How to define a two-dimensional array

Question

I want to define a two-dimensional array without an initialized length like this   Matrix          but it does not work     I ve tried the code below  but it is wrong too   Matrix    5  5    Error   Traceback      IndexError  list index out of range   What is my mistake

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To declare a matrix of zeros  ones    numpy zeros  x  y     e g    gt  gt  gt  numpy zeros  3  5       array    0    0    0    0    0         0    0    0    0    0         0    0    0    0    0       or     numpy ones  x  y   e g    gt  gt  gt  np ones  3  5   array    1    1    1    1    1         1    1    1    1    1         1    1    1    1    1       Even three dimensions are possible   http   www astro ufl edu  warner prog python html see --  Multi-dimensional arrays

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l   0   L  for   in range W     Will be faster than   l     0 for x in range L   for y in range W

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You re technically trying to index an uninitialized array  You have to first initialize the outer list with lists before adding items  Python calls this  list comprehension      Creates a list containing 5 lists  each of 8 items  all set to 0 w  h   8  5  Matrix     0 for x in range w   for y in range h      You can now add items to the list   Matrix 0  0    1 Matrix 6  0    3   error  range     Matrix 0  6    3   valid   Note that the matrix is  y  address major  in other words  the  y index  comes before the  x index    print Matrix 0  0    prints 1 x  y   0  6  print Matrix x  y    prints 3  be careful with indexing     Although you can name them as you wish  I look at it this way to avoid some confusion that could arise with the indexing  if you use  x  for both the inner and outer lists  and want a non-square Matrix

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In Python you will be creating a list of lists   You do not have to declare the dimensions ahead of time  but you can   For example   matrix      matrix append     matrix append     matrix 0  append 2  matrix 1  append 3    Now matrix 0  0     2 and matrix 1  0     3   You can also use the list comprehension syntax   This example uses it twice over to build a  two-dimensional list    from itertools import count  takewhile matrix     i for i in takewhile lambda j  j  lt   k 1    10  count k 10    for k in range 10

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If all you want is a two dimensional container to hold some elements  you could conveniently use a dictionary instead   Matrix        Then you can do   Matrix 1 2    15 print Matrix 1 2    This works because 1 2 is a tuple  and you re using it as a key to index the dictionary  The result is similar to a dumb sparse matrix   As indicated by osa and Josap Valls  you can also use Matrix   collections defaultdict lambda 0  so that the missing elements have a default value of 0   Vatsal further points that this method is probably not very efficient for large matrices and should only be used in non performance-critical parts of the code

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Creates a list containing 5 lists initialized to 0 Matrix     0  5  5   Be careful about this short expression  see full explanation down in  F J s answer

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If you want to create an empty matrix  the correct syntax is  matrix          And if you want to generate a matrix of size 5 filled with 0    matrix     0 for i in xrange 5   for i in xrange 5

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The accepted answer is good and correct  but it took me a while to understand that I could also use it to create a completely empty array   l        for   in range 3     results in

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I read in comma separated files like this   data    for l in infile      l   split          data append l    The list  data  is then a list of lists with index data row  col

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A rewrite for easy reading     2D array  matrix    5 rows  5 cols rows count   5 cols count   5    create       creation looks reverse       create an array of  cols count  cols  for each of the  rows count  rows          all elements are initialized to 0 two d array     0 for j in range cols count   for i in range rows count      index is from 0 to 4       for both rows  amp  cols       since 5 rows  5 cols    use two d array 0  0    1 print two d array 0  0     prints 1     1st row  1st col  top-left element of matrix   two d array 1  0    2 print two d array 1  0     prints 2     2nd row  1st col  two d array 1  4    3 print two d array 1  4     prints 3     2nd row  last col  two d array 4  4    4 print two d array 4  4     prints 4     last row  last col  right  bottom element of matrix

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rows   int input    cols   int input     matrix      for i in range rows     row        for j in range cols       row append 0    matrix append row   print matrix    Why such a long code  that too in Python you ask   Long back when I was not comfortable with Python  I saw the single line answers for writing 2D matrix and told myself I am not going to use 2-D matrix in Python again   Those single lines were pretty scary and It didn t give me any information on what Python was doing  Also note that I am not aware of these shorthands    Anyways  here s the code for a beginner whose coming from C  CPP and Java background  Note to Python Lovers and Experts  Please do not down vote just because I wrote a detailed code

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This is how I usually create 2D arrays in python   col   3 row   4 array     0    col for   in range row     I find this syntax easy to remember compared to using two for loops in a list comprehension

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I m on my first Python script  and I was a little confused by the square matrix example so I hope the below example will help you save some time      Creates a 2 x 5 matrix  Matrix     0 for y in xrange 5   for x in xrange 2     so that  Matrix 1  4    2   Valid Matrix 4  1    3   IndexError  list index out of range

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Using NumPy you can initialize empty matrix like this   import numpy as np mm   np matrix       And later append data like this   mm   np append mm    1 2    axis 1

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If you don t have size information before start then create two one-dimensional lists   list 1  To store rows list 2  Actual two-dimensional matrix   Store the entire row in the 1st list  Once done  append list 1 into list 2   from random import randint  coordinates    temp    points int raw input  Enter No Of Coordinates  gt     for i in range 0 points       randomx randint 0 1000      randomy randint 0 1000      temp        temp append randomx      temp append randomy      coordinates append temp   print coordinates   Output   Enter No Of Coordinates  gt 4   522  96    378  276    349  741    238  439

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If you really want a matrix  you might be better off using numpy  Matrix operations in numpy most often use an array type with two dimensions  There are many ways to create a new array  one of the most useful is the zeros function  which takes a shape parameter and returns an array of the given shape  with the values initialized to zero    gt  gt  gt  import numpy  gt  gt  gt  numpy zeros  5  5   array    0    0    0    0    0             0    0    0    0    0             0    0    0    0    0             0    0    0    0    0             0    0    0    0    0       Here are some other ways to create 2-d arrays and matrices  with output removed for compactness    numpy arange 25  reshape  5  5             create a 1-d range and reshape numpy array range 25   reshape  5  5       pass a Python range and reshape numpy array  5    25  reshape  5  5        pass a Python list and reshape numpy empty  5  5                          allocate  but don t initialize numpy ones  5  5                           initialize with ones   numpy provides a matrix type as well  but it is no longer recommended for any use  and may be removed from numpy in the future

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Try this   rows   int input  Enter rows n    my list      for i in range rows       my list append list map int  input   split

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Here is a shorter notation for initializing a list of lists   matrix     0  5 for i in range 5     Unfortunately shortening this to something like 5  5  0   doesn t really work because you end up with 5 copies of the same list  so when you modify one of them they all change  for example    gt  gt  gt  matrix   5  5  0    gt  gt  gt  matrix   0  0  0  0  0    0  0  0  0  0    0  0  0  0  0    0  0  0  0  0    0  0  0  0  0    gt  gt  gt  matrix 4  4    2  gt  gt  gt  matrix   0  0  0  0  2    0  0  0  0  2    0  0  0  0  2    0  0  0  0  2    0  0  0  0  2

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Here is the code snippet for creating a matrix in python     get the input rows and cols rows   int input  rows       cols   int input  Cols          initialize the list l   0  cols for i in range rows      fill some random values in it for i in range 0 rows       for j in range 0 cols           l i  j    i j    print the list for i in range 0 rows       print       for j in range 0 cols           print l i  j  end        Please suggest if I have missed something

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You can create an empty two dimensional list by nesting two or more square bracing or third bracket      separated by comma  with a square bracing  just like below  Matrix             Now suppose you want to append 1 to Matrix 0  0  then you type  Matrix 0  append 1   Now  type Matrix and hit Enter  The output will be    1        If you entered the following statement instead Matrix 1  append 1   then the Matrix would be       1

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Use   matrix     0  5 for i in range 5     The  5 for the first dimension works because at this level the data is immutable

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You should make a list of lists  and the best way is to use nested comprehensions    gt  gt  gt  matrix     0 for i in range 5   for j in range 5    gt  gt  gt  pprint pprint matrix    0  0  0  0  0     0  0  0  0  0     0  0  0  0  0     0  0  0  0  0     0  0  0  0  0     On your  5  5  example  you are creating a list with an integer  5  inside  and try to access its 5th item  and that naturally raises an IndexError because there is no 5th item    gt  gt  gt  l    5   gt  gt  gt  l 5  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  IndexError  list index out of range

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That s what dictionary is made for  matrix        You can define keys and values in two ways  matrix 0 0    value  or matrix      0 0     value    Result       value   value   value   value   value        value   value   value   value   value

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If you want to be able to think it as a 2D array rather than being forced to think in term of a list of lists  much more natural in my opinion   you can do the following   import numpy Nx 3  Ny 4 my2Dlist  numpy zeros  Nx Ny   tolist     The result is a list  not a NumPy array   and you can overwrite the individual positions with numbers  strings  whatever

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Use   import copy  def ndlist  args  init 0       dp   init     for x in reversed args           dp    copy deepcopy dp  for   in range x       return dp  l   ndlist 1 2 3 4    4 dimensional list initialized with 0 s l 0  1  2  3    1   I do think NumPy is the way to go  The above is a generic one if you don t want to use NumPy

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In case if you need a matrix with predefined numbers you can use the following code   def matrix rows  cols  start 0       return   c   start   r   cols for c in range cols   for r in range rows     assert matrix 2  3  1       1  2  3    4  5  6

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by using list     matrix in python       Roy  80 75 85 90 95    John  75 80 75 85 100    Dave  80 80 80 90 95     by using dict  you can also store this info in the hash table for fast searching like   matrix      1   0 0     2   0 1   3   0 2   4     1 0   5   1 1   6   1 2   7   2 0   8   2 1   9   2 2      matrix  1   will give you  result in O 1  time    nb  you need to deal with a collision in the hash table

[python] How to define a two-dimensional array?

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