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Fix BitArray.from_counts/from_samples to not fail for input with only 0 outcome and num_bits=None (backport #12800) #12821

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Jul 26, 2024
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Fix BitArray.from_counts/from_samples to not fail for input with only 0 outcome and num_bits=None (backport #12800) #12821

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7 changes: 5 additions & 2 deletions qiskit/primitives/containers/bit_array.py
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Original file line number Diff line number Diff line change
Expand Up @@ -231,7 +231,7 @@ def from_counts(
Args:
counts: One or more counts-like mappings with the same number of shots.
num_bits: The desired number of bits per shot. If unset, the biggest value found sets
this value.
this value, with a minimum of one bit.

Returns:
A new bit array with shape ``()`` for single input counts, or ``(N,)`` for an iterable
Expand Down Expand Up @@ -275,7 +275,7 @@ def from_samples(
Args:
samples: A list of bitstrings, a list of integers, or a list of hexstrings.
num_bits: The desired number of bits per sample. If unset, the biggest sample provided
is used to determine this value.
is used to determine this value, with a minimum of one bit.

Returns:
A new bit array.
Expand All @@ -298,6 +298,9 @@ def from_samples(
# we are forced to prematurely look at every iterand in this case
ints = list(ints)
num_bits = max(map(int.bit_length, ints))
# convention: if the only value is 0, represent with one bit:
if num_bits == 0:
num_bits = 1

num_bytes = _min_num_bytes(num_bits)
data = b"".join(val.to_bytes(num_bytes, "big") for val in ints)
Expand Down
7 changes: 7 additions & 0 deletions releasenotes/notes/fix-bitarray-fromcounts-nobits-82958a596b3489ec.yaml
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Original file line number Diff line number Diff line change
@@ -0,0 +1,7 @@
---
fixes:
- |
Fixed a bug in :meth:`.BitArray.from_counts` and :meth:`.BitArray.from_samples`.
Previously these would raise an error if given data containing only zeros, and no
value for the optional argument ``num_bits``. Now they produce a :class:`.BitArray`
with :attr:`.BitArray.num_bits` set to 1.
14 changes: 14 additions & 0 deletions test/python/primitives/containers/test_bit_array.py
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Original file line number Diff line number Diff line change
Expand Up @@ -222,6 +222,7 @@ def convert(counts: Counts):

counts1 = convert(Counts({"0b101010": 2, "0b1": 3, "0x010203": 4}))
counts2 = convert(Counts({1: 3, 2: 6}))
counts3 = convert(Counts({0: 2}))

bit_array = BitArray.from_counts(counts1)
expected = BitArray(u_8([[0, 0, 42]] * 2 + [[0, 0, 1]] * 3 + [[1, 2, 3]] * 4), 17)
Expand All @@ -238,6 +239,10 @@ def convert(counts: Counts):
]
self.assertEqual(bit_array, BitArray(u_8(expected), 17))

bit_array = BitArray.from_counts(counts3)
expected = BitArray(u_8([[0], [0]]), 1)
self.assertEqual(bit_array, expected)

def test_from_samples_bitstring(self):
"""Test the from_samples static constructor."""
bit_array = BitArray.from_samples(["110", "1", "1111111111"])
Expand All @@ -246,6 +251,9 @@ def test_from_samples_bitstring(self):
bit_array = BitArray.from_samples(["110", "1", "1111111111"], 20)
self.assertEqual(bit_array, BitArray(u_8([[0, 0, 6], [0, 0, 1], [0, 3, 255]]), 20))

bit_array = BitArray.from_samples(["000", "0"])
self.assertEqual(bit_array, BitArray(u_8([[0], [0]]), 1))

def test_from_samples_hex(self):
"""Test the from_samples static constructor."""
bit_array = BitArray.from_samples(["0x01", "0x0a12", "0x0105"])
Expand All @@ -254,6 +262,9 @@ def test_from_samples_hex(self):
bit_array = BitArray.from_samples(["0x01", "0x0a12", "0x0105"], 20)
self.assertEqual(bit_array, BitArray(u_8([[0, 0, 1], [0, 10, 18], [0, 1, 5]]), 20))

bit_array = BitArray.from_samples(["0x0", "0x0"])
self.assertEqual(bit_array, BitArray(u_8([[0], [0]]), 1))

def test_from_samples_int(self):
"""Test the from_samples static constructor."""
bit_array = BitArray.from_samples([1, 2578, 261])
Expand All @@ -262,6 +273,9 @@ def test_from_samples_int(self):
bit_array = BitArray.from_samples([1, 2578, 261], 20)
self.assertEqual(bit_array, BitArray(u_8([[0, 0, 1], [0, 10, 18], [0, 1, 5]]), 20))

bit_array = BitArray.from_samples([0, 0, 0])
self.assertEqual(bit_array, BitArray(u_8([[0], [0], [0]]), 1))

def test_reshape(self):
"""Test the reshape method."""
# this creates incrementing bitstrings from 0 to 360 * 32 - 1
Expand Down
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