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

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José  Cancela
José Cancela on 9 Jan 2022
Edited: Matt J on 9 Jan 2022
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?
  1 Comment
dpb
dpb on 9 Jan 2022
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.

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Answers (3)

John D'Errico
John D'Errico on 9 Jan 2022
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.

Image Analyst
Image Analyst on 9 Jan 2022
"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).

Matt J
Matt J on 9 Jan 2022
Edited: Matt J on 9 Jan 2022
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

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