How to create a fix size list in python

Question

In C    I can create a array like     int  a   new int 10     in python I just know that I can declare a list than append some items or like    l    1 2 3 4  l   range 10    Can I initialize a list by a given size like c   and do not do any assignment

User · Answer

This is more of a warning than an answer  Having seen in the other answers my list    None    10  I was tempted and set up an array like this speakers               10 and came to regret it immensely as the resulting list did not behave as I thought it should  I resorted to   speakers      for i in range 10       speakers append            As             10 appears to create an list where subsequent elements are a copy of the first element  for example    gt  gt  gt  n           10  gt  gt  gt  n                                                                                                       gt  gt  gt  n 0  0     abc   gt  gt  gt  n    abc          abc          abc          abc          abc          abc          abc          abc          abc          abc         gt  gt  gt  n 0  1     True   gt  gt  gt  n    abc    True      abc    True      abc    True      abc    True      abc    True      abc    True      abc    True      abc    True      abc    True      abc    True      Whereas with the  append option    gt  gt  gt  n     gt  gt  gt  for i in range 10        n append                gt  gt  gt  n                                                                                                       gt  gt  gt  n 0  0     abc   gt  gt  gt  n    abc                                                                                                   gt  gt  gt  n 0  1     True   gt  gt  gt  n    abc    True                                                                                                I m sure that the accepted answer by ninjagecko does attempt to mention this  sadly I was too thick to understand  Wrapping up  take care

User · Answer

tl dr  The exact answer to your question is numpy empty or numpy empty like  but you likely don t care and can get away with using myList    None  10000     Simple methods  You can initialize your list to all the same element  Whether it semantically makes sense to use a non-numeric value  that will give an error later if you use it  which is a good thing  or something like 0  unusual  maybe useful if you re writing a sparse matrix or the  default  value should be 0 and you re not worried about bugs  is up to you    gt  gt  gt   None for   in range 10    None  None  None  None  None  None  None  None  None  None     Here   is just a variable name  you could have used i    You can also do so like this    gt  gt  gt   None  10  None  None  None  None  None  None  None  None  None  None    You probably don t need to optimize this  You can also append to the array every time you need to    gt  gt  gt  x       gt  gt  gt  for i in range 10    gt  gt  gt     x append i      Performance comparison of simple methods  Which is best    gt  gt  gt  def initAndWrite test         x    None  10000      for i in range 10000         x i    i       gt  gt  gt  def initAndWrite2 test         x    None for   in range 10000        for i in range 10000         x i    i       gt  gt  gt  def appendWrite test         x           for i in range 10000         x append i    Results in python2 7    gt  gt  gt  import timeit  gt  gt  gt  for f in  initAndWrite test  initAndWrite2 test  appendWrite test        print     takes    usec loop  format f   name    timeit timeit f  number 1000  1000        initAndWrite test takes 714 596033096 usec loop initAndWrite2 test takes 981 526136398 usec loop appendWrite test takes 908 597946167 usec loop   Results in python 3 2   initAndWrite test takes 641 3581371307373 usec loop initAndWrite2 test takes 1033 6499214172363 usec loop appendWrite test takes 895 9040641784668 usec loop   As we can see  it is likely better to do the idiom  None  10000 in both python2 and python3  However  if one is doing anything more complicated than assignment  such as anything complicated to generate or process every element in the list   then the overhead becomes a meaninglessly small fraction of the cost  That is  such optimization is premature to worry about if you re doing anything reasonable with the elements of your list     Uninitialized memory  These are all however inefficient because they go through memory  writing something in the process  In C this is different  an uninitialized array is filled with random garbage memory  sidenote  that has been reallocated from the system  and can be a security risk when you allocate or fail to mlock and or fail to delete memory when closing the program   This is a design choice  designed for speedup  the makers of the C language thought that it was better not to automatically initialize memory  and that was the correct choice   This is not an asymptotic speedup  because it s O N    but for example you wouldn t need to first initialize your entire memory block before you overwrite with stuff you actually care about  This  if it were possible  is equivalent to something like  pseudo-code  x   list size 10000    If you want something similar in python  you can use the numpy numerical matrix N-dimensional-array manipulation package  Specifically  numpy empty or numpy empty like  That is the real answer to your question

User · Answer

Python has nothing built-in to support this. Do you really need to optimize it so much as I don't think that appending will add that much overhead.

However, you can do something like l = [None] * 1000.

Alternatively, you could use a generator.

User · Answer

It s not really the python way to initialize lists like this  Anyway  you can initialize a list like this    gt  gt  gt  l    None    4  gt  gt  gt  l  None  None  None  None

User · Answer

Note also that when you used arrays in C++ you might have had somewhat different needs, which are solved in different ways in Python:

You might have needed just a collection of items; Python lists deal with this usecase just perfectly.
You might have needed a proper array of homogenous items. Python lists are not a good way to store arrays.

Python solves the need in arrays by NumPy, which, among other neat things, has a way to create an array of known size:

from numpy import *

l = zeros(10)

User · Answer

your list    None  size required

User · Answer

You can do it using array module  array module is part of python standard library   from array import array from itertools import repeat  a   array  i   repeat 0  10     or a   array  i    0  10    repeat function repeats 0 value 10 times  It s more memory efficient than  0  10  since it doesn t allocate memory  but repeats returning the same number x number of times

User · Answer

You can use this   None    10  But this won t be  fixed size  you can still append  remove     This is how lists are made   You could make it a tuple  tuple  None    10   to fix its width  but again  you won t be able to change it  not in all cases  only if the items stored are mutable    Another option  closer to your requirement  is not a list  but a collections deque with a maximum length  It s the maximum size  but it could be smaller   import collections max 4 items   collections deque  None    4  maxlen 4    But  just use a list  and get used to the  pythonic  way of doing things

User · Answer

fix array   numpy empty n  dtype   object    where n is the size of your array  though it works  it may not be the best idea as you have to import a library for this purpose  Hope this helps

[python] How to create a fix size list in python?

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