[R] Computing sums of the columns of an array

[Duncan Murdoch]

On 8/5/2005 12:43 PM, Uwe Ligges wrote:
> Duncan Murdoch wrote:
> 
>> On 8/5/2005 12:16 PM, Martin C. Martin wrote:
>> 
>>>Hi,
>>>
>>>I have a 5x731 array A, and I want to compute the sums of the columns.  
>>>Currently I do:
>>>
>>>apply(A, 2, sum)
>>>
>>>But it turns out, this is slow: 70% of my CPU time is spent here, even 
>>>though there are many complicated steps in my computation.
>>>
>>>Is there a faster way?
>> 
>> 
>> You'd probably do better with matrix multiplication:
>> 
>> rep(1, nrow(A)) %*% A
> 
> 
> No, better use colSums(), which has been optimized for this purpose:
> 
>   A <- matrix(seq(1, 10000000), ncol=10000)
>   system.time(colSums(A))
>   # ~ 0.1 sec.
>   system.time(rep(1, nrow(A)) %*% A)
>   # ~ 0.5 sec.

I didn't claim my solution was the best, only better. :-)

One point of interest:  I think your example exaggerates the difference 
by using a matrix of integers.  On my machine I get a ratio something 
like yours with the same example

 > A <- matrix(seq(1, 10000000), ncol=10000)
 >   system.time(colSums(A))
[1] 0.08 0.00 0.08   NA   NA
 >   system.time(rep(1, nrow(A)) %*% A)
[1] 0.25 0.01 0.23   NA   NA

but if I make A floating point, there's much less difference:

 > A <- matrix(as.numeric(seq(1, 10000000)), ncol=10000)
 >   system.time(colSums(A))
[1] 0.09 0.00 0.09   NA   NA
 >   system.time(rep(1, nrow(A)) %*% A)
[1] 0.11 0.00 0.12   NA   NA

Still, colSums is the winner in both cases.

Duncan Murdoch