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This page presents the numerous possibilities offered by operator() to index sub-set of rows and columns. This API has been introduced in Eigen 3.4. It supports all the feature proposed by the block API , and much more. In particular, it supports slicing that consists in taking a set of rows, columns, or elements, uniformly spaced within a matrix or indexed from an array of indices.

# Overview

All the aforementioned operations are handled through the generic DenseBase::operator()(const RowIndices&, const ColIndices&) method. Each argument can be:

• An integer indexing a single row or column, including symbolic indices.
• The symbol Eigen::all representing the whole set of respective rows or columns in increasing order.
• An ArithmeticSequence as constructed by the Eigen::seq, Eigen::seqN, or Eigen::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].

More generally, it can accepts any object exposing the following two member functions:

<integral type> operator[](<integral type>) const;
<integral type> size() const;

where <integral type> stands for any integer type compatible with Eigen::Index (i.e. std::ptrdiff_t).

# Basic slicing

Taking a set of rows, columns, or elements, uniformly spaced within a matrix or vector is achieved through the Eigen::seq or Eigen::seqN functions where "seq" stands for arithmetic sequence. Their signatures are summarized below:

function description example
seq(firstIdx,lastIdx)
auto seq(FirstType f, LastType l, IncrType incr)
represents the sequence of integers ranging from firstIdx to lastIdx
seq(2,5) <=> {2,3,4,5}
seq(firstIdx,lastIdx,incr)
same but using the increment incr to advance from one index to the next
seq(2,8,2) <=> {2,4,6,8}
seqN(firstIdx,size)
ArithmeticSequence< typename internal::cleanup_index_type< FirstType >::type, typename internal::cleanup_index_type< SizeType >::type, typename internal::cleanup_seq_incr< IncrType >::type > seqN(FirstType first, SizeType size, IncrType incr)
Definition: ArithmeticSequence.h:162
represents the sequence of size integers starting from firstIdx
seqN(2,5) <=> {2,3,4,5,6}
seqN(firstIdx,size,incr)
same but using the increment incr to advance from one index to the next
seqN(2,3,3) <=> {2,5,8}

The firstIdx and 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(). Here are some examples for a 2D array/matrix A and a 1D array/vector v.

Intent Code Block-API equivalence
Bottom-left corner starting at row i with n columns
A(seq(i,last), seqN(0,n))
static const symbolic::SymbolExpr< internal::symbolic_last_tag > last
Definition: IndexedViewHelper.h:38
A.bottomLeftCorner(A.rows()-i,n)
Block starting at i,j having m rows, and n columns
A(seqN(i,m), seqN(i,n)
A.block(i,j,m,n)
Block starting at i0,j0 and ending at i1,j1
A(seq(i0,i1), seq(j0,j1)
A.block(i0,j0,i1-i0+1,j1-j0+1)
Even columns of A
A(all, seq(0,last,2))
static const Eigen::internal::all_t all
Definition: IndexedViewHelper.h:171
First n odd rows A
A(seqN(1,n,2), all)
The last past one column
A(all, last-1)
A.col(A.cols()-2)
The middle row
A(last/2,all)
A.row((A.rows()-1)/2)
Last elements of v starting at i
v(seq(i,last))
v.tail(v.size()-i)
Last n elements of v
v(seq(last+1-n,last))
v.tail(n)

As seen in the last exemple, referencing the last n elements (or rows/columns) is a bit cumbersome to write. This becomes even more tricky and error prone with a non-default increment. Here comes Eigen::lastN(size) , and Eigen::lastN(size,incr) :

Intent Code Block-API equivalence
Last n elements of v
v(lastN(n))
auto lastN(SizeType size, IncrType incr) -> decltype(seqN(Eigen::last-(size-fix< 1 >()) *incr, size, incr))
Definition: ArithmeticSequence.h:331
v.tail(n)
Bottom-right corner of A of size m times n
v(lastN(m), lastN(n))
A.bottomRightCorner(m,n)
Bottom-right corner of A of size m times n
v(lastN(m), lastN(n))
A.bottomRightCorner(m,n)
Last n columns taking 1 column over 3
A(all, lastN(n,3))

# Compile time size and increment

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:

v(seq(last-fix<7>, last-fix<2>))

In this example Eigen knowns at compile-time that the returned expression has 6 elements. It is equivalent to:

v(seqN(last-7, fix<6>))

We can revisit the even columns of A example as follows:

A(all, seq(0,last,fix<2>))

# Reverse order

Row/column indices can also be enumerated in decreasing order using a negative increment. For instance, one over two columns of A from the column 20 to 10:

A(all, seq(20, 10, fix<-2>))

The last n rows starting from the last one:

A(seqN(last, n, fix<-1>), all)

You can also use the ArithmeticSequence::reverse() method to reverse its order. The previous example can thus also be written as:

A(lastN(n).reverse(), all)

# Array of indices

The generic operator() can also takes as input an arbitrary list of row or column indices stored as either an ArrayXi, a std::vector<int>, std::array<int,N>, etc.

Example:Output:
std::vector<int> ind{4,2,5,5,3};
MatrixXi A = MatrixXi::Random(4,6);
cout << "Initial matrix A:\n" << A << "\n\n";
cout << "A(all,ind):\n" << A(all,ind) << "\n\n";
static const RandomReturnType Random()
Definition: Random.h:113
Initial matrix A:
-10   1   4   7   4  -2
-8  -6   9 -10 -10   4
5 -10  -2  -9  -2   2
-1   4   0   1  -9   9

A(all,ind):
4   4  -2  -2   7
-10   9   4   4 -10
-2  -2   2   2  -9
-9   0   9   9   1



You can also directly pass a static array:

Example:Output:
#if EIGEN_HAS_STATIC_ARRAY_TEMPLATE
MatrixXi A = MatrixXi::Random(4,6);
cout << "Initial matrix A:\n" << A << "\n\n";
cout << "A(all,{4,2,5,5,3}):\n" << A(all,{4,2,5,5,3}) << "\n\n";
#endif
Initial matrix A:
-10   1   4   7   4  -2
-8  -6   9 -10 -10   4
5 -10  -2  -9  -2   2
-1   4   0   1  -9   9

A(all,{4,2,5,5,3}):
4   4  -2  -2   7
-10   9   4   4 -10
-2  -2   2   2  -9
-9   0   9   9   1



or expressions:

Example:Output:
ArrayXi ind(5); ind<<4,2,5,5,3;
MatrixXi A = MatrixXi::Random(4,6);
cout << "Initial matrix A:\n" << A << "\n\n";
cout << "A(all,ind-1):\n" << A(all,ind-1) << "\n\n";
Initial matrix A:
-10   1   4   7   4  -2
-8  -6   9 -10 -10   4
5 -10  -2  -9  -2   2
-1   4   0   1  -9   9

A(all,ind-1):
7   1   4   4   4
-10  -6 -10 -10   9
-9 -10  -2  -2  -2
1   4  -9  -9   0



When passing an object with a compile-time size such as Array4i, std::array<int,N>, or a static array, then the returned expression also exhibit compile-time dimensions.

# Custom index list

More generally, operator() can accept as inputs any object ind of type T compatible with:

Index s = ind.size(); or Index s = size(ind);
i = ind[i];
EIGEN_DEFAULT_DENSE_INDEX_TYPE Index
The Index type as used for the API.
Definition: Meta.h:74

This means you can easily build your own fancy sequence generator and pass it to operator(). Here is an exemple enlarging a given matrix while padding the additional first rows and columns through repetition:

Example:Output:
Index size() const { return out_size; }
Index operator[] (Index i) const { return std::max<Index>(0,i-(out_size-in_size)); }
Index in_size, out_size;
};
Matrix3i A;
A.reshaped() = VectorXi::LinSpaced(9,1,9);
cout << "Initial matrix A:\n" << A << "\n\n";
MatrixXi B(5,5);
static EIGEN_DEPRECATED const RandomAccessLinSpacedReturnType LinSpaced(Sequential_t, Index size, const Scalar &low, const Scalar &high)
Definition: CwiseNullaryOp.h:244
Initial matrix A:
1 4 7
2 5 8
3 6 9

1 1 1 4 7
1 1 1 4 7
1 1 1 4 7
2 2 2 5 8
3 3 3 6 9