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 ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
