From: Gavin Simpson <gavin.simpson_at_ucl.ac.uk>

Date: Sat 06 Aug 2005 - 02:39:03 EST

Date: Sat 06 Aug 2005 - 02:39:03 EST

On Fri, 2005-08-05 at 12:16 -0400, 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?
*

Yes, colSums()

e.g.:

*> set.seed(1234)
*

> dat <- matrix(runif(5*731), ncol = 731)

> system.time(for(i in 1:1000) apply(dat, 2, sum), gcFirst = TRUE)

[1] 8.05 0.00 9.89 0.00 0.00

> system.time(for(i in 1:1000) colSums(dat), gcFirst = TRUE)

[1] 0.09 0.01 0.09 0.00 0.00

But neither is that slow on my system. What is A?

**HTH
**
Gav

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