Python Program to Construct n*m Matrix from List

We are given a list we need to construct a n*m matrix from that list. For example, a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12] we need to construct a 3*4 matrix so that resultant output becomes [[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]] .

Using List Comprehension

This method uses list comprehension to group elements into sub-lists of length m to form the matrix.

a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
n, m = 3, 4

#This list comprehension divides the list 'a' into sublists of length 'm'. 

m = [a[i:i + m] for i in range(0, n * m, m)]
print(m)

[[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]]

Explanation:

• List comprehension creates sublists of length m (4) by iterating over the range from 0 to n * m (12) with a step size of m (4).
• Resulting list m will contain 3 sublists, each consisting of 4 elements from the original list a

Using numpy

We can create an n*m matrix using NumPy by reshaping a 1D array into a 2D array with the desired dimensions. The numpy.reshape() function enables us to transform the list into a matrix with n rows and m columns, while keeping the total number of elements unchanged.

import numpy as np
a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
n, m = 3, 4

# Convert the list 'a' into a NumPy array, then reshape it.
m = np.array(a).reshape(n, m)
print(m)

[[ 1  2  3  4]
 [ 5  6  7  8]
 [ 9 10 11 12]]

Explanation:

• np.array(a) converts the list a into a NumPy array.
• .reshape(n, m) reshapes the array into a matrix with n rows and m columns, where n * m should match the total number of elements in a.

Using a For Loop

By using a for loop, a list can be split into sublists of size m by iterating through the list in steps of m and appending each slice to a new list. This approach manually creates a matrix by grouping elements into rows of size m.

a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
n, m = 3, 4
mat = []  
for i in range(n):
    # Slice the list 'a' into sublists of size 'm' and append to 'matrix'
    mat.append(a[i*m : (i+1)*m])
print(mat)

[[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]]

Explanation:

• Code iterates n times (3 times) slicing the list a into sublists of length m (4 elements each) during each iteration using a[i*m : (i+1)*m].
• Each sliced sublist (representing a row of the matrix) is appended to the list mat creating a 3x4 matrix.

Using zip() and iter()

This approach groups elements using zip and iter

a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
n, m = 3, 4

# Create a list 'mat' by grouping elements of 'a' into sublists of size 'm' 
mat = [list(x) for x in zip(*[iter(a)] * m)]

print(mat)

[[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]]

Explanation:

• zip(*[iter(a)] * m) creates an iterator over the list a, grouping elements into tuples of size m by using the iter() function to create an iterator and then zipping them together.
• Each tuple of size m is converted into a list and added to mat, resulting in a 3x4 matrix (3 rows and 4 columns).