Finding sequences and choose first one from that sequence
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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.
1 Commento
Kevin Phung
il 15 Feb 2019
Modificato: Kevin Phung
il 15 Feb 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]?
Risposta accettata
Cris LaPierre
il 15 Feb 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
Più risposte (2)
Cris LaPierre
il 17 Feb 2019
Modificato: Cris LaPierre
il 17 Feb 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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