Q:

Eigen conservativeResize strange behavior

I am using m.conservativeResize() to do the equivalent in Eigen as the reshape function in MatLab. So let N = 3, and then...
static MatrixXd m(N*N,1);

and then I assign this matrix some values, and it looks like this:
1
1
0
1
0
1
0
1
1

and then try to reshape it...
m.conservativeResize(N,N);

So now the same values should be there, but now in N rows and N columns rather than N*N rows and one column.
However that's not what I get. The first column has the first three values in the column vector - OK so far - but then the remaining values just look like garbage values from uninitialized memory:
1  3.08116e-309      0.420085
1 -2.68156e+154    1.2461e-47
0 -2.68156e+154      0.634626

Any idea what I am doing wrong?

A:

conservativeResize() doesn't ``move" the elements around (in other words, doesn't work like MATLABs reshape, since it performs memory re-allocation even if the initial and final sizes are the same). From the documentation:

Resizes the matrix to rows x cols while leaving old values untouched.
...
Matrices are resized relative to the top-left element. In case values need to be appended to the matrix they will be uninitialized.

These statements seem a bit confusing. What it means is the following: think about the initial matrix as a rectangle, of size A x B. Then think about the resized matrix as another rectangle of size C x D. Then mentally overlap the two rectangles, making sure the top-left corner is common to both. The common elements of the intersection are the ones that are preserved by the conservativeResize. The rest just correspond to uninitialized memory.
In case you want a true reshaping, use resize() instead (making absolutely sure that A x B == C x D, otherwise reallocation takes place and all bets are off).

