MATLAB: How to solve the following exercise

neighbours

Hye!

I have to solve the following problem:

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

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

Best Answer

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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MATLAB: Delete rows in 3d-matrix

Thought about this last night in bed and couldn't understand the stupid mindtwist I made, it's just simply:

 stuff(1,:,:)=[]

And indeed it's the upper slice. My bad. Thanks anyway!

MATLAB: How to get numbers as answers? Values show NaN

Because you chose bad variable names, you didn't see what was causing the error. Look at how i goes with m -- they're rows. And j goes with n -- they're columns. But when you come to index the array, you swap the rows and columns:

A(j,:)=A(i,:);

See how j is the first index, which should be the rows? But you associated j and n with columns, meaning they should come in the second place, not the first index place. This code is closer but it's still messed up because of the other problem : no comments. So because of that I can't figure out what you intend, but at least with proper variable names you should be able to figure it out. So dump your code and start repairing this:

function my_rref(A)
[rows, columns]=size(A);
for col=1:columns-1
	for row=col+1:rows
		if abs(A(row,col))>abs(A(col,col))
			t=A(row,:);
			A(row,:)=A(col,:);
			A(col,:)=t;
		end
	end
end
for col=1:columns-1
	for row=col+1:rows
		x=A(row,col)/A(row,row);
		for k=col:rows+1
			A(row,k)=A(row,k)-x*A(col,k);
		end
	end
end
for row=rows:-1:2
	for col=row-1:-1:1
		A(col,:)=A(col,:)-A(row,:)*(A(col,row)/A(row,row));
	end
end
for col=1:columns
	A(col,:)=A(col,:)/A(col,col);
end
disp('Rref of A is')
A

Best Answer

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MATLAB: How to get numbers as answers? Values show NaN

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