Initialising an array of fixed size in python

Question

I would like to know how i can initialise an array or list   yet to be populated with values  to have a defined size   for example in C   int x 5      declared without adding elements     How do i do that in python   Thanks

User · Answer

If you are working with bytes you could use the builtin bytearray. If you are working with other integral types look at the builtin array.

Specifically understand that a list is not an array.

If, for example, you are trying to create a buffer for reading file contents into you could use bytearray as follows (there are better ways to do this but the example is valid):

with open(FILENAME, 'rb') as f:
    data = bytearray(os.path.getsize(FILENAME))
    f.readinto(data)

In this snippet the bytearray memory is preallocated with the fixed length of FILENAMEs size in bytes. This preallocation allows the use of the buffer protocol to more efficiently read the file into a mutable buffer without an array copy. There are yet better ways to do this but I believe this provides one answer to your question.

User · Answer

Well I would like to help you by posting a sample program and its output  Program   t   input     x    None  t y     None  t  t  for i in range 1  t 1       x i-1    i       for j in range 1  t 1           y i-1  j-1    j   print x print y   Output  -  2  1  2    1  2    1  2     I hope this clears some very basic concept of yours regarding their declaration  To initialize them with some other specific values  like initializing them with 0   you can declare them as   x    0  10   Hope it helps

User · Answer

The best bet is to use the numpy library   from numpy import ndarray  a   ndarray  5   int

User · Answer

gt  gt  gt  import numpy  gt  gt  gt  x   numpy zeros  3 4    gt  gt  gt  x array    0    0    0    0             0    0    0    0             0    0    0    0      gt  gt  gt  y   numpy zeros 5      gt  gt  gt  y array   0    0    0    0    0      x is a 2-d array  and y is a 1-d array  They are both initialized with zeros

User · Answer

You can use    gt  gt  gt  lst    None    5  gt  gt  gt  lst  None  None  None  None  None

User · Answer

Why don t these questions get answered with the obvious answer   a   numpy empty n  dtype object    This creates an array of length n that can store objects   It can t be resized or appended to   In particular  it doesn t waste space by padding its length  This is the Python equivalent of Java s  Object   a   new Object n     If you re really interested in performance and space and know that your array will only store certain numeric types then you can change the dtype argument to some other value like int  Then numpy will pack these elements directly into the array rather than making the array reference int objects

User · Answer

One thing I find easy to do is i set an array of empty strings for the size I prefer  for example  Code   import numpy as np  x  np zeros 5 str  print x   Output                      Hope this is helpful

User · Answer

gt  gt  gt  n   5                      length of list  gt  gt  gt  list    None    n          populate list  length n with n entries  None   gt  gt  gt  print list   None  None  None  None  None    gt  gt  gt  list append 1              append 1 to right side of list  gt  gt  gt  list   list -n             redefine list as the last n elements of list  gt  gt  gt  print list   None  None  None  None  1    gt  gt  gt  list append 1              append 1 to right side of list  gt  gt  gt  list   list -n             redefine list as the last n elements of list  gt  gt  gt  print list   None  None  None  1  1    gt  gt  gt  list append 1              append 1 to right side of list  gt  gt  gt  list   list -n             redefine list as the last n elements of list  gt  gt  gt  print list   None  None  1  1  1    or with really nothing in the list to begin with    gt  gt  gt  n   5                      length of list  gt  gt  gt  list                        create list  gt  gt  gt  print list       gt  gt  gt  list append 1              append 1 to right side of list  gt  gt  gt  list   list -n             redefine list as the last n elements of list  gt  gt  gt  print list   1    on the 4th iteration of append    gt  gt  gt  list append 1              append 1 to right side of list  gt  gt  gt  list   list -n             redefine list as the last n elements of list  gt  gt  gt  print list   1 1 1 1    5 and all subsequent    gt  gt  gt  list append 1              append 1 to right side of list  gt  gt  gt  list   list -n             redefine list as the last n elements of list  gt  gt  gt  print list   1 1 1 1 1

User · Answer

Do this    gt  gt  gt  d       None for y in range  2     for x in range  2      gt  gt  gt  d   None  None    None  None    gt  gt  gt  d 0  0    1  gt  gt  gt  d   1  None    None  None     The other solutions will lead to this kind of problem    gt  gt  gt  d       None     2     2  gt  gt  gt  d   None  None    None  None    gt  gt  gt  d 0  0    1  gt  gt  gt  d   1  None    1  None

User · Answer

You can try using Descriptor  to limit the size  class fixedSizeArray object       def   init   self  arraySize 5           self arraySize   arraySize         self array    None    self arraySize      def   repr   self           return str self array       def   get   self  instance  owner           return self array      def append self  index None  value None           print  Append Operation cannot be performed on fixed size array          return      def insert self  index None  value None           if not index and index - 1 not in xrange self arraySize               print  invalid Index or Array Size Exceeded              return         try              self array index    value         except              print  This is Fixed Size Array  Please Use the available Indices    arr   fixedSizeArray 5  print arr arr append 100  print arr arr insert 1  200  print arr arr insert 5  300  print arr   OUTPUT    None  None  None  None  None  Append Operation cannot be performed on fixed size array  None  None  None  None  None   None  200  None  None  None  This is Fixed Size Array  Please Use the available Indices  None  200  None  None  None

User · Answer

An easy solution is x    None  length  but note that it initializes all list elements to None  If the size is really fixed  you can do x  None None None None None  as well  But strictly speaking  you won t get undefined elements either way because this plague doesn t exist in Python

[python] Initialising an array of fixed size in python

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