Extract {mmapcharr}

Create an Implementation of [ For Custom Matrix-Like Types

Description

extract is a function that converts different index types such as negative integer vectors or logical vectors passed to the [ function as i (e.g. X[i]) or i and j (e.g. X[i, j]) into positive integer vectors. The converted indices are provided as the i parameter of extract_vector or i and j parameters of extract_matrix to facilitate implementing the extraction mechanism for custom matrix-like types.

Usage

Extract(extract_vector, extract_matrix)

Arguments

extract_vector

A function in the form of function(x, i) that takes a subset of x based on a single vector of indices i and returns a vector.

extract_matrix

A function in the form of function(x, i, j) that takes a subset of x based on two vectors of indices i and j and returns a matrix.

Details

The custom type must implement methods for dim for this function to work. Implementing methods for nrow and ncol is not necessary as the default method of those generics calls dim internally.

This idea initially comes from package crochet.

Value

A function in the form of function(x, i, j, ..., drop = TRUE) that is meant to be used as a method for [ for a custom type.