Python cookbook (data structure and algorithm) method for saving the last N elements, pythoncookbook

Python cookbook (data structure and algorithm) method for saving the last N elements, pythoncookbook

This example describes how to save the last N elements in Python. We will share this with you for your reference. The details are as follows:

Problem:We hope to make a limited history statistics for the last few records during iteration or other forms of processing.

Solution:Select collections. deque.

The following code performs a simple text match on a series of text lines. When a match is found, the current match line and the last N lines of text checked are output:

from collections import dequedef search(lines, pattern, history=5):  previous_lines = deque(maxlen=history)  for line in lines:    if pattern in line:      yield line, previous_lines    previous_lines.append(line)# Example use on a fileif __name__ == '__main__':  with open('somefile.txt') as f:    for line, prevlines in search(f, 'python', 5):      for pline in prevlines:        print(pline, end='')      print(line, end='')      print('-'*20)

Just like the code above, when writing code that searches for a record, a generator function containing the yield keyword is usually used, decouple the code that processes the search process from the code that uses the search results. For details about the generators, refer to the content related to the iterators and generators on this site.

deque(maxlen=N)Create a queue with a fixed length. When a new element is added and the queue is full, the oldest record is automatically removed:

>>> from collections import deque>>> q=deque(maxlen=3)>>> q.append(1)>>> q.append(2)>>> q.append(3)>>> qdeque([1, 2, 3], maxlen=3)>>> q.append(4)>>> qdeque([2, 3, 4], maxlen=3)>>> q.append(5)>>> qdeque([3, 4, 5], maxlen=3)>>>

Although you can manually perform such operations (append and del) on the list, the queue solution is much more elegant and faster.

If no queue length is specified, a non-boundary queue is obtained. You can add and pop-up operations at both ends:

>>> q=deque()>>> qdeque([])>>> q.append(1)>>> q.append(2)>>> q.append(3)>>> qdeque([1, 2, 3])>>> q.appendleft(4)>>> qdeque([4, 1, 2, 3])>>> q.pop()3>>> qdeque([4, 1, 2])>>> q.popleft()4>>> qdeque([1, 2])>>>