问题
Given a matrix
mat = matrix(round(runif(min=0,max=1,n=9*9)),ncol=9,nrow=9)
say you want all the values of 1 using array indexing
indx.1 = which(mat == 1, arr.ind=TRUE)
How do you manipulate those index values within your matrix?
The below doesn't accomplish what I am after:
result.i.dont.want = mat
result.i.dont.want[indx.1[,1],indx.1[,2]] = NA
because, as far as I can tell, R indexes over every combination of indx.1[,1], and indx.1[,2].
I know this is very easy if you use arr.ind=FALSE, however, I am curious for arr.ind=TRUE. For example:
result.i.do.want = mat
result.i.do.want[which(mat == 1)] = NA
Thanks for the help!
回答1:
You are asking about matrix indexing. indx.1 returned by which is a matrix of 2 columns; you can use it directly to address matrix elements. This is known as matrix indexing. So try mat[index.1].
Also consider this toy example:
A <- matrix(1:9, 3, 3)
A[1:2, 1:2]
# [,1] [,2]
#[1,] 1 4
#[2,] 2 5
A[cbind(1:2, 1:2)]
# [1] 1 5
来源：https://stackoverflow.com/questions/39300325/how-to-manipulate-array-indexed-values-from-matrix-in-r