Finding sequences and choose first one from that sequence
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GMDI
el 15 de Feb. de 2019
Comentada: GMDI
el 17 de Feb. de 2019
I have a matrix like this :
A=[1 7; 1 8; 2 2; 2 3; 2 4; 2 8; 4 3; 4 4; 4 5; 4 6];
I want to find a matrix (say B) from A as like bellow:
B= [1 7; 2 2; 2 8; 4 3];
That means, if there is a sequence (not sure if this si the exact word to describe the problem!) in the second column of A, then I want only first element of this sequence. For example: I have 1 7 and 1 8 in A..so I want 1 7. Similarly; I have 2 2; 2 3; 2 4; 2 8..so from here I want 2 2 and 2 8. Finally, for 4 3; 4 4; 4 5; 4 6--I want 4 3.
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Kevin Phung
el 15 de Feb. de 2019
Editada: Kevin Phung
el 15 de Feb. de 2019
"For example: I have 1 7 and 1 8 in A..so I want 1 7. Similarly; I have 2 2; 2 3; 2 4; 2 8..so from here I want 2 2 and 2 8. "
This is very contradicting...if youre only taking the first value for each sequence, why are you taking 2 8?
shouldn't your expected result be:
B = [1 7; 2 2; 4 3]?
Respuesta aceptada
Cris LaPierre
el 15 de Feb. de 2019
I agree, it was a bit confusing at first. Group by the first column and then by the max of the diff of values in the second column or 1.
To handle the case where there is a single row in a group ([2,8]), I had to specify min as an anonymous fxn so that I could specify the direction.
A=[1 7; 1 8; 2 2; 2 3; 2 4; 2 8; 4 3; 4 4; 4 5; 4 6];
G = findgroups(A(:,1),[1;max(1,diff(A(:,2)))])
f = @(x) min(x,[],1)
B = splitapply(f,A,G)
B = 4×2
1 7
2 2
2 8
4 3
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Cris LaPierre
el 17 de Feb. de 2019
Editada: Cris LaPierre
el 17 de Feb. de 2019
Simplifying somewhat. Try this
% find magnitude of largest number in column 1
multFac = 10^ceil(log10(max(A(:,1))))
% Plan is to combine the 2 columns with multFac*A(:,1) + A(:,2), essentially grouping them by column 1.
% Take the difference, and remove any rows with sequential values (diff==1)
% This leaves the first value of each sequence
idx = diff(multFac*[0;A(:,1)]+[0;A(:,2)])~=1;
B = A(idx==1,:)
B = 17×2
1 15
1 21
2 8
10 11
10 15
16 15
19 20
19 24
19 28
20 4
22 9
22 15
31 16
37 24
61 26
64 10
76 18
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