[R] R conditional matrix operations - advanced condition

[Lukasz Kielpinski]

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


Hope I will get some suggestions,

Lukasz