Constraint inconsistency? <Pertains to CVXPY, not CVX, so this is on the wrong forum>

MIDCP
1

I have been working with different formulations and variations of the bin packing problem. All of my experiments for this infamous problem were done with Python 3+ and cvxpy (vers 1.1.5). While testing my code I came across the following that IMHO is an inconsistency in defining a cvxpy constraint. Of course, I know that “True” has a value of “1”; but, there is only a single True for the 2nd constraint shown, while the 1st constraint is for m-True’s. The constraint in question has the following form:

\sum_{j=1}^n X_{i,j} = 1; \ i = 1,2,...,m

Here are two different forms of this constraint that I have used:

X = cvxpy.Variable((m,n),boolean=True)
u = numpy.ones((n,1))
e = numpy.ones((m,1))

cst = [X @ u == e] # works as expected (right side is an m x 1 array)

and the other,

cst = [X @ u == 1] # also works but why? (right side is a scalar)

2

This is the CVX forum. CVXPY is a different tool, and has a forum at https://groups.google.com/forum/#!forum/cvxpy

3

This is called broadcasting and is completely standard. It will apply your scalar constraint to each entry of the vector (or bigger shape).

4

Please be aware Mark that I am a member of the cvxpy forum and this is where I submitted my message – perhaps I am missing something. Please provide information on exactly how I should submit a question to the cvxpy forum.
Thanks

5

Thank you Michal. I am aware of “broadcasting” but was unaware that this feature was part of cvxpy. Would you please point me to where this information is provided in the cvxpy documentation.
Thanks again :slightly_smiling_face:

6

If you are posting at https://groups.google.com/forum/#!forum/cvxpy and the post is showing up here, something is wrong. http://ask.cvxr.com/ is the CVX forum, NOT the CVXPY forum.

7

Sorry Mark, :upside_down_face:
I have now found the proper way to submit my comment to the cvxpy forum. I will try to not let this happen again :slightly_smiling_face:

8

I have been looking through the API documentation for cvxpy and did find the following:

…shape (3, 2), then X[:,0] has shape (3,). CVXPY behavior follows NumPy semantics in all cases, with the exception that broadcasting only works when one argument is 0D. Several CVXPY atoms have been renamed: mul_elemwise to multiply ma…

What is 0D? and when was this change made?