Welcome to OGeek Q&A Community for programmer and developer-Open, Learning and Share
Welcome To Ask or Share your Answers For Others

Categories

0 votes
274 views
in Technique[技术] by (71.8m points)

python - How to create a dynamic array

As I understand, the list type in Python is a dynamic pointer array, which will increase it's capacity when items are appended to it. And an array in NumPy uses a continuous memory area to hold all the data of the array.

Are there any types that dynamically increase its capacity as a list, and stores the value as a NumPy array? Something like List in C#. And it's great if the type has the same interface as a NumPy array.

I can create a class which wraps a NumPy array inside, and resize this array when it's full, such as:

class DynamicArray(object):
    def __init__(self):
        self._data = np.zeros(100)
        self._size = 0

    def get_data(self):
        return self._data[:self._size]

    def append(self, value):
        if len(self._data) == self._size:
            self._data = np.resize(self._data, int(len(self._data)*1.25))
        self._data[self._size] = value
        self._size += 1

but DynamicArray can't be used as a NumPy array, and I think all the views returned by get_data() before np.resize() will hold the old array.

Edit: array type in array module is dynamic array. The following program test the increase factor of list and array:

from array import array
import time
import numpy as np
import pylab as pl

def test_time(func):
    arrs = [func() for i in xrange(2000)]
    t = []
    for i in xrange(2000):
        start = time.clock()
        for a in arrs:
            a.append(i)
        t.append(time.clock()-start)
    return np.array(t)

t_list = test_time(lambda:[])
t_array = test_time(lambda:array("d"))
pl.subplot(211)
pl.plot(t_list, label="list")
pl.plot(t_array, label="array")
pl.legend()
pl.subplot(212)
pl.plot(np.where(t_list>2*np.median(t_list))[0])
pl.plot(np.where(t_array>2*np.median(t_array))[0])
pl.show()

enter image description here

from the graph: the increase factor of list is bigger than array.

See Question&Answers more detail:os

与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
Welcome To Ask or Share your Answers For Others

1 Reply

0 votes
by (71.8m points)

You may be interested to know that the Python standard library also includes an array module which sounds like just what you want:

This module defines an object type which can compactly represent an array of basic values: characters, integers, floating point numbers. Arrays are sequence types and behave very much like lists, except that the type of objects stored in them is constrained.


与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
OGeek|极客中国-欢迎来到极客的世界,一个免费开放的程序员编程交流平台!开放,进步,分享!让技术改变生活,让极客改变未来! Welcome to OGeek Q&A Community for programmer and developer-Open, Learning and Share
Click Here to Ask a Question

1.4m articles

1.4m replys

5 comments

57.0k users

...