4 ms·

Show me an example that uses `reduce` with a function other than sum/product that makes it clearer than the equivalent loop.

ramblenode 10y ago
Reduce doesn't have side effects, handles an empty collection, doesn't need to reference the index of the iterable, and self-documents the code as a fold. I find the last part nice because a loop or a Python comprehension would need to add a comment to explain what would otherwise be clear from the syntax. Here's an alternative example where I think it makes sense (checking if the size of all elements in a collection are equal): reduce(lambda a, b: len(a) == len(b), ls)
- dalke 10y ago
  I don't think that does what you think it does: >>> reduce(lambda a, b: len(a) == len(b), ["a", "b", "c"]) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 1, in <lambda> TypeError: object of type 'bool' has no len() It's trying to do len(len("a") == len("b")) == len("c"), which becomes len(True) == len("c"), and len(True) doesn't work. Some variations which do work are: len(set(map(len, ls))) == 1 len(set(len(x) for x in ls)) == 1 all(len(ls[0]) == len(x) for x in ls) len(min(ls, key=len)) == len(max(ls, key=len)) Append "if ls else True" to handle the empty list.
  - reinhardt 10y ago
    > I don't think that does what you think it does: Heh, just proved my point. Reduce won't be missed.
  - ramblenode 10y ago
    Ouch, good catch! I can't think of a nice way to salvage reduce but these are some clever alternatives. Point anti-reduce.
    - dalke 10y ago
      This doesn't salvage it in any meaningful sense, but it does work: class EqualLengths: def __init__(self, size): self.size = size def __nonzero__(self): return True def __repr__(self): return "True" def equal_lengths(a, b): if isinstance(a, EqualLengths): if a.size == len(b): return a return False if a is False: return a n = len(a) if n == len(b): return EqualLengths(n) return False >>> reduce(equal_lengths, ["a", "b", "cc", "d"]) False >>> reduce(equal_lengths, ["a", "b", "c", "d"]) True However, I could do something similar with sum(): class SumEqual: def __init__(self): self.size = None self.is_equal = True def __add__(self, other): if self.is_equal: if self.size is None: self.size = len(other) else: self.is_equal = (self.size == len(other)) return self def __nonzero__(self): return self.is_equal def __repr__(self): return repr(self.is_equal) >>> sum(["a", "b", "cc", "d"], SumEqual()) False >>> sum(["a", "b", "c", "d"], SumEqual()) True and unlike the first case, this sum() solution will work when there are fewer than two items in the list.

ramblenode 10y ago

Reduce doesn't have side effects, handles an empty collection, doesn't need to reference the index of the iterable, and self-documents the code as a fold. I find the last part nice because a loop or a Python comprehension would need to add a comment to explain what would otherwise be clear from the syntax. Here's an alternative example where I think it makes sense (checking if the size of all elements in a collection are equal): reduce(lambda a, b: len(a) == len(b), ls)

dalke 10y ago
I don't think that does what you think it does: >>> reduce(lambda a, b: len(a) == len(b), ["a", "b", "c"]) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 1, in <lambda> TypeError: object of type 'bool' has no len() It's trying to do len(len("a") == len("b")) == len("c"), which becomes len(True) == len("c"), and len(True) doesn't work. Some variations which do work are: len(set(map(len, ls))) == 1 len(set(len(x) for x in ls)) == 1 all(len(ls[0]) == len(x) for x in ls) len(min(ls, key=len)) == len(max(ls, key=len)) Append "if ls else True" to handle the empty list.
- reinhardt 10y ago
  > I don't think that does what you think it does: Heh, just proved my point. Reduce won't be missed.
- ramblenode 10y ago
  Ouch, good catch! I can't think of a nice way to salvage reduce but these are some clever alternatives. Point anti-reduce.
  - dalke 10y ago
    This doesn't salvage it in any meaningful sense, but it does work: class EqualLengths: def __init__(self, size): self.size = size def __nonzero__(self): return True def __repr__(self): return "True" def equal_lengths(a, b): if isinstance(a, EqualLengths): if a.size == len(b): return a return False if a is False: return a n = len(a) if n == len(b): return EqualLengths(n) return False >>> reduce(equal_lengths, ["a", "b", "cc", "d"]) False >>> reduce(equal_lengths, ["a", "b", "c", "d"]) True However, I could do something similar with sum(): class SumEqual: def __init__(self): self.size = None self.is_equal = True def __add__(self, other): if self.is_equal: if self.size is None: self.size = len(other) else: self.is_equal = (self.size == len(other)) return self def __nonzero__(self): return self.is_equal def __repr__(self): return repr(self.is_equal) >>> sum(["a", "b", "cc", "d"], SumEqual()) False >>> sum(["a", "b", "c", "d"], SumEqual()) True and unlike the first case, this sum() solution will work when there are fewer than two items in the list.