[R] How to apply vector value function to a multidimensional array indexed by the remaining dimensions?

[Charles C. Berry]

On Fri, 1 Oct 2010, yunjiangster wrote:

>
> Hi,
>  I am looking for some generalization of colSums and rowSums for general
> vector valued functions, and for arrays of more than 2 dimensions.
>  So as a concrete example, suppose I have a 3 dimensional array, given by x
> = array(1:100,c(3,4,5)).
> and I want to sum the 3rd index of x to obain a 3 by 4 matrix. Using rowSums
> would return a vector of length 3 because it treats the last two indices as
> a single index.
>

see the help page for colSums, esp. the 'dims' arg

>  Besides summation, let's say if I define a vector valued function
> f<-function(x){min(x[1],min(x[2],x[3]))}, and I want to apply f to the last
> index of x, meaning for each 1<= i <= 3, 1<=j <=4, I want to compute
> f(x[i,j,]), and then put them in a 3 by 4 matrix. How would I be able to do
> that without using a for loop?
>

?apply

>  thanks.
>
>  Sincerely,
>  John
> -- 
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Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901