Reshape

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.

Reshaped 2D views

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:
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: 1804289383 -1550966999 1365180540 336465782 -465790871 -1122281286 304089172 -1868760786 -189735855 -1364114958 35005211 -2309581 719885386 2044897763 -1852781081 1101513929 Here is m.reshaped(2, 8): 1804289383 -189735855 -1550966999 -1364114958 1365180540 35005211 336465782 -2309581 -465790871 719885386 -1122281286 2044897763 304089172 -1852781081 -1868760786 1101513929

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.

1D linear views

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:
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;
Output:
Here is the matrix m: 1804289383 -1550966999 1365180540 336465782 -465790871 -1122281286 304089172 -1868760786 -189735855 -1364114958 35005211 -2309581 719885386 2044897763 -1852781081 1101513929 Here is m.reshaped().transpose(): 1804289383 -465790871 -189735855 719885386 -1550966999 -1122281286 -1364114958 2044897763 1365180540 304089172 35005211 -1852781081 336465782 -1868760786 -2309581 1101513929 Here is m.reshaped<RowMajor>().transpose(): 1804289383 -1550966999 1365180540 336465782 -465790871 -1122281286 304089172 -1868760786 -189735855 -1364114958 35005211 -2309581 719885386 2044897763 -1852781081 1101513929

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.

TutorialReshapeInPlace

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:
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;
Output:
Here is the matrix m: 1804289383 -1550966999 1365180540 336465782 -465790871 -1122281286 304089172 -1868760786 -189735855 -1364114958 35005211 -2309581 719885386 2044897763 -1852781081 1101513929 Here is m.reshaped(2, 8): 1804289383 -189735855 -1550966999 -1364114958 1365180540 35005211 336465782 -2309581 -465790871 719885386 -1122281286 2044897763 304089172 -1852781081 -1868760786 1101513929 Here is the matrix m after m.resize(2,8): 1804289383 -189735855 -1550966999 -1364114958 1365180540 35005211 336465782 -2309581 -465790871 719885386 -1122281286 2044897763 304089172 -1852781081 -1868760786 1101513929

However beware that unlike reshaped, the result of resize depends on the input storage order. It thus behaves similarly to reshaped<AutoOrder>:

Example:
Matrix<int, Dynamic, Dynamic, RowMajor> 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; 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;
Output:
Here is the matrix m: 1804289383 -465790871 -189735855 719885386 -1550966999 -1122281286 -1364114958 2044897763 1365180540 304089172 35005211 -1852781081 336465782 -1868760786 -2309581 1101513929 Here is m.reshaped(2, 8): 1804289383 1365180540 -465790871 304089172 -189735855 35005211 719885386 -1852781081 -1550966999 336465782 -1122281286 -1868760786 -1364114958 -2309581 2044897763 1101513929 Here is m.reshaped<AutoOrder>(2, 8): 1804289383 -465790871 -189735855 719885386 -1550966999 -1122281286 -1364114958 2044897763 1365180540 304089172 35005211 -1852781081 336465782 -1868760786 -2309581 1101513929 Here is the matrix m after m.resize(2,8): 1804289383 -465790871 -189735855 719885386 -1550966999 -1122281286 -1364114958 2044897763 1365180540 304089172 35005211 -1852781081 336465782 -1868760786 -2309581 1101513929

Finally, assigning a reshaped matrix to itself is currently not supported and will result to undefined-behavior because of aliasing . The following is forbidden:

A = A.reshaped(2,8); 

This is OK:

A = A.reshaped(2,8).eval();