Return the maximun of a matrix

Question

I want to return the max value of a matrix. For example this is my matrix:

matrix = [[0, 1, 10, 0, 0], [0, 0, 6, 0, 1], [0, 1, 4, 0, 0]]

I want to return the max so here '10'

This is my code but I have an error:

max = 0
for i in range(len(matrix)+1):
   for j in range(len(matrix[0])+1):
      if matrix[i][j] > matrix[i+1][j+1]:
         max =  matrix[i][j]
print(max)

Thanks in advance

Lists of lists are really inefficient for storing matrices. You should consider using numpy instead. — wim, Nov 11 '20 at 15:54
Related/dupe [Most efficient method to find maximum element and list of maximum elements from a 2-d array?](https://stackoverflow.com/q/48195041/674039) — wim, Nov 11 '20 at 18:02

score 6 · Answer 1 · answered Nov 11 '20 at 15:56

6

There are several issues with your code, I suggest you use the max function:

matrix = [[0, 1, 10, 0, 0], [0, 0, 6, 0, 1], [0, 1, 4, 0, 0]]

result = max(max(row) for row in matrix)
print(result)

Output

answered Nov 11 '20 at 15:56

Dani Mesejo

61,499
6
49
76

Pranav Hosangadi · Answer 2 · 2020-11-11T16:47:18.507

Multiple ways to do it:

Fix your method. In Python, lists are zero-based so you need to only iterate from i = 0 to len(matrix) - 1. Doing for i in range(len(matrix)): does this for you. You don't need to do range(len(matrix) + 1)). Also, you should only replace the current maxval if the element you're looking at is greater than maxval.

So,

maxval = -9999999
for i in range(len(matrix)):
   for j in range(len(matrix[i])):
      if matrix[i][j] > maxval: 
         maxval =  matrix[i][j]
print(maxval)
# Out: 10

Or, a more pythonic way is to iterate over the elements instead of accessing them through their indices

maxval = -9999999
for row in matrix:
    for element in row:
        if element > maxval:
            maxval = element

# maxval: 10

Notice I use maxval instead of max so as not to shadow python's inbuilt max() function.

Use numpy (if you're already using it for other things). Like wim mentioned in their comment, a numpy array is a much better way to store matrices instead of lists of lists. Why? See this question

import numpy as np

matrix = [[0, 1, 10, 0, 0], [0, 0, 6, 0, 1], [0, 1, 4, 0, 0]]

maxval = np.max(matrix)
# maxval: 10

Iterate over rows, create a list of max values in each row. Then find the max of this list

matrix = [[0, 1, 10, 0, 0], [0, 0, 6, 0, 1], [0, 1, 4, 0, 0]]

rowmax = [max(row) for row in matrix]
maxval = max(rowmax)

# or in one line:
maxval = max(max(row) for row in matrix)

Use map. This is essentially the same as the previous method.

matrix = [[0, 1, 10, 0, 0], [0, 0, 6, 0, 1], [0, 1, 4, 0, 0]]

maxval = max(map(max, matrix))

score 1 · Answer 3 · answered Nov 11 '20 at 15:58

1

you can try this.

matrix = [[0, 1, 10, 0, 0], [0, 0, 6, 0, 1], [0, 1, 4, 0, 0]]
max1 = 0
for sub_list in  matrix:
    for item in sub_list:
        if item>max1:
            max1=item

answered Nov 11 '20 at 15:58

Amit Kumar

613
3
15

Return the maximun of a matrix

3 Answers3