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Proposal for multiplying a NumPy array by a scalar TransferFunction #1076
Description
Currently, multiplying a SISO TransferFunction object by a NumPy array results in a dimension error:
import control
import numpy as np
tf = control.TransferFunction([1], [1, 0])
arr = np.array([
[1, 0],
[2, 1],
])
print(tf * arr)
print(arr * tf)ValueError: C = A * B: A has 1 column(s) (input(s)), but B has 2 row(s) (output(s)).
I think it would be convenient if output was a MIMO TransferFunction object of the array's dimension. So, for example,
G = control.TransferFunction(
[
[[1], [0]],
[[2], [1]],
],
[
[[1, 0], [1, 0]],
[[1, 0], [1, 0]],
],
)I would be happy to implement this and do a PR, but I want to make sure it's something the maintainers actually want. I would think all SISO systems (state-space, etc) should behave in the same way.
This appears to have been discussed in #459 , but I'm not sure what the conclusion was.
Activity
murrayrm commentedon Dec 6, 2024
I think it makes sense that if a "scalar" I/O system (= SISO system) is used in an operation involving either an ndarray or a MIMO transfer function, it should broadcast to the right shape.
Specific examples/comments:
@to get matrix multiplication) => maybe we want to implement__matmul__?__matmul__, then I think <2x2 MIMO system> @ <1x1 SISO system> should probably generate an error, as it does with ndarrays of those sizes.For now, perhaps we just implement the SISO → MIMO broadcast functionality and leave out
_matmul__? Note that all of this should work for all I/O systems, not just transfer functions. That might be tricky forNonlinearIOSystem's, so perhaps we just implement forLTIsystems (FrequencyResponseData,StateSpace,TransferFuncion).sdahdah commentedon Dec 6, 2024
^ I very much agree with this
^ I'm not sure I follow these two points
^ I do think that this should raise an error if
__matmul__is implemented. My gut feeling is that implementing__matmul__would be confusing though, sinceseries()is essentially "multiplying transfer matrices." Is the idea that*would allow broadcasting and@would not, but otherwise they would function identically?^ I will take a look at this next week and see if I can find a nice way to do it for all I/O systems. If not, I will start small with just LTI.
Should I also look at #1075 at the same time?
murrayrm commentedon Dec 6, 2024
I'll probably take a look at this over the week and if the fix is simple, will submit a PR that you can build on. But if I haven't gotten to it by the time you start, it would be great to take care of #1075 at the same time (it will be in the same block of code).
murrayrm commentedon Dec 7, 2024
#1075 is fixed. Make sure to build on PR #1078 when you make the changes for this issue.
sdahdah commentedon Dec 12, 2024
Thanks. I started working on it earlier this week, and I will probably be ready to do a PR next week.