2D List Mystery

Question

Can someone please walk me through how to get these outputs for this 2D List Mystery code? I'm having trouble following it...

def mystery(data, pos, n):
result = set()
for i in range(0, n):
for j in range(0, n):
result.add(data[i + pos][j + pos])
return result

Suppose that a variable called grid has been declared as follows:
grid = [[8, 2, 7, 8, 2, 1], [1, 5, 1, 7, 4, 7], [5, 9, 6, 7, 3, 2], [7, 8, 7, 7, 7, 9], [4, 2, 6, 9, 2, 3], [2, 2, 8, 1, 1, 3]]

which means it will store the following 6-by-6 grid of values:
827821
151747
596732
787779
426923
228113

For each call below, indicate what value is returned. If the function call results in an error, write "error" instead.

Function Call
mystery(grid, 2, 2)
mystery(grid, 0, 2)
mystery(grid, 3, 3)

Contents of Set Returned:
{6, 7}
{1, 2, 5, 8}
{1, 2, 3, 7, 9}

Answer 1

The position (pos) tells you where to start in that array, e.g. for 0 you start at data[0][0] = 8.
Then n is the number of rows and columns you want to consider (thus a sub-square-matrix so to say). For n=2, range(0,n) means that i and j can have the values 0 and 1 (or in general from 0 to n-1).

Note, that the result is a set, so only numbers that are not yet added to your set will be considered (as {1,2,2} is the same as {1,2}).

Now again if you stride from position data[0][0], the inner loop goes through the columns first, i.e.
data[0][0+0], data[0][1+0]
For the given grid it is 8 and 2.
Then i goes to next row (outer loop)
data[1+0][0+0], data[1+0][1+0]
which is 1 and 5.

You add those numbers together in a set and thus {1,2,5,8}.

For pos=2 and n=2, you consider the numbers
67
77
(figure out why, based on what I explained above)

When added together in a set, it would be {6,7,7,7}, but this is the same as {6,7}, thus the given output.