How to iterate through a section of a 2D list

Question

I want to go through just a section of a 2D list, rather than the whole thing. Here's essentially what it is I want to do: Let's say the user inputs the coordinates [1,1] (so, row 1 column 1) If I have the 2D list:

[[1,3,7],
[4,2,9],
[13,5,6]]

Then I want to iterate through all the elements adjacent to the element at [1,1]

Furthermore, if the element is at a corner or the edge of the 2D list, (so basically if the user enters [0,0], for example) then I want to just want to get back the elements at [0,0] , [0,1] , [1,0] , and [1,1]

So essentially I just want elements adjacent to a specific to a certain point in the 2D array.

Here's what I've done so far: I've made it so that it assigns 4 variables at the start of the code: starting_row , ending_row , starting_column , and ending_column . These variables are assigned values based off of which coordinates the user wants to input (if they the row is 0 or len(list) then the for loop runs accordingly. The same goes for the columns). Then, I use a nested for loop to go through every element

for row in range(row_start, row_end+1):
        for column in range(column_start, column_end+1):
            print(lst[row,column])            

Only thing is, it doesn't seem to work correctly and often outputs the whole entire 2D list when enter a list size of more than 3x3 elements (all the lists will be square lists)

Answer 1

You can slice the list of lists according to the given row and column. For the lower bounds, use max with 0 to avoid slicing with a negative index, but not so for the upper bounds since it is okay for the stopping index of a slice to be out of the range of a list:

def get_adjacent_items(matrix, row, col):
    output = []
    for r in matrix[max(row - 1, 0): row + 2]:
        for i in r[max(col - 1, 0): col + 2]:
            output.append(i)
    return output

or, with a list comprehension:

def get_adjacent_items(matrix, row, col):
    return [i for r in matrix[max(row - 1, 0): row + 2] for i in r[max(col - 1, 0): col + 2]]

so that given:

m = [[1, 3, 7],
     [4, 2, 9],
     [13, 5, 6]]

get_adjacent_items(m, 0, 0) returns: [1, 3, 4, 2]

get_adjacent_items(m, 1, 1) return: [1, 3, 7, 4, 2, 9, 13, 5, 6]

get_adjacent_items(m, 2, 1) returns: [4, 2, 9, 13, 5, 6]

get_adjacent_items(m, 2, 2) returns: [2, 9, 5, 6]