=3)
  M
  #      [,1] [,2] [,3]
  # [1,]    1    4    7
  # [2,]    2    5    8
  # [3,]    3    6    9

  X2 <- function(x){ x^2 }
  apply(M,2,X2)
  #      [,1] [,2] [,3]
  # [1,]    1   16   49
  # [2,]    4   25   64
  # [3,]    9   36   81

  SX2 <- function(x){ sum(x^2) }
  apply(M,2,SX2)
  # [1]  14  77 194

Not ythe use of apply(), not lapply(); also that the result is
not a matrix (with 1 row) but a vector (dimensionless):

  dim(apply(M,2,SX2))
  # NULL

If you want the result to be a 1-row vector, then you need to
force this explicitly:

  N <- apply(M,2,SX2)
  # dim(N)<-c(1,3)

  N
  #      [,1] [,2] [,3]
  # [1,]   14   77  194

or (in this case) more simply:

  rbind(NULL,apply(M,2,SX2))
  #      [,1] [,2] [,3]
  # [1,]   14   77  194

If that approach does not work with your matrix and your function,
then there must be something special about one or the other!
Ted.

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E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
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Date: 24-Jan-10                                       Time: 00:49:12
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