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rdiagonal

Main diagonal into anti-diagonal

I need to transform main diagonal

{matrix(
1 1 1 1,
0 2 2 2,
0 0 3 3,
0 0 0 4)
}

into:

{matrix(
0 0 0 1,
0 0 1 2,
0 1 2 3,
1 2 3 4)
}

I tried all operators I could find t(), arev(), flipud(), apply(x,2,rev) and so on. Without a positive result. Hope you can help me.

Solution

  • Does this work for you? Takes each column and 'rotates' (for lack of a better word) x places, where x is the column index.

    res <- sapply(1:ncol(input),function(x){
  #get relevant column
  base <- input[,x]
  n <- length(base)
  indices <- 1:n
  #reshuffle indices: first above x, then below x
  out <- base[c(indices[indices>x],indices[indices<=x])]
  out
})

all(res==output)
[1] TRUE

    data used:

    input <- structure(c(1, 0, 0, 0, 1, 2, 0, 0, 1, 2, 3, 0, 1, 2, 3, 4), .Dim = c(4L, 
4L))
output <- structure(c(0, 0, 0, 1, 0, 0, 1, 2, 0, 1, 2, 3, 1, 2, 3, 4), .Dim = c(4L, 
4L))