-
Using block operations
- rvalue, i.e. it was only read from
- lvalues, i.e. you can assign to a block
- Columns and rows
- Corner-related operations
- Block operations for vectors
- Using block operations
| Block operation | Version constructing a dynamic-size block expression |
Version constructing a fixed-size block expression |
|---|---|---|
Block of size (p,q), starting at (i,j)
|
matrix.block(i,j,p,q);
|
matrix.block<p,q>(i,j);
|
- rvalue, i.e. it was only read from
int main()
{
Eigen::MatrixXf m(4,4);
m << 1, 2, 3, 4,
5, 6, 7, 8,
9,10,11,12,
13,14,15,16;
cout << "Block in the middle" << endl;
cout << m.block<2,2>(1,1) << endl << endl;
for (int i = 1; i <= 3; ++i)
{
cout << "Block of size " << i << "x" << i << endl;
cout << m.block(0,0,i,i) << endl << endl;
}
}
Block in the middle
6 7
10 11
6 7
10 11
Block of size 1x1
1
Block of size 2x2
1 2
5 6
Block of size 3x3
1 2 3
5 6 7
9 10 11
- lvalues, i.e. you can assign to a block
int main()
{
Array22f m;
m << 1,2,3,4;
Array44f a = Array44f::Constant(0.6);
cout << "Here is the array a:" << endl << a << endl << endl;
a.block<2,2>(1,1) = m;
cout << "Here is now a with m copied into its central 2x2 block:" << endl << a << endl << endl;
a.block(0,0,2,3) = a.block(2,1,2,3);
cout << "Here is now a with bottom-right 2x3 block copied into top-left 2x2 block:" << endl << a << endl << endl;
}
Here is the array a:
0.6 0.6 0.6 0.6
0.6 0.6 0.6 0.6
0.6 0.6 0.6 0.6
0.6 0.6 0.6 0.6
0.6 0.6 0.6 0.6
0.6 0.6 0.6 0.6
0.6 0.6 0.6 0.6
0.6 0.6 0.6 0.6
Here is now a with m copied into its central 2x2 block:
0.6 0.6 0.6 0.6
0.6 1 2 0.6
0.6 3 4 0.6
0.6 0.6 0.6 0.6
Here is now a with bottom-right 2x3 block copied into top-left 2x2 block:
3 4 0.6 0.6
0.6 0.6 0.6 0.6
0.6 3 4 0.6
0.6 0.6 0.6 0.6
int main()
{
Eigen::MatrixXf m(3,3);
m << 1,2,3,
4,5,6,
7,8,9;
cout << "Here is the matrix m:" << endl << m << endl;
cout << "2nd Row: " << m.row(1) << endl;
m.col(2) += 3 * m.col(0);
cout << "After adding 3 times the first column into the third column, the matrix m is:\n";
cout << m << endl;
}
1 2 3
4 5 6
7 8 9
2nd Row: 4 5 6
After adding 3 times the first column into the third column, the matrix m is:
1 2 6
4 5 18
7 8 30
4 5 6
7 8 9
2nd Row: 4 5 6
After adding 3 times the first column into the third column, the matrix m is:
1 2 6
4 5 18
7 8 30
- Corner-related operations
| Block operation | Version constructing a dynamic-size block expression |
Version constructing a fixed-size block expression |
|---|---|---|
| Top-left p by q block * |
matrix.topLeftCorner(p,q);
|
matrix.topLeftCorner<p,q>();
|
| Bottom-left p by q block * |
matrix.bottomLeftCorner(p,q);
|
matrix.bottomLeftCorner<p,q>();
|
| Top-right p by q block * |
matrix.topRightCorner(p,q);
|
matrix.topRightCorner<p,q>();
|
| Bottom-right p by q block * |
matrix.bottomRightCorner(p,q);
|
matrix.bottomRightCorner<p,q>();
|
| Block containing the first q rows * |
matrix.topRows(q);
|
matrix.topRows<q>();
|
| Block containing the last q rows * |
matrix.bottomRows(q);
|
matrix.bottomRows<q>();
|
| Block containing the first p columns * |
matrix.leftCols(p);
|
matrix.leftCols<p>();
|
| Block containing the last q columns * |
matrix.rightCols(q);
|
matrix.rightCols<q>();
|
int main()
{
m << 1, 2, 3, 4,
5, 6, 7, 8,
9, 10,11,12,
13,14,15,16;
cout << "m.leftCols(2) =" << endl << m.leftCols(2) << endl << endl;
cout << "m.bottomRows<2>() =" << endl << m.bottomRows<2>() << endl << endl;
m.topLeftCorner(1,3) = m.bottomRightCorner(3,1).transpose();
cout << "After assignment, m = " << endl << m << endl;
}
m.leftCols(2) =
1 2
5 6
9 10
13 14
1 2
5 6
9 10
13 14
m.bottomRows<2>() =
9 10 11 12
13 14 15 16
After assignment, m =
8 12 16 4
5 6 7 820:37:13
9 10 11 12
13 14 15 16
- Block operations for vectors
| Block operation | Version constructing a dynamic-size block expression |
Version constructing a fixed-size block expression |
|---|---|---|
Block containing the first n elements *
|
vector.head(n);
|
vector.head<n>();
|
Block containing the last n elements *
|
vector.tail(n);
|
vector.tail<n>();
|
Block containing n elements, starting at position i *
|
vector.segment(i,n);
|
vector.segment<n>(i);
|
int main()
{
Eigen::ArrayXf v(6);
v << 1, 2, 3, 4, 5, 6;
cout << "v.head(3) =" << endl << v.head(3) << endl << endl;
cout << "v.tail<3>() = " << endl << v.tail<3>() << endl << endl;
v.segment(1,4) *= 2;
cout << "after 'v.segment(1,4) *= 2', v =" << endl << v << endl;
}
v.head(3) =
1
2
3
1
2
3
v.tail<3>() =
4
5
6
after 'v.segment(1,4) *= 2', v =
1
4
6
8
10
6