HELP : Subscript indices must either be real positive integers or logicals - AGAIN

I need your help to solve the problem of subscript index that must either be positive integer or logical. Can any one help to correct this ?

A = magic(5);
  M1 = size(A,1);
  M2 = size(A,2);
  E = [5 8 20];
  for x = 1:M1
    for y = 1:M2

    if A(A(x,y) == E)
        A(x-1,y) = 5
      end
    end
    end

Thank you!

Answer 1

[Adam]

By including 'again' in the title I hope that you are learning from previous solutions and that this is a different type of cause to your previous such problems.

The line

A(x-1,y) = 5

fails first time round the loop because x is 1 so x-1 is 0 which is not a valid array index (this is what the error message tells you - it must be a real positive integer or a logical - 0 is neither of those).

How to fix it depends on your intent. Starting your for loop indexed from 2 instead of 1 would remove the error but may not be the solution you want.

[Gobert]

Thank you very much!

[Gobert]

@Adam,

I have another question. See the following code, first:

   A = magic(5);
    M1 = size(A,1);
    M2 = size(A,2);
    E = [5 20];

    for x = 2:M1
       for y = 2:M2

for k = 1:length(E)

        if A(A(x,y) == E(k))
           A(x-1,y) = E(k);
  end
        end
      end
    end
A

I can successfully modify the pixels values at (x-1,y) with reference to (x,y), according to the code provided above. Can you help/tell me how to modify the pixels also at (x+1,y), (x,y-1), and (x,y+ 1) - so that the output of A be equal to [17 5 1 8 15; 5 5 5 20 16; 4 5 20 20 20; 10 12 19 20 3; 11 18 25 2 9] or looks like shown below ?

17    5     1     8     15
 5     5     5     20    16
 4     5     20    20    20
 10    12    19    20     3
 11    18    25     2     9

[Adam]

You should just be able to add the equivalent lines of code to what you already have after

A(x-1,y) = E(k);

[Gobert]

@Adam,

Thanks again but how exactly - so that I can achieve the above mentioned/expected result ?

[Adam]

for x = 2:(M1-1)
   for y = 2:(M2-1)
      for k = 1:length(E)
         if A(A(x,y) == E(k))
            A(x-1,y) = E(k);
            A(x+1,y) = E(k);
            A(x,y-1) = E(k);
            A(x,y+1) = E(k);
         end
      end
   end
end

should probably do the job, though I'm sure there are neater ways.

[Gobert]

The above code did not unfortunately yield the results expected. See the results, it yielded:

A =

      17     5     5     5    15
       5     5     5     5     5
       5     5     5     5     5
       5     5     5     5     5
      11     5     5     5     9

The expected results are shown below:

   A = 
           17    5     1     8     15
            5     5     5     20    16
            4     5     20    20    20
          10    12    19    20     3
          11    18    25     2     9