The `morder`

function returns a permutation of row indices which can be used
to rearrangea an object
according to the values in the specified columns (a multi-column ordering).
The `mpermute`

function actually
reorders the rows of a `big.matrix`

or `matrix`

based on
an order vector or a desired ordering on a set of columns.

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`x` |
A |

`cols` |
The columns of |

`na.last` |
for controlling the treatment of |

`decreasing` |
logical. Should the sort order be increasing or decreasing? |

`order` |
A vector specifying the reordering of rows, i.e. the result of a call to |

`allow.duplicates` |
ff |

`...` |
optional parameters to pass to |

The `morder`

function behaves similar to `order`

, returning
a permutation of `1:nrow(x)`

which rearranges objects according to the
values in the specified columns. However, `morder`

takes
a `big.matrix`

or an **R** `matrix`

(with numeric type) and a set of
columns (`cols`

) with which to determine the ordering; `morder`

does
not incur the same memory overhead required by `order`

, and runs more quickly.

The `mpermute`

function changes the row ordering of a `big.matrix`

or `matrix`

based on a vector `order`

or an ordering based
on a set of columns specifed by `cols`

. It should be noted that
this function has side-effects, that is `x`

is changed when this
function is called.

`morder`

returns an ordering vector.

`mpermute`

returns nothing but does change the contents of `x`

.
This type of a side-effect is generally frowned upon in **R**, but we “break”
the rules here to avoid memory overhead and improve performance.

Michael J. Kane <bigmemoryauthors@gmail.com>

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