jenswirf - 1 year ago 70

R Question

Is there an easy way to extract a vector of an 'offset' and 'reverse' "diagonal" (the x's) of a matrix in R?

`[,1] [,2] [,3] [,4] [,5]`

[1,] x 0 0 0 0

[2,] 0 0 0 0 x

[3,] 0 0 0 x 0

[4,] 0 0 x 0 0

[5,] 0 x 0 0 0

I tried

`diag()`

Answer Source

1) The ** i-th offset reverse diagonal** of square matrix

`m`

:```
off.rev.diag <- function(m, i = 0) m[ (row(m) + col(m) - 1) %% ncol(m) == i ]
```

For example:

```
> m <- matrix(1:25, 5); m
[,1] [,2] [,3] [,4] [,5]
[1,] 1 6 11 16 21
[2,] 2 7 12 17 22
[3,] 3 8 13 18 23
[4,] 4 9 14 19 24
[5,] 5 10 15 20 25
> off.rev.diag(m, 1)
[1] 1 10 14 18 22
> off.rev.diag(m, 2)
[1] 2 6 15 19 23
```

2) We can also write a **replacement function**:

```
"off.rev.diag<-" <- function(m, i = 0, value) {
m.ix <- matrix(seq_along(m), nrow(m))
replace(m, off.rev.diag(m.ix, i), value)
}
```

For example,

```
> off.rev.diag(m, 1) <- -(1:5)
> m
[,1] [,2] [,3] [,4] [,5]
[1,] -1 6 11 16 21
[2,] 2 7 12 17 -5
[3,] 3 8 13 -4 23
[4,] 4 9 -3 19 24
[5,] 5 -2 15 20 25
```