Compare matrix values and add them to another (python)

Question

I am not able to make the structure to compare whether the value at the[l][c] position of one matrix is greater than at the same position of another, and if so, this value goes to that same position in a third matrix:

matrixOne = [[1, 12, 45], [58, 524, 78], [1, 2, 3], [45, 456, 8]]

matrixTwo = [[0, 1, 2], [85, 1, 74], [27, 63, 21], [25, 47, 962]]

matrixTrhee = [[], [], [], []]

for l in range(0, 4):

    for c in range(0, 4):

        if matrixOne[l][c] > matrixTwo[l][c]:

            matrixTrhee[l][c] = matrixOne[l][c]

        else:

            matrixTrhee[l][c] = matrixTwo[l][c]

for l in range(0, 4):

    for c in range(0, 4):

        print(f'[{matrixTrhee}{[l]}{[c]}:^5]', end=' ')

    print()

Answer 1

Running the question code:

matrixOne = [
[ 1,  12, 45], 
[58, 524, 78], 
[ 1,   2,  3],
[45, 456,  8]]

matrixTwo = [
[ 0,  1,  2], 
[85,  1,  74], 
[27, 63,  21], 
[25, 47, 962]]

#Inicializa a lista com todas as posições definidas
matrixTrhee = [[0,0,0] for _ in range(4)]

for l in range(0, 4):   
    #Aqui são três colunas e não quatro. 
    for c in range(0, 3):    
        if matrixOne[l][c] > matrixTwo[l][c]:    
            matrixTrhee[l][c]=matrixOne[l][c]    
        else:    
            matrixTrhee[l][c]=matrixTwo[l][c]

print(*matrixTrhee, sep="\n")
#[ 1,  12,  45]
#[85, 524,  78]
#[27,  63,  21]
#[45, 456, 962]

A more compact alternative would be to flatten (convert to vector 1D) the matrices and with the standard library method itertools, chain.from_iterable(iterable) and compare the corresponding pairs of matrix items.

The algorithm is simple:

• Flatten a matrix and a enumerates the elements.
• Flatten the other matrix.
• Transition by the enumeration matrix and compare taking advantage of the indices which the higher value.
• Regroup the result in the required matrix structure.

Example:

from itertools import chain

matrixOne = [
[ 1,  12, 45], 
[58, 524, 78], 
[ 1,   2,  3],
[45, 456,  8]]

matrixTwo = [
[ 0,  1,  2], 
[85,  1,  74], 
[27, 63,  21], 
[25, 47, 962]]


m1 = enumerate(chain.from_iterable(matrixOne))             #Achata e enumera matrixOne .
m2 = list(chain.from_iterable(matrixTwo))                  #Achata matrixTwo.
m3 = [max(v, m2[i]) for i, v in m1]                        #Itera por m1 e encontra o maior valor.

matrixTrhee= [m3[r: r+3] for r in range(0, len(m3), 3)]  #Reorganiza o resultado em uma matriz 4x3.

print(*matrixTrhee, sep="\n")
#[ 1,  12,  45]
#[85, 524,  78]
#[27,  63,  21]
#[45, 456, 962]

^{Test the code on Ideone}

Answer 2

One of the valid ways to resolve this issue is:

matrixOne = [[1, 12, 45], [58, 524, 78], [1, 2, 3], [45, 456, 8]]
matrixTwo = [[0, 1, 2], [85, 1, 74], [27, 63, 21], [25, 47, 962]]

matrixTrhee = []
for l in range(4):
    line = []
    for c in range(3):
        if matrixOne[l][c] > matrixTwo[l][c]:
            line.append(matrixOne[l][c])
        else:
            line.append(matrixTwo[l][c])
    matrixTrhee.append(line)

print(matrixTrhee)

Note that the first block for traverses the range(4) - equivalent to the number of rows in the matrix matrixTrhee. Then the second block for traverses the range(3) - equivalent to the number Dae columns of the matrix matrixTrhee - by checking whether each element of the matrixOne is greater than the corresponding element of matrixTwo. If yes, it will be added in line the element matrixOne[l][c]. Otherwise, the item matrixTwo[l][c] is added in line.

For each closure of 2nd block for is added line à matrixTrhee.

Finalizing the activities of the two blocks for the following output is displayed.

[[1, 12, 45], [85, 524, 78], [27, 63, 21], [45, 456, 962]]

---

Now, if you prefer to display the array in tabular form, you can use the numpy library. This way the code will be:

import numpy as np

matrixOne = [[1, 12, 45], [58, 524, 78], [1, 2, 3], [45, 456, 8]]
matrixTwo = [[0, 1, 2], [85, 1, 74], [27, 63, 21], [25, 47, 962]]

matrixTrhee = []
for l in range(4):
    line = []
    for c in range(3):
        if matrixOne[l][c] > matrixTwo[l][c]:
            line.append(matrixOne[l][c])
        else:
            line.append(matrixTwo[l][c])
    matrixTrhee.append(line)

print(np.array(matrixTrhee))

After performing all operations the following output will be displayed:

[[  1  12  45]
 [ 85 524  78]
 [ 27  63  21]
 [ 45 456 962]]

---

If you prefer you can also use List Comprehension. This way the code will stay:

import numpy as np

matrixOne = [[1, 12, 45], [58, 524, 78], [1, 2, 3], [45, 456, 8]]
matrixTwo = [[0, 1, 2], [85, 1, 74], [27, 63, 21], [25, 47, 962]]

matrixTrhee = [[matrixOne[l][c] if matrixOne[l][c] > matrixTwo[l][c] else matrixTwo[l][c]
                for c in range(3)] for l in range(4)]

print(np.array(matrixTrhee))

Producing the following output:

[[  1  12  45]
 [ 85 524  78]
 [ 27  63  21]
 [ 45 456 962]]

---

If there is no need to display delimiters - [] - we can use Generating Expressions. This way the code would be:

matrixOne = [[1, 12, 45], [58, 524, 78], [1, 2, 3], [45, 456, 8]]
matrixTwo = [[0, 1, 2], [85, 1, 74], [27, 63, 21], [25, 47, 962]]

matrixTrhee = ((matrixOne[l][c] if matrixOne[l][c] > matrixTwo[l][c] else matrixTwo[l][c]
                for c in range(3)) for l in range(4))

for i in matrixTrhee:
    for j in i:
        print(f'{j:>5}', end='')
    print()

Getting the following output:

    1   12   45
   85  524   78
   27   63   21
   45  456  962

Or, we can simply compare the values and display the results.

matrixOne = [[1, 12, 45], [58, 524, 78], [1, 2, 3], [45, 456, 8]]
matrixTwo = [[0, 1, 2], [85, 1, 74], [27, 63, 21], [25, 47, 962]]

print(f'A matrixTrhee é:')
for l in range(4):
    for c in range(3):
        if matrixOne[l][c] > matrixTwo[l][c]:
            print(f'{matrixOne[l][c]:>5}', end='')
        else:
            print(f'{matrixTwo[l][c]:>5}', end='')
    print()

Then we get the following output:

A matrixTrhee é:
    1   12   45
   85  524   78
   27   63   21
   45  456  962