[R] R conditional matrix operations - advanced condition

[Lukasz Kielpinski]

Thanks a lot!
now my program works about 10x faster (and I will have results before deadline!)
to sum up more generally - if you want matrix operation to be
conditional it is advisable to have results pre-computed. (?)

Lukasz

On 23 March 2012 19:50, Berend Hasselman <bhh at xs4all.nl> wrote:
>
> On 23-03-2012, at 17:29, Lukasz Kielpinski wrote:
>
>> Hello List!
>>
>> I stumbled across an efficiency problem - calculation that would be
>> probably done very fast as a matrix operation I must perform as a
>> for-loop.
>> My intention was to do a conditional operation in matrix depending on
>> the information in first column (summing as many data points from
>> vector my_data as the number specified in the first column of the
>> matrix) but the result is that the function takes the condition only
>> from the first row of column for calculations in every row.
>> Is it possible to solve this problem as a matrix calculation or I have
>> to iterate over each row? (which I suppose is much slower)
>>
>> #problem looks like:
>> my_mat <- matrix(1:50,ncol=2)
>> my_mat <- cbind(my_mat,0)     #here I have a matrix with empty third
>> column where I want to store my results
>> my_data <- rnorm(25) #this is a dataset I want to use for filling the
>> third column
>> #and I did
>> my_mat[,3] <- sum(my_data[1:my_mat[,1]]) + my_mat[,2]
>> #which didn't work as I expected
>
> If I understand that this correctly I think you should do this
>
> my_data.csum <- cumsum(my_data)
> my_mat[,3] <- my_data.csum[my_mat[,1]] + my_mat[,2]
>
> Berend
>