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Update slicing tutorial docs to reflect Eigen::placeholders namespace

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libeigen/eigen!2360

Closes #3064

Co-authored-by: Rasmus Munk Larsen <rmlarsen@gmail.com>
This commit is contained in:
Rasmus Munk Larsen
2026-03-29 18:51:02 -07:00
parent 09581fda38
commit 2776ba55eb
1 changed files with 17 additions and 2 deletions
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19
doc/TutorialSlicingIndexing.dox
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@@ -18,6 +18,21 @@ Each argument can be:
- An ArithmeticSequence as constructed by the Eigen::seq, Eigen::seqN, or Eigen::placeholders::lastN functions.
- Any 1D vector/array of integers including %Eigen's vector/array, expressions, std::vector, std::array, as well as plain C arrays: `int[N]`.
The symbols \c last, \c lastp1, \c end, \c all, and \c lastN live in the Eigen::placeholders namespace,
while \c seq, \c seqN, and \c fix live directly in the Eigen namespace.
For convenience, they are all re-exported via the Eigen::indexing namespace.
To make the code examples on this page compile, add one of the following:
\code
using namespace Eigen::indexing;
// or individually:
using Eigen::placeholders::last;
using Eigen::placeholders::all;
using Eigen::placeholders::lastN;
using Eigen::seq;
using Eigen::seqN;
using Eigen::fix;
\endcode
More generally, it can accepts any object exposing the following two member functions:
\code
<integral type> operator[](<integral type>) const;
@@ -57,7 +72,7 @@ Taking a set of rows, columns, or elements, uniformly spaced within a matrix or
</tr>
</table>
The \c firstIdx and \c lastIdx parameters can also be defined with the help of the Eigen::last symbol representing the index of the last row, column or element of the underlying matrix/vector once the arithmetic sequence is passed to it through operator().
The \c firstIdx and \c lastIdx parameters can also be defined with the help of the Eigen::placeholders::last symbol representing the index of the last row, column or element of the underlying matrix/vector once the arithmetic sequence is passed to it through operator().
Here are some examples for a 2D array/matrix \c A and a 1D array/vector \c v.
<table class="manual">
<tr>
@@ -148,7 +163,7 @@ Here comes \ref Eigen_placeholders_lastN :
In terms of performance, %Eigen and the compiler can take advantage of compile-time size and increment.
To this end, you can enforce compile-time parameters using Eigen::fix<val>.
Such compile-time value can be combined with the Eigen::last symbol:
Such compile-time value can be combined with the Eigen::placeholders::last symbol:
\code v(seq(last-fix<7>, last-fix<2>))
\endcode
In this example %Eigen knowns at compile-time that the returned expression has 6 elements.