How can i call the first point that appears in a matrix with a certain value?

How can I create a cicle that reads a matrix and gives back the matrix line with the first appearence in the matrix of a certain value

A=[1 2 3; 4 5 6;7 6 9;6 4 7;9 0 10;3 4 6]
[lin col]=size(A)
for i= 1:lin
  for j= 1:col
    if A(i,j)==6
      firstappearence = i
    end
  end
end

I tried this way however once there is more than one 6 in the matrix firstappearence value would be incorrect. Can you help on this please?

[dpb]

Look at the break instruction...altho NB: it only stops execution of the one loop; as constructed your code will need some additional logic to terminate the second loop.

An alternative to avoid looping could be

>> A=[1 2 3; 4 5 6;7 6 9;6 4 7;9 0 10;3 4 6];
>> MAGICNUMBER=6;
>> [c,r]=find(A.'==6,1)
c =
          3.00
r =
          2.00
>> 

Also above NB: the transpose of A since MATLAB storage/search order is by column. Transposing is then same as searching by row; but then also have to reverse the output variables to refer back to the original array order.

Answer 1

[John D'Errico]

A=[1 2 3; 4 5 6;7 6 9;6 4 7;9 0 10;3 4 6]

A = 6×3

1 2 3 4 5 6 7 6 9 6 4 7 9 0 10 3 4 6

firstappearance = find(A == 6,1,'first')

firstappearance = 4

So the first appearance of a 6 in that matrix is actually the 4th element in the matrix, in the sequence MATLAB stores the elements of a 2-dimensional matrix. Thus the 4th row, first column.

[rowind,colind] = ind2sub(size(A),firstappearance)

rowind = 4

colind = 1

If you wanted MATLAB to look at the rows first, then transpose the matrix before doing the find operation.

Answer 2

[Image Analyst]

"I tried this way however once there is more than one 6 in the matrix firstappearence value would be incorrect."

So, if you want ALL occurrences you can also use find(). Just don't tell it 'first', and get both outputs instead of only 1 output.

A=[1 2 3; 4 5 6;7 6 9;6 4 7;9 0 10;3 4 6]

A = 6×3

1 2 3 4 5 6 7 6 9 6 4 7 9 0 10 3 4 6

[rows, columns] = find(A == 6)

rows = 4×1

4 3 2 6

columns = 4×1

1 2 3 3

So you have 6's at (row, column) = (4, 1) and (3, 2) and (2, 3) and (6, 3).

Answer 3

[Matt J]

A=[1 2 3; 4 5 6;7 6 9;6 4 7;9 0 10;3 4 6];
[~,firstappearance]=max(A==6)

firstappearance = 1×3

4 3 2