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BitArray.get_counts() fails given an empty classical register #12062

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garrison opened this issue Mar 21, 2024 · 3 comments · May be fixed by #12317
Open

BitArray.get_counts() fails given an empty classical register #12062

garrison opened this issue Mar 21, 2024 · 3 comments · May be fixed by #12317
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bug Something isn't working mod: primitives Related to the Primitives module

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@garrison
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garrison commented Mar 21, 2024 •
edited

Environment

  • Qiskit version: main
  • Python version: 3.11
  • Operating system: Linux

What is happening?

BitArray is not fully tested with empty classical registers. For instance, get_counts() fails during the reshape if the output has zero bits. It appears that get_int_counts() will have the same problem, too.

How can we reproduce the issue?

import numpy as np
from qiskit.primitives.containers import BitArray

data = BitArray(np.zeros([1024, 0], dtype=np.uint8), 0)
data.get_counts()

results in

Traceback (most recent call last):
  File "/home/garrison/Qiskit/empty-creg-counts.py", line 7, in <module>
    data.get_counts()
  File "/home/garrison/serverless/.direnv/python-3.11.0/lib/python3.11/site-packages/qiskit/primitives/containers/bit_array.py", line 292, in get_counts
    return self._get_counts(loc=loc, converter=converter)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/garrison/serverless/.direnv/python-3.11.0/lib/python3.11/site-packages/qiskit/primitives/containers/bit_array.py", line 146, in _get_counts
    arr = self._array.reshape(-1, self._array.shape[-1]) if loc is None else self._array[loc]
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: cannot reshape array of size 0 into shape (0)

What should happen?

I expect a sensible result, such as {"": 1024} for get_counts() or {0: 1024} for get_int_counts().

Any suggestions?

No response
@garrison garrison added the bug Something isn't working label Mar 21, 2024
@garrison garrison mentioned this issue Mar 21, 2024
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@atharva-satpute
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Shouldn't it return {} instead of some key-value pairs considering the fact that np.zeros([1024, 0]) is an empty array

@garrison
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Shouldn't it return {} instead of some key-value pairs considering the fact that np.zeros([1024, 0]) is an empty array

That array may be "empty" but it still has a shape. It just means there are 1024 instances of nothing.

Also, following my suggestion means there are fewer special cases a user must worry about; for instance, it maintains that the dict's values will sum up to the total number of shots, even if the register is empty.

@jakelishman jakelishman added the mod: primitives Related to the Primitives module label Apr 14, 2024
@atharva-satpute atharva-satpute linked a pull request May 1, 2024 that will close this issue
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@ihincks
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ihincks commented May 2, 2024

Thanks @garrison . It looks like from_samples also breaks with num_bits=0, so we'd need to fix that too for full support.

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Update BitArray.get_counts() to support zero classical registers atharva-satpute/qiskit-terra
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@garrison @ihincks @jakelishman @atharva-satpute