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Python code reading (Part 44): find the location of qualified elements

2022-01-30 08:09:10 FelixZ

Python Code reading ( The first 44 piece )_ Find the location of the eligible element -cover.jpg

Python Code reading collection Introduction : Why not recommend Python Beginners directly look at the project source code

The code read in this article implements the search for the location of qualified elements in the list .

The code snippet read in this article comes from 30-seconds-of-python.

find_index

def find_index(lst, fn):
  return next(i for i, x in enumerate(lst) if fn(x))

# EXAMPLES
find_index([1, 2, 3, 4], lambda n: n % 2 == 1) # 0
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find_index Function receives a list and a discriminant function , Returns the subscript of the first element that makes the discriminant function true .

The function uses a list derivation 、enumerate() and next() Come back to lst First of all Is a discriminant function fn The return of is True The index of the element .

next(iterator[, default]) Get the next element of the iterator . If the iterator runs out , Returns the given default, If there is no default, trigger StopIteration

enumerate(iterable, start=0) Returns an enumeration object .enumerate() Of the iterator returned __next__() Method returns a tuple , It contains a count value ( from start Start , The default is 0) And through iteration iterable Value gained . If start from 0 Start , Equivalent to getting the subscript of the list .

>>> seasons = ['Spring', 'Summer', 'Fall', 'Winter']
>>> list(enumerate(seasons))
[(0, 'Spring'), (1, 'Summer'), (2, 'Fall'), (3, 'Winter')]
>>> list(enumerate(seasons, start=1))
[(1, 'Spring'), (2, 'Summer'), (3, 'Fall'), (4, 'Winter')]
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find_last_index

Allied , Can achieve find_last_index function , Use lst[::-1] Slice form , Reverse the list . And then use len(lst) And the position of the first element in the inverted list that conforms to the discriminant function , Calculate the position of the element in the original list .

def find_last_index(lst, fn):
  return len(lst) - 1 - next(i for i, x in enumerate(lst[::-1]) if fn(x))

# EXAMPLES
find_last_index([1, 2, 3, 4], lambda n: n % 2 == 1) # 2
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