I am trying to get a particular diagonal from my data. This is my code :
a2008=c(30,50,65)
a2009=c(40,90,NA)
a2010=c(55,NA,NA)
mydata=rbind(a2008,a2009,a2010)
mydata
[,1] [,2] [,3]
a2008 30 50 65
a2009 40 90 NA
a2010 55 NA NA
how can we get the diagonal 65, 90, 55 ?
I already used the diag function but i got 30, 90, NA.
Some help would be appreciated
To get the main anti-diagonal, you can do this. (rev to get stated order)
mydata[row(mydata) + col(mydata) == ncol(mydata) + 1]
You can get the diagonal below that with
mydata[row(mydata) + col(mydata) == ncol(mydata) + 2]
and so on
you can try to get the antidiagonal by
diag(mydata[,nrow(mydata):1])
[1] 65 90 55
or
diag(as.matrix(rev(as.data.frame(mydata))))
You can get that diagonal by flipping your matrix before calling diag:
diag(mydata[ , 3:1]) # [1] 65 90 55
Doing mydata[ , 3:1] is indexing the columns of your matrix in reverse order.
That's a matrix, not a data frame. Which is good! Diagonals of data frames don't make much sense. I would do it like this:
mydata[cbind(1:3, 3:1)]
# [1] 65 90 55
You can use a matrix to index a matrix, this is saying you want the values from rows 1, 2, 3 and columns 3, 2, 1.
You can achieve this by first flipping the matrix and them finding the diagonal. That is
diag(apply(mydata, 1, rev))
I hope this helps!
