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PERF: RangeIndex.is_monotonic_inc/dec #13749

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PERF: RangeIndex.is_monotonic_inc/dec #13749

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1 change: 1 addition & 0 deletions doc/source/whatsnew/v0.19.0.txt
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Expand Up @@ -649,6 +649,7 @@ Performance Improvements
- Improved performance of float64 hash table operations, fixing some very slow indexing and groupby operations in python 3 (:issue:`13166`, :issue:`13334`)
- Improved performance of ``DataFrameGroupBy.transform`` (:issue:`12737`)
- Improved performance of ``Index.difference`` (:issue:`12044`)
- Improved performance of ``RangeIndex.is_monotonic_increasing`` and ``is_monotonic_decreasing`` (:issue:`13749`)
- Improved performance of datetime string parsing in ``DatetimeIndex`` (:issue:`13692`)
- Improved performance of hashing ``Period`` (:issue:`12817`)

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8 changes: 8 additions & 0 deletions pandas/indexes/range.py
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Expand Up @@ -221,6 +221,14 @@ def is_unique(self):
""" return if the index has unique values """
return True

@cache_readonly
def is_monotonic_increasing(self):
return self._step > 0 or len(self) <= 1

@cache_readonly
def is_monotonic_decreasing(self):
return self._step < 0 or len(self) <= 1

@property
def has_duplicates(self):
return False
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10 changes: 10 additions & 0 deletions pandas/tests/indexes/test_range.py
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Expand Up @@ -315,6 +315,16 @@ def test_is_monotonic(self):
self.assertTrue(index.is_monotonic_increasing)
self.assertTrue(index.is_monotonic_decreasing)

index = RangeIndex(2, 1)
self.assertTrue(index.is_monotonic)
self.assertTrue(index.is_monotonic_increasing)
self.assertTrue(index.is_monotonic_decreasing)

index = RangeIndex(1, 1)
self.assertTrue(index.is_monotonic)
self.assertTrue(index.is_monotonic_increasing)
self.assertTrue(index.is_monotonic_decreasing)

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Maybe add a test for the empty case as well?

In [23]: pd.RangeIndex().is_monotonic_increasing
Out[23]: True

In [24]: pd.RangeIndex().is_monotonic_decreasing
Out[24]: True

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Although this should maybe rather raise an error (@jreback)

Other index types to raise without arguments, and range() does as well.

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@jorisvandenbossche empty case is coverd by (1, 1).

pd.RangeIndex(1, 1).values
# array([], dtype=int64)

Let me issue a separate PR to prohibit empty construction.

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Ah yes, assumed that RangeIndex() does not has a step yet, but that is of course wrong :-)

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yeah I don't think RangeIndex() should be permitted, though I seem to remember needing it for unpickling or somesuch.......

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let's make a new issue about the empty RangeIndex()

def test_equals(self):
equiv_pairs = [(RangeIndex(0, 9, 2), RangeIndex(0, 10, 2)),
(RangeIndex(0), RangeIndex(1, -1, 3)),
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