Description Usage Arguments Details Value

`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.

1 | ```
Extract(extract_vector, extract_matrix)
``` |

`extract_vector` |
A function in the form of |

`extract_matrix` |
A function in the form of |

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.**

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.

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