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

[David Winsemius]

On Dec 23, 2009, at 9:46 PM, Francesco Napolitano 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)
>    }
>
In R the loop is not necessary, even confusing as you are demonstrating:

 > mat <- matrix(rnorm(100), nrow=10)  # 10 x 10
 > colSums(mat[1:5, ])                 # sums of 1st 5 rows
  [1]  3.33331735  0.86672248 -3.10971483  1.23620455 -0.31887421  
-0.50544837
  [7]  3.13636155  0.02175862 -2.18816961 -1.31760196


> 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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David Winsemius, MD
Heritage Laboratories
West Hartford, CT