Eigen
3.4.90 (git rev a4098ac676528a83cfb73d4d26ce1b42ec05f47c)
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Since the version 3.4, Eigen exposes convenient methods to reshape a matrix to another matrix of different sizes or vector. All cases are handled via the DenseBase::reshaped(NRowsType,NColsType)
and DenseBase::reshaped()
functions. Those functions do not perform in-place reshaping, but instead return a view on the input expression.
The more general reshaping transformation is handled via: reshaped(nrows,ncols)
. Here is an example reshaping a 4x4 matrix to a 2x8 one:
Example: | Output: |
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Matrix4i m = Matrix4i::Random();
cout << "Here is the matrix m:" << endl << m << endl;
cout << "Here is m.reshaped(2, 8):" << endl << m.reshaped(2, 8) << endl;
| Here is the matrix m: -10 1 4 7 -8 -6 9 -10 5 -10 -2 -9 -1 4 0 1 Here is m.reshaped(2, 8): -10 5 1 -10 4 -2 7 -9 -8 -1 -6 4 9 0 -10 1 |
By default, the input coefficients are always interpreted in column-major order regardless of the storage order of the input expression. For more control on ordering, compile-time sizes, and automatic size deduction, please see de documentation of DenseBase::reshaped(NRowsType,NColsType)
that contains all the details with many examples.
A very common usage of reshaping is to create a 1D linear view over a given 2D matrix or expression. In this case, sizes can be deduced and thus omitted as in the following example:
Example: |
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Matrix4i m = Matrix4i::Random();
cout << "Here is the matrix m:" << endl << m << endl;
cout << "Here is m.reshaped().transpose():" << endl << m.reshaped().transpose() << endl;
cout << "Here is m.reshaped<RowMajor>().transpose(): " << endl << m.reshaped<RowMajor>().transpose() << endl;
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Output: |
Here is the matrix m: -10 1 4 7 -8 -6 9 -10 5 -10 -2 -9 -1 4 0 1 Here is m.reshaped().transpose(): -10 -8 5 -1 1 -6 -10 4 4 9 -2 0 7 -10 -9 1 Here is m.reshaped<RowMajor>().transpose(): -10 1 4 7 -8 -6 9 -10 5 -10 -2 -9 -1 4 0 1 |
This shortcut always returns a column vector and by default input coefficients are always interpreted in column-major order. Again, see the documentation of DenseBase::reshaped() for more control on the ordering.
The above examples create reshaped views, but what about reshaping inplace a given matrix? Of course this task in only conceivable for matrix and arrays having runtime dimensions. In many cases, this can be accomplished via PlainObjectBase::resize(Index,Index):
Example: |
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MatrixXi m = Matrix4i::Random();
cout << "Here is the matrix m:" << endl << m << endl;
cout << "Here is m.reshaped(2, 8):" << endl << m.reshaped(2, 8) << endl;
m.resize(2,8);
cout << "Here is the matrix m after m.resize(2,8):" << endl << m << endl;
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Output: |
Here is the matrix m: -10 1 4 7 -8 -6 9 -10 5 -10 -2 -9 -1 4 0 1 Here is m.reshaped(2, 8): -10 5 1 -10 4 -2 7 -9 -8 -1 -6 4 9 0 -10 1 Here is the matrix m after m.resize(2,8): -10 5 1 -10 4 -2 7 -9 -8 -1 -6 4 9 0 -10 1 |
However beware that unlike reshaped
, the result of resize
depends on the input storage order. It thus behaves similarly to reshaped<AutoOrder>
:
Example: |
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cout << "Here is the matrix m:" << endl << m << endl;
cout << "Here is m.reshaped(2, 8):" << endl << m.reshaped(2, 8) << endl;
cout << "Here is m.reshaped<AutoOrder>(2, 8):" << endl << m.reshaped<AutoOrder>(2, 8) << endl;
m.resize(2,8);
cout << "Here is the matrix m after m.resize(2,8):" << endl << m << endl;
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Output: |
Here is the matrix m: -10 -8 5 -1 1 -6 -10 4 4 9 -2 0 7 -10 -9 1 Here is m.reshaped(2, 8): -10 4 -8 9 5 -2 -1 0 1 7 -6 -10 -10 -9 4 1 Here is m.reshaped<AutoOrder>(2, 8): -10 -8 5 -1 1 -6 -10 4 4 9 -2 0 7 -10 -9 1 Here is the matrix m after m.resize(2,8): -10 -8 5 -1 1 -6 -10 4 4 9 -2 0 7 -10 -9 1 |
Finally, assigning a reshaped matrix to itself is currently not supported and will result to undefined-behavior because of aliasing . The following is forbidden:
This is OK: