30 lines
917 B
Markdown
30 lines
917 B
Markdown
---
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title: Unfold list
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type: snippet
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language: python
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tags: [function,list]
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cover: painters-desk
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dateModified: 2020-11-02T19:28:35+02:00
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---
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Builds a list, using an iterator function and an initial seed value.
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- The iterator function accepts one argument (`seed`) and must always return a list with two elements ([`value`, `nextSeed`]) or `False` to terminate.
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- Use a generator function, `fn_generator`, that uses a `while` loop to call the iterator function and `yield` the `value` until it returns `False`.
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- Use a list comprehension to return the list that is produced by the generator, using the iterator function.
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```py
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def unfold(fn, seed):
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def fn_generator(val):
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while True:
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val = fn(val[1])
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if val == False: break
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yield val[0]
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return [i for i in fn_generator([None, seed])]
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```
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```py
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f = lambda n: False if n > 50 else [-n, n + 10]
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unfold(f, 10) # [-10, -20, -30, -40, -50]
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```
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