Collect double values of a matrix

8 views (last 30 days)
Hi,
I have got a question. I have a matrix with three columns (X,Y,Z coordinates) and nearly 60000 rows. Now, I would like to compare the X and Z values of all 60000 rows. All rows whose X and Z values only occur once should be deleted, only the rows whose X and Z value occur twice or more should be regarded. Next, the rows with the equal X and Z value should be compared and the row with the higher Y value should be written into a new matrix.
I thought about using a loop for the search of the rows with equal X and Z values but the computing time is quite high, is there a better way? And how do I define the "do"?
for k = 1:end
for m = k+1:end
if A(k,1) == A(m,1) && A(k,3) ==A(m,3)
do something
end
end
end
I haven't thought about the loop for the search of the higher Y value, yet.
Yours,
Tobias

Accepted Answer

Andrei Bobrov
Andrei Bobrov on 9 Oct 2019
Edited: Andrei Bobrov on 9 Oct 2019
Let xyz - your array (n x 3);
[~,~,c] = unique(xyz(:,[1,3]),'rows','stable'); % c - serial numbers of rows with same values in first and last columns.
d = accumarray(c,1); % d - the number of specific rows with the same first and last columns. Here first row of 'd' c = 1, nth row - c = n.
[lo,i] = ismember(c,find(d > 1)); % duplicate rows in xyz.
j = accumarray(i(lo),find(lo),[],@(x)x(max(xyz(x,2)) == xyz(x,2))); % index of the row with the maximum value in the second column.
out = xyz(j,:);
  3 Comments
Tobias Riewendt
Tobias Riewendt on 9 Oct 2019
Ok, I will do it. Thanks for your answer.

Sign in to comment.

More Answers (2)

Martin C.
Martin C. on 8 Oct 2019
hmm well to count occurences you could somehow use a hashmap
containers.Map in matlab. that would be efficient.

Turlough Hughes
Turlough Hughes on 8 Oct 2019
Edited: Turlough Hughes on 9 Oct 2019
Ok Following your comment I've edited my answer:
I would first find rows which have equal X and Z values.
c=1;
for ii=1:size(A,1)
if A(ii,1)==A(ii,3)
id_s(c)=ii;
c=c+1;
end
end
B=A(id_s,:);
Rows in A with equal X and Z value and are assigned to B.
To ensure a given X (and hence a given Z) of a row is repeated in another row I find the rows where this is not the case and delete them.
[~,ix] = unique(B(:,1)); % so B(ix,1) includes all values in B(:,1) without repeating any.
Xtw=B(:,1);
Xtw(ix)=[]; % Xtw - includes all values in X that occurred two or more times.
[~,i1x]=setdiff(B(:,1),Xtw); % Gets index for rows in B where the x value is not repeated elsewhere.
B(i1x,:)=[];
  1 Comment
Tobias Riewendt
Tobias Riewendt on 9 Oct 2019
That is not exactly what I want. I try to give an example:
[X , Y , Z]:
[1 2 3
2 5 3
7 7 7
7 7 9
1 5 3]
The result should be a matrix of the rows with an equal X AND Z value. All other rows should be deleted. This means:
[1 2 3
1 5 3]

Sign in to comment.

Categories

Find more on Loops and Conditional Statements in Help Center and File Exchange

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!