Dirk Eddelbuettel — written Jan 2, 2013 — updated Nov 19, 2016 — James Joseph Balamuta — source

Subsetting in armadillo is a popular topic that frequently appears on StackOverflow. Prinicipally, the occurrences are due to the unique approach that armadillo takes on performing subset operations. Within this topic, light will be shed on the applicable use of armadillo’s subsetting capabilities as it relates to non-contiguous submatrix views.

To begin, we start with a recent inquiring on StackOverflow that dealt with
the proposition of isolating pre-known locations and assigning the locations
new values. The *R* equivalent would be in the lines of:

[1] 1 2 3 4 5 6 7 8 9 10

[1] 1 6 3 3 5 6 7 8 9 1

Within `armadillo`

, the `submatrix views`

that we are most interested in to replicate this behavior is `.elem()`

.
The main difference of subsetting with this function is the need to use an
`arma::uvec`

containing the locations of elements to extract.
The `u`

prepended to the `vec`

object standards for `unsigned int`

, which are
only able to store positive integer numbers (e.g. 0, 1, 2,…, 42, and so on.)

To execute this code, we would use:

[1] 1 2 3 4 5 6 7 8 9 10

[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10] [1,] 1 6 3 3 5 6 7 8 9 1

Another common operation is to use logical indexing to ascertain where within
a vector certain conditions are met. For instance, to find values that are
greater than or equal to 0.5 in *R* the operation is governed by:

[1] 0.37667253 0.92201944 0.99187774 0.97723602 0.23629728 0.10706678 [7] 0.03915213 0.72802690 0.13023496 0.39995203

[1] 0.37667253 1.00000000 1.00000000 1.00000000 0.23629728 0.10706678 [7] 0.03915213 1.00000000 0.13023496 0.39995203

Within `armadillo`

, the use of logical values to subset is not possible without `arma::find`

to replicate *R*’s logical index subset behavior. However, per the previous
discussion, the values that `.elem()`

takes requires indices. Therefore,
the `arma::find`

function is taking logical values and convert them into index positions. This
operation is akin to the process behind *R*’s `which()`

function. Furthermore,
to specify a specific value that should be assigned to the subset, the use of
`.fill()`

is required.

[1] 0.37667253 0.92201944 0.99187774 0.97723602 0.23629728 0.10706678 [7] 0.03915213 0.72802690 0.13023496 0.39995203

[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [1,] 0.3766725 1 1 1 0.2362973 0.1070668 0.03915213 1 0.130235 [,10] [1,] 0.399952

As was the case with vectors, elements can be extracted from a matrix using
both the `arma::find`

function
to obtain indices and the `.elem()`

function in submatrix views
to extract values. For example, to know the values of `M * M'`

that are greater
or equal to 100, one could do:

The result:

[,1] [,2] [,3] [1,] 1 4 7 [2,] 2 5 8 [3,] 3 6 9

[,1] [1,] 108 [2,] 108 [3,] 126

Another form of subsetting within matrices is done using matrix notation. Matrix notation allows for the specification of an element based on their row and column location. This question recently popped up on StackOverflow as the non-continuous submatrix views were initially causing the user problems.

[,1] [,2] [,3] [1,] 1 4 7 [2,] 2 5 8 [3,] 3 6 9

[,1] [,2] [1,] 1 2 [2,] 3 1 [3,] 2 3

[1] 4 3 8

As one StackOverflow user discovered,
the non-continuous submatrix views obtained via
`X.submat( vector_of_row_indices, vector_of_column_indices )`

does **not** provide
the traditional *R* matrix subset. Within `armadillo`

7.200, the ability to
supply multiple matrix notations to
`sub2ind()`

was added. However, it requires a different matrix format than *R*. Instead of
supplying the row and column as one *row* in the matrix, one must supply
each observation as a *column* in the matrix. That is, the subscript matrix
must be \(2 \times n\), where \(n\) is the number of matrix locations. The first
row is associated with row location of the element in the matrix and
the second row is requires the column location of the element.

The result:

[,1] [,2] [,3] [1,] 0 2 1 [2,] 1 0 2

[,1] [,2] [,3] [1,] 4 3 8

`umat`

matrix types to standard `arma`

types.Awhile back, on StackOverflow
question,
someone asked how convert to convert from `arma::umat`

to `arma::mat`

. The
former is the format we have seen used for `arma::find`

.

For the particular example at hand, a call to the `conv_to<T>()`

converter provided the solution. For posterity, we rewrite the answer here.

[,1] [,2] [,3] [1,] 1 4 7 [2,] 2 5 8 [3,] 3 6 9

[,1] [,2] [,3] [1,] 0 0 0 [2,] 1 0 0 [3,] 1 1 0Tweet

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