Matlab Matrix: Eliminating Duplicate Entries

Hello, all, thanks for reading this post.

I have a problem with a point coordinate matrix I create. I output the matrix correctly, but one of the bugs in the program I inherited is every point after point 1 is duplicated. I looks something like:

         0         0         0
         0         0    3.0000
         0         0    3.0000
         0    1.4225    4.8659
         0    1.4225    4.8659
         0   -1.4269    4.8520
         0   -1.4269    4.8520
    1.1125    2.3073    6.0264
    1.1125    2.3073    6.0264
   -1.1160    2.3007    6.0177
   -1.1160    2.3007    6.0177
    1.1086   -2.3144    6.0039
    1.1086   -2.3144    6.0039
   -1.1120   -2.3078    5.9953
   -1.1120   -2.3078    5.9953

for 8 coordinates (and 7 connections, as this is a binary tree).

Is there a way to output a new point coordinate matrix where I have 8 rows, and no duplicate points?

Thanks

Edit: unique(A,'rows') will work in my case (because the point coordinates are exactly the same), but when I use it it outputs the coordinates in alpha-numeric order. Is there a way to output the coordinates in their original order, minus the duplicates?

Answer 1

[Fangjun Jiang]

unique(A,'rows')

[Walter Roberson]

Note: if the above does not work, then the duplicates might not be *exact* duplicates -- e.g., the difference between them might be on the order of 1E-15 . If that is the case, you might need a different strategy.

[Brian]

Actually, unique(A,'rows') will work in my case (because the point coordinates are exactly the same), but when I use it it outputs the coordinates in alpha-numeric order. Is there a way to output the coordinates in their original order, minus the duplicates?

[Fangjun Jiang]

Yes. the output of unique() is sorted. You can do this to get the original order.

a=[9 8 9 7 6 7 5 4 8];

[b,ind]=unique(a,'first');

b=a(sort(ind));

[Brian]

Thanks, I tried that but when I did it it only sorted the first column and deleted the other three (making it a 1 by 8 array, effectively deleting the y and z coordinates).

What I mean is: it turned:

0 0 0

0 0 3.0000

0 0 3.0000

0 1.4225 4.8659

0 1.4225 4.8659

0 -1.4269 4.8520

0 -1.4269 4.8520

1.1125 2.3073 6.0264

1.1125 2.3073 6.0264

-1.1160 2.3007 6.0177

-1.1160 2.3007 6.0177

1.1086 -2.3144 6.0039

1.1086 -2.3144 6.0039

-1.1120 -2.3078 5.9953

-1.1120 -2.3078 5.9953

into:

0

0

0

0

1.1125

-1.1160

1.1086

-1.1120

Which is right for the x coordinates. Is it possible to extend the sort to the entire matrix, not just the first column?

[Walter Roberson]

Using the indexing I suggested in my answer would deal with these issues much more readily...

[Fangjun Jiang]

It's a matter of selecting all the columns.

b=a(sort(ind),:)

Answer 2

[Walter Roberson]

A(1:2:end,:)

This does not have the problem with comparing nearly equal quantities, but it does assume that the second row of each pair is acceptable

[Brian]

Thanks, this worked exactly as I wanted to! Sorry I didn't see your comment until now, but this worked exactly as I needed it to.