[R] Newbie: colSums() compared with Matlab's sum()

[Henrik Bengtsson]

Use

   submat <- data[1:i,, drop=FALSE]

/H

On Wed, Dec 23, 2009 at 6:46 PM, Francesco Napolitano
<franapoli at gmail.com> wrote:
> Hi all,
>
> I'm trying to learn R after years of Matlab's experience. Here is an
> issue I couldn't solve today.
>
> Consider the following piece of code (written by memory):
>
> for(i in 1:n){
>    submat <- data[1:i,]
>    C <- colSums(submat)
>    }
>
> The problem is that at the first iteration, data[1:1,] reduces to a
> vector and colSums returns an error. This sounds really strange to me
> because a sum-over-columns operation should have no problem working with
> columns of length 1. Matlab's "sum()" works just fine in such case.
>
> The error says that I need an at least 2D array. So I try using
> matrix(data[1:i,]). Unfortunately this returns a column instead of a
> row. So I could do t(matrix(data[1:i,])), but this would transpose the
> also the sub-matrices from the 2nd iteration on. I could fix everything
> with some "if" but it would be really terrible code :-/.
>
> I'm sure I'm missing something because my neurons stick with Matlab
> behaviour. Can you help me to understand what's wrong with my reasoning?
>
> Thank you very much,
> Francesco.
>
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