Alex Miller - 1 year ago 78
R Question

How to Manipulate Array Indexed Values from Matrix in R

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!

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
``````
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