How to input matrix (2D list) in Python?

Question

I tried to create this code to input an m by n matrix. I intended to input [[1,2,3],[4,5,6]] but the code yields [[4,5,6],[4,5,6]. Same things happen when I input other m by n matrix, the code yields an m by n matrix whose rows are identical.

Perhaps you can help me to find what is wrong with my code.

m = int(input('number of rows, m = '))
n = int(input('number of columns, n = '))
matrix = []; columns = []
# initialize the number of rows
for i in range(0,m):
  matrix += [0]
# initialize the number of columns
for j in range (0,n):
  columns += [0]
# initialize the matrix
for i in range (0,m):
  matrix[i] = columns
for i in range (0,m):
  for j in range (0,n):
    print ('entry in row: ',i+1,' column: ',j+1)
    matrix[i][j] = int(input())
print (matrix)

Solution 1

The problem is on the initialization step.

for i in range (0,m):
  matrix[i] = columns

This code actually makes every row of your matrix refer to the same columns object. If any item in any column changes - every other column will change:

>>> for i in range (0,m):
...     matrix[i] = columns
... 
>>> matrix
[[0, 0, 0], [0, 0, 0]]
>>> matrix[1][1] = 2
>>> matrix
[[0, 2, 0], [0, 2, 0]]

You can initialize your matrix in a nested loop, like this:

matrix = []
for i in range(0,m):
    matrix.append([])
    for j in range(0,n):
        matrix[i].append(0)

or, in a one-liner by using list comprehension:

matrix = [[0 for j in range(n)] for i in range(m)]

or:

matrix = [x[:] for x in [[0]*n]*m]

See also:

• How to initialize a two-dimensional array in Python?

Hope that helps.

Solution 2

you can accept a 2D list in python this way ...

simply

arr2d = [[j for j in input().strip()] for i in range(n)] 
# n is no of rows

for characters

n = int(input().strip())
m = int(input().strip())
a = [[0]*n for _ in range(m)]
for i in range(n):
    a[i] = list(input().strip())
print(a)

or

n = int(input().strip())
n = int(input().strip())
a = []
for i in range(n):
    a[i].append(list(input().strip()))
print(a)

for numbers

n = int(input().strip())
m = int(input().strip())
a = [[0]*n for _ in range(m)]
for i in range(n):
    a[i] = [int(j) for j in input().strip().split(" ")]
print(a)

where n is no of elements in columns while m is no of elements in a row.

In pythonic way, this will create a list of list

Solution 3

If the input is formatted like this,

1 2 3
4 5 6
7 8 9

a one liner can be used

mat = [list(map(int,input().split())) for i in range(row)] 

explanation with example:

1. input() takes a string as input. "1 2 3"
2. split() splits the string by whitespaces and returns a
   list of strings. ["1", "2", "3"]
3. list(map(int, ...)) transforms/maps the list of strings into a list of ints. [1, 2, 3]
4. All these steps are done row times and these lists are stored in another list.[[1, 2, 3], [4, 5, 6], [7, 8, 9]], row = 3

Solution 4

If you want to take n lines of input where each line contains m space separated integers like:

1 2 3
4 5 6 
7 8 9 

Then you can use:

a=[] // declaration 
for i in range(0,n):   //where n is the no. of lines you want 
 a.append([int(j) for j in input().split()])  // for taking m space separated integers as input

Then print whatever you want like for the above input:

print(a[1][1]) 

O/P would be 5 for 0 based indexing

Solution 5

Apart from the accepted answer, you can also initialise your rows in the following manner - matrix[i] = [0]*n

Therefore, the following piece of code will work -

m = int(input('number of rows, m = '))
n = int(input('number of columns, n = '))
matrix = []
# initialize the number of rows
for i in range(0,m):
    matrix += [0]
# initialize the matrix
for i in range (0,m):
    matrix[i] = [0]*n
for i in range (0,m):
    for j in range (0,n):
        print ('entry in row: ',i+1,' column: ',j+1)
        matrix[i][j] = int(input())
print (matrix)

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Comments

[executable]

Can you explain why this should answer OP question ?

[Taufiq Ahommed Neloy]

would you please give me any reference link or explain how this statement work??

[Julkar9]

@TaufiqAhommedNeloy I have added an explanation.

[Taufiq Ahommed Neloy]

thank you so much for the explanation @Julkar9