Lets create matrix m.
m <- matrix(1:9, 3,3, T); m
[,1] [,2] [,3]
[1,] 1 2 3
[2,] 4 5 6
[3,] 7 8 9
m[3,1] # 7
m[3][1] # 7
Why does second indexing notation work? Is there difference between these two notations? Is it safe to use?
But sequences behave differently:
m[1:2, 1:2] # works as expected, return matrix
m[1:2][1:2] # return vector 1 4, why?
A matrix is a vector with dim attributes. By doing the m[3], it returns only the 3rd element. If we want to use a chained extract, then extract the row with column index blank after the , (drop = FALSE - in case we want to avoid coercing the matrix to vector) and select the first element which is the first column
m[3,, drop = FALSE][1]
#[1] 7
In the OP's first option, it uses the row index and column index with 3, 1 which selects the element based on both index
In the updated example, OP specified row index as first 2 rows and columns as first 2 columns. So, it returns a matrix omitting the 3rd row and 3rd column
m[1:2, 1:2]
# [,1] [,2]
#[1,] 1 2
#[2,] 4 5
But, in the second case
m[1:2]
#[1] 1 4
extracts the first two elements
likewise, if we do
m[1:5]
#[1] 1 4 7 2 5
is the first five elements following the columnwise order
Therefore,
m[1:2][1:2]
returns only 1, 4 because from the first Extract, it is only extracting 1 and 4. Then, the second extract is based on that subset and it also have 2 elements. If we increase the index, those positions are not available and filled by NA
m[1:2][1:4]
#[1] 1 4 NA NA
The elementwise indexing is acting on the vector
c(m)
#[1] 1 4 7 2 5 8 3 6 9
where the first two elements are 1 and 4