How to initialize a two-dimensional array in Python

Question

I m beginning python and I m trying to use a two-dimensional list  that I initially fill up with the same variable in every place  I came up with this   def initialize twodlist foo       twod list          new          for i in range  0  10           for j in range  0  10               new append foo          twod list append new          new        It gives the desired result  but feels like a workaround  Is there an easier shorter more elegant way to do this

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If you use numpy  you can easily create 2d arrays   import numpy as np  row   3 col   5 num   10 x   np full  row  col   num    x  array   10  10  10  10  10           10  10  10  10  10           10  10  10  10  10

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To initialize a two-dimensional array in Python   a     0 for x in range columns   for y in range rows

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twod list     foo for   in range m   for   in range n     for n is number of rows  and m is the number of column  and foo is the value

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You can do just this     element    numcols    numrows   For example    gt  gt  gt     a    3    2    a    a    a      a    a    a      But this has a undesired side effect    gt  gt  gt  b      a   3  3  gt  gt  gt  b    a    a    a      a    a    a      a    a    a     gt  gt  gt  b 1  1   a   gt  gt  gt  b 1  1     b   gt  gt  gt  b    a    b    a      a    b    a      a    b    a

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from random import randint l       for i in range 10       k        for j in range 10           a  randint 1 100          k append a       l append k      print l  print max l 2     b      for i in range 10       a   l i  5      b append a   print min b

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The important thing I understood is  While initializing an array in any dimension  We should give a default value to all the positions of array  Then only initialization completes  After that  we can change or receive new values to any position of the array  The below code worked for me perfectly  N 7 F 2   INITIALIZATION of 7 x 2 array with deafult value as 0 ar   0  F for x in range N     RECEIVING NEW VALUES TO THE INITIALIZED ARRAY for i in range N       for j in range F           ar i  j  int input    print ar

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If it s a sparsely-populated array  you might be better off using a dictionary keyed with a tuple   dict      key    a b  dict key    value

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row 5 col 5   x  col for x in  b for b in range row      The above will give you a 5x5 2D array     0  0  0  0  0     1  1  1  1  1     2  2  2  2  2     3  3  3  3  3     4  4  4  4  4     It is using nested list comprehension  Breakdown as below     x  col for x in  b for b in range row       x  col --  final expression that is evaluated for x in --  x will be the value provided by the iterator  b for b in range row    --  Iterator    b for b in range row    this will evaluate to  0 1 2 3 4  since row 5 so now it simplifies to     x  col for x in  0 1 2 3 4     This will evaluate to    0  5 for x in  0 1 2 3 4   --  with x 0 1st iteration   1  5 for x in  0 1 2 3 4   --  with x 1 2nd iteration   2  5 for x in  0 1 2 3 4   --  with x 2 3rd iteration   3  5 for x in  0 1 2 3 4   --  with x 3 4th iteration   4  5 for x in  0 1 2 3 4   --  with x 4 5th iteration

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t      0  10 for i in  0  10    for each element a new  0  10 will be created

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foo for x in xrange 10   for y in xrange 10

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Code   num rows  num cols   4  2 initial val   0 matrix     initial val    num cols for   in range num rows   print matrix       0  0    0  0    0  0    0  0     initial val must be immutable

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As  Arnab and  Mike pointed out  an array is not a list  Few differences are 1  arrays are fixed size during initialization 2  arrays normally support lesser operations than a list   Maybe an overkill in most cases  but here is a basic 2d array implementation that leverages hardware array implementation using python ctypes c libraries   import ctypes class Array      def   init   self size foo    foo is the initial value         self  size   size         ArrayType   ctypes py object   size         self  array   ArrayType           for i in range size               self  array i    foo     def   getitem   self index           return self  array index      def   setitem   self index value           self  array index    value     def   len   self           return self  size  class TwoDArray      def   init   self columns rows foo           self  2dArray   Array rows foo          for i in range rows               self  2dArray i    Array columns foo       def numRows self           return len self  2dArray      def numCols self           return len  self  2dArray  0       def   getitem   self indexTuple           row   indexTuple 0          col   indexTuple 1          assert row  gt   0 and row  lt  self numRows                    and col  gt  0 and col  lt  self numCols                     Array script out of range          return   self  2dArray  row   col   if   name         main          twodArray   TwoDArray 4 5 5  sample input     print twodArray 2 3

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This way is faster than the nested list comprehensions   x    for x in   foo    10    10       for immutable foo    Here are some python3 timings  for small and large lists   python3 -m timeit   x    for x in   1    10    10   1000000 loops  best of 3  1 55 usec per loop    python3 -m timeit    1 for i in range 10   for j in range 10    100000 loops  best of 3  6 44 usec per loop    python3 -m timeit   x    for x in   1    1000    1000   100 loops  best of 3  5 5 msec per loop    python3 -m timeit    1 for i in range 1000   for j in range 1000    10 loops  best of 3  27 msec per loop   Explanation     foo  10  10 creates a list of the same object repeated 10 times  You can t just use this  because modifying one element will modify that same element in each row   x    is equivalent to list X  but is a bit more efficient since it avoids the name lookup  Either way  it creates a shallow copy of each row  so now all the elements are independent   All the elements are the same foo object though  so if foo is mutable  you can t use this scheme   you d have to use  import copy   copy deepcopy foo  for x in range 10   for y in range 10     or assuming a class  or function  Foo that returns foos    Foo   for x in range 10   for y in range 10

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A pattern that often came up in Python was  bar      for item in some iterable      bar append SOME EXPRESSION    which helped motivate the introduction of list comprehensions  which convert that snippet to  bar    SOME EXPRESSION for item in some iterable    which is shorter and sometimes clearer  Usually you get in the habit of recognizing these and often replacing loops with comprehensions   Your code follows this pattern twice  twod list                                                                    for i in range  0  10                                       new                         can be replaced          this too     for j in range  0  10        with a list                    new append foo          comprehension              twod list append new

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This is the best I ve found for teaching new programmers  and without using additional libraries  I d like something better though   def initialize twodlist value       list        for row in range 10           list append  value  10      return list

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Here is an easier way    import numpy as np twoD   np array     m  n    For initializing all cells with any  x  value use    twoD   np array   x  m  n

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Initializing a 2D matrix of size m X n with 0 m n   map int input   split    l     0 for i in range m   for j in range n   print l

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Often I use this approach for initializing a 2-dimensional array  n   int x  for x in input   split    for i in range int input

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The general pattern to add dimensions could be drawn from this series   x   0 mat1      for i in range 3       mat1 append x      x  1 print mat1    x 0 mat2      for i in range 3       tmp          for j in range 4           tmp append x          x  1     mat2 append tmp   print mat2    x 0 mat3      for i in range 3       tmp          for j in range 4           tmp2              for k in range 5               tmp2 append x              x  1         tmp append tmp2      mat3 append tmp   print mat3

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lst   0  m for i in range n    initialize all matrix n rows and m columns

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You can use a list comprehension   x     foo for i in range 10   for j in range 10     x is now a 10x10 array of  foo   which can depend on i and j if you want

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Don t use   v  n  n  it is a trap    gt  gt  gt  a     0  3  3  gt  gt  gt  a   0  0  0    0  0  0    0  0  0    gt  gt  gt  a 0  0  1  gt  gt  gt  a   1  0  0    1  0  0    1  0  0     but       t      0  3 for i in range 3     works great

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Incorrect Approach    None m  n   gt  gt  gt  m  n   map int  raw input   split    5 5  gt  gt  gt  x 0  0    34  gt  gt  gt  x   34  None  None  None  None    34  None  None  None  None    34  None  None  None  None    34  None  None  None  None    34  None  None  None  None    gt  gt  gt  id x 0  0   140416461589776  gt  gt  gt  id x 3  0   140416461589776  With this approach  python does not allow creating different  address space for the outer columns and  will lead to various misbehaviour than your expectation  Correct Approach but with exception  y     0 for i in range m   for j in range n    gt  gt  gt  id y 0  0      id y 1  0   False  It is good approach but there is exception if you set default value to None  gt  gt  gt  r     None for i in range 5   for j in range 5    gt  gt  gt  r   None  None  None  None  None    None  None  None  None  None    None  None  None  None  None    None  None  None  None  None    None  None  None  None  None    gt  gt  gt  id r 0  0      id r 2  0   True  So set your default value properly using this approach  Absolute correct  Follow the mike s reply of double loop

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To initialize a 2-dimensional array use  arr       m for i in range n    actually  arr       m  n will create a 2D array in which all n arrays will point to same array  so any change in value in any element will be reflected in all n lists  for more further explanation visit   https   www geeksforgeeks org python-using-2d-arrays-lists-the-right-way

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You can try this   0  10  10  This will return the 2d array of 10 rows and 10 columns with value 0 for each cell

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Another way is to use a dictionary to hold a two-dimensional array   twoD      twoD 0 0    0 print twoD 0 0         gt  prints 0   This just can hold any 1D  2D values and to initialize this to 0 or any other int value  use collections   import collections twoD   collections defaultdict int  print twoD 0 0        gt  prints 0 twoD 1 1    1 print twoD 1 1        gt  prints 1

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Matrix    for i in range 0 3     for j in range 0 3       Matrix i j    raw input  Enter the matrix

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use the simplest think to create this   wtod list        and add the size   wtod list     0 for x in xrange 10   for x in xrange 10     or if we want to declare the size firstly  we only use      wtod list     0 for x in xrange 10   for x in xrange 10

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Usually when you want multidimensional arrays you don t want a list of lists  but rather a numpy array or possibly a dict   For example  with numpy you would do something like  import numpy a   numpy empty  10  10   a fill foo

[python] How to initialize a two-dimensional array in Python?

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