how to assign rank to each row?

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Rounak Saha Niloy am 13 Sep. 2023
Bearbeitet: Stephen23 am 13 Sep. 2023
I have a matrix as follows:
A=[1 4;
1 4;
4 1;
4 1;
2 2;
2 3;
2 3;
3 2;
3 3];
I want to have another matrix like this-
B=[1;1;2;2;3;4;4;5;6];
Basically, I want to check each row and increase the corresponding value by 1 if iot does not matches with the preceding one. if it matche with the preceding one, I want to keep it as before.
How can I do this?
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Torsten am 13 Sep. 2023
Bearbeitet: Torsten am 13 Sep. 2023
For beginners, a for loop over the rows of A, inside the loop: a comparison of row i with row i-1 and an if-statement that adds 1 to element B(i-1) to get element B(i) if the two rows are not equal and that sets B(i) to B(i-1) if they are equal.

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Akzeptierte Antwort

Stephen23 am 13 Sep. 2023
A = [1,4;1,4;4,1;4,1;2,2;2,3;2,3;3,2;3,3]
A = 9×2
1 4 1 4 4 1 4 1 2 2 2 3 2 3 3 2 3 3
B = cumsum([1;any(diff(A,1,1),2)])
B = 9×1
1 1 2 2 3 4 4 5 6
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Les Beckham am 13 Sep. 2023
Note that diff(A,1,1) is the same as diff(A) so this can be simplified as
A = [1,4;1,4;4,1;4,1;2,2;2,3;2,3;3,2;3,3]
A = 9×2
1 4 1 4 4 1 4 1 2 2 2 3 2 3 3 2 3 3
B = cumsum([1;any(diff(A),2)])
B = 9×1
1 1 2 2 3 4 4 5 6
Stephen23 am 13 Sep. 2023
Bearbeitet: Stephen23 am 13 Sep. 2023
"Note that diff(A,1,1) is the same as diff(A) so this can be simplified as"
Not quite. Consider what happens if one day A happens to have exactly one row... my code still works correctly for any size of matrix A, whereas not specifying the dimension is a latent bug.
For the same reason experienced MATLAB users often prefer to specify the (optional) dimension with many common functions e.g. MAX, MIN, MEAN, etc. when used with non-vector data.

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