55 lines
2.8 KiB
Markdown
55 lines
2.8 KiB
Markdown
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# Use the built-in reversed() and Unpack with join()
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```python
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def reverse(text):
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return (''.join(reversed(text)))
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```
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This approach calls the built-in `reversed()` function to return a [reverse iterator](https://docs.python.org/3/library/functions.html#reversed) that is then unpacked by `str.join()`.
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This is equivalent to using a reverse slice, but incurs a bit of extra overhead due to the unpacking/iteration needed by `str.join()`.
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This takes `O(n)` time and `O(n)` space for the reversed copy.
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```python
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def reverse(text):
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output = ''
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for index in reversed(range(len(text))):
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output += text[index]
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return output
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```
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This version uses `reversed()` to reverse a `range()` object rather than feed a start/stop/step to `range()` itself.
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It then uses the reverse range to iterate over the input string and concatenate each code point to a new 'output' string.
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This has over-complicated `reversed()`, as it can be called directly on the input string with almost no overhead.
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This has also incurs the performance hit of repeated concatenation to the 'output' string.
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While this approach _looks_ as if it would be similar to the first, it is actually `O(n**2)` in time complexity due to string concatenation.
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It was also the slowest in benchmarks.
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## Timings vs Reverse Slice
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As a (very) rough comparison, below is a timing table for these functions vs the canonical reverse slice:
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While `reversed()` is very fast, the call to `join()` to unpack slows things down compared to using a reverse slice.
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For long strings, this slight overhead starts to become significant.
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Using `reversed()` but concatenating to a string is non-performant in this context.
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| **string length >>>>** | 5 | 11 | 22 | 52 | 66 | 86 | S142 | 1420 | 14200 | 142000 |
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|------------------------ |:--------: |:--------: |:--------: |:--------: |:--------: |:--------: |:--------: |:--------: |:--------: |:--------: |
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| reverse slice | 1.70e-07 | 1.78e-07 | 1.89e-07 | 2.10e-07 | 2.25e-07 | 2.40e-07 | 3.56e-07 | 1.52e-06 | 1.22e-05 | 1.20e-04 |
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| reverse reversed | 3.71e-07 | 4.77e-07 | 6.78e-07 | 1.20e-06 | 1.63e-06 | 1.01e-06 | 2.78e-06 | 2.47e-05 | 2.44e-04 | 2.40e-03 |
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| reverse reversed range | 6.34e-07 | 1.05e-06 | 1.85e-06 | 3.85e-06 | 4.73e-06 | 6.10e-06 | 9.77e-06 | 1.44e-04 | 1.53e-03 | 1.89e-02 |
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Measurements were taken on a 3.1 GHz Quad-Core Intel Core i7 Mac running MacOS Ventura.
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Tests used `timeit.Timer.autorange()`, repeated 3 times.
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Time is reported in seconds taken per string after calculating the 'best of' time.
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The [`timeit`][timeit] module docs have more details, and [note.nkmk.me][note_nkmk_me] has a nice summary of methods.
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[note_nkmk_me]: https://note.nkmk.me/en/python-timeit-measure/
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[timeit]: https://docs.python.org/3/library/timeit.html#python-interface
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