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

[Brian G. Peterson]

Francesco Napolitano wrote:
> Il giorno mer, 23/12/2009 alle 22.14 -0500, David Winsemius ha scritto:
>>> 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
> 
> It would be unnecessary also in Matlab. In the real code I need to store
> the partial sums (the example overwrites them, so it looks like wasted
> loops).

try:

data <- matrix(rnorm(100), nrow=10)  # 10 x 10
apply(data,2,cumsum)

if you want to store the partial sums
(that was an important bit of information you left out)