How to use row number inside apply function in R

I want to process data frame as follows, where I want to get the sum of 2 vectors and append it to a data frame as a row vector. 2 vectors are row vector of considering row and column vector which start just below the considering row with a fixed length.

 data
  A b1 b2 b3
  1 2  2  2
  2 3  3  3  
  3 4  4  4
  4 5  5  5
  5 6  6  6

 output (expected)
  A b1 b2 b3
  1 4  5  6
  2 6  7  8
  3 8  9  -
  4 10 -  -
  5 -  -  -

In the example if 1st row is considered, two vectors are

 row vector r- [2 2 2]
 column vector c - [2,3,4]

After getting the transpose of column vector I can add tow vectors and append it to a new data frame. This process must be done to all the rows.

Easiest way to do this is looping, but in R loops are not efficient, instead apply function can be used. However in this scenario, to do that need to know what is the current row number.

Is there a way to do this efficiently in R

r

Answers

answered 4 months ago G. Grothendieck #1

1) rollapply We can use rollapply to form the matrix of subvectors of A and then add that together with an initial column of zero to m. Note that we pad A with NA values so that the result of rollapply is the appropriate shape.

library(zoo)

m <- cbind(A = 1:5, b1 = 2:6, b2 = 2:6, b3 = 2:6)  # input matrix

nc1 <- ncol(m) - 1
A <- c(m[, 1], rep(NA, nc1))
cbind(0, rollapply(A[-1], nc1, c)) + m

giving:

     A b1 b2 b3
[1,] 1  4  5  6
[2,] 2  6  7  8
[3,] 3  8  9 NA
[4,] 4 10 NA NA
[5,] 5 NA NA NA

2) base This solution is similar but does not use any packages. The first two lines are the same as in (1).

nc1 <- ncol(m) - 1
A <- c(m[, 1], rep(NA, nc1))
cbind(0, embed(A[-1], nc1)[, seq(nc1, 1)]) + m

giving:

     A b1 b2 b3
[1,] 1  4  5  6
[2,] 2  6  7  8
[3,] 3  8  9 NA
[4,] 4 10 NA NA
[5,] 5 NA NA NA

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