How to solve the following exercise?

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Ellen De Jonghe 2020-1-14

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https://ww2.mathworks.cn/matlabcentral/answers/500222-how-to-solve-the-following-exercise

编辑： Andrei Bobrov 2020-1-14

采纳的回答： Andrei Bobrov

Hye!

I have to solve the following problem:

Consider a matrix M with only the numbers 1 to 9 as elements.

M =
   9     3     2     4
   6     4     8     5
   7     1     6     4
   8     9     5     1

Consider one of the elements M(i,j) that's not on the edge of the matrix. Such element always has 8 neighbours. If M(i,j) > 1 and each number from 1 to M(i,j)-1 is one of the 8 neighbours, we say that element is neighboring. If M(i,j) = 1, the element is automatically neighboring.

For example, M(2,2) is neighboring because 1,2,3,4 and 5 are one of the element's neighbours. M(3,4) on the other hand isn't neighboring because 2 and 3 don't occur around the element.

Now, I have to write a function that has 3 inputs: a matrix M and a row- and column index. The function has to control whether de element is neighboring or not and has a logical 0 or 1 as output.

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Guillaume 2020-1-14

@Ellen, you've got the correct algorithm. As you suspec and Bob pointed out, the construction of n can be done in just one line with simple indexing.

@Bob, no the code also works for 1. m would be empty, so ismember will return empty. sum(empty) is 0 which is also the length of empty.

For the record, the one-liner I was talking about is

res = all(ismember(1:matrix(row, col)-1, matrix(row-1:row+1, col-1:col+1)))

Ellen De Jonghe 2020-1-14

Thanks!

I indeed forgot to check the case where M(i,j) = 1, I will try to figure that out.

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采纳的回答

Andrei Bobrov 2020-1-14

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/500222-how-to-solve-the-following-exercise#answer_410087

编辑：Andrei Bobrov 2020-1-14

function out = find_neighbor(M,i,j)
    out = all(ismember(1:M(i,j)-1,M(i-1:i+1,j-1:j+1)));
end

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Guillaume 2020-1-14

The if is not needed, out is true anyway, if M(i,j) is 1, since all([]) is true.

Andrei Bobrov 2020-1-14

Thanks Guillaume! I'm fixed.

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