emkornfield commented on issue #35576:
URL: https://github.com/apache/arrow/issues/35576#issuecomment-1547404763
the first part of the error is down-casting from float64 (python default
representation to)->float32
```
a = pa.array([545803904.0], type=pa.float32())
a
<pyarrow.lib.FloatArray object at 0x3ebb737bb400>
[
545803900
]
```
Same happens with numpy:
```
numpy.float32(545803904.0)
545803900.0
```
The second part of the error I think is likely due to implementation which
looks like we somehow might do an extra cast through an intermediate value:
```
a = pa.array([545803900.0], type=pa.float64())
print(a.cast(pa.decimal128(38, 18)))
print(a.cast(pa.float32()).cast(pa.decimal128(38, 18)))
```
gives:
[
545803899.999999976169013248
]
[
545803886.966396699654750208
]
I think the second source of error might be:
https://github.com/apache/arrow/blob/cd6e2a4d2b9373b942da18b4cc82cb41431764d9/cpp/src/arrow/util/decimal.cc#L158
since this looks like it it is done in float space (instead of casting to
double) which potentially causes further loss of precision.
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