Sum between elements of a vector using indices of start and end positions

Question

I have a vector (in this case extracted from a data frame) that I would like to calculate a some sums from using a two additional vectors containing start and end indices of the data vector to be used. For this, I am working in R.

For example, my vector to use for calculations is:

Data Vector:
[1] 1.45
[2] 1.56
[3] 1.57
[4] 1.40
[5] 3.45
[6] 1.45
[7] 1.66
[8] 2.03
[9] 1.33

Using other information in my data frame, I have calculated two other vectors containing the index positions of the above vector which I have called "start" and "stop" to denote the ranges between which I want to sum values. For example:

Start: 
[1] 1 
[2] 4
[3] 7

End: 
[1] 3 
[2] 6
[3] 9

I would like to use these two index vectors to sum between elements 1-3, 4-6, and 7-9 in my data vector. I'm struggling with a way to implement this across a data frame with several hundred rows.

I'm now attempting to write a function to do this, but wanted to put this up in case there is a simpler solution that I am missing.

Answer 1

Maybe there is a better solution, but I wrote some quick code that does what you want for a single vector, could rewrite some of it to calculate sums for multiple columns I am sure, if that is what you want.

 sum<-c(rep(0,length(start)))
 
 for (i in 1:length(start)){
     for (j in start[i]:end[i]){
        if(is.na(data[j] == FALSE)){
        sum[i]<-sum[i]+data[j]
        }
    }
 }

Edited to work with NAs. Again probably a better way to do it (I'm not R expert either) but that should work.

Answer 2

This will work without too many checks and does not cover N/As (hopefully you will have filtered them out from the data.frame the number vector came from).

subSums <- function(vector, start_vector, end_vector){
  if (length(start_vector) != length(end_vector)){
    print("Start and End Point vectors are not the same length")
    return()
  }
  
  result <- NULL
  for (index in seq_along(start_vector)) {
    result[index] <- sum(vector[start_vector[index]:end_vector[index]])
  }
  
  return(result)
}

example of use:

vec<-1:20
subSums(vec, c(1,3), c(2,4))