Pdist2 inside for

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Riccardo Rossi
Riccardo Rossi 2019-1-16
编辑： Jan 2019-1-17
Hi everybody,
i have two 3D matrix A and B with different lengths. I need to build a for loop to calculate the pdist2 between the first row of A and all the rows of B, the second row of A and all the rows of B, ...., the n row of A and all the rows of B.
An example of the two matrix:
A
X Y Z
0.2 0.4 0.5
1.4 0.6 4.5
0.3 0.5 3.1
B
X Y Z
0.8 0.7 0.5
1.4 0.9 1.5
0.2 3.5 2.1
0.3 0.7 0.5
2.4 0.3 0.5
0.2 3.5 3.1
Thank you very much!

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Jan
Jan 2019-1-16
编辑：Jan 2019-1-16
Why? pdist2 works directly with providing a [M1 x N] and a [M2 x N] matrix. So why do you want to write a loop and process one matrix rowwise?
D = pdist2(A, B)
But you write "3D matrix". Maybe you mean "3D array", because matrices are 2D by definition. Then the shown examples are misleading. If A and B are 3D arrays, you for got to mention what you want to do with the 3rd dimension.
  2 个评论
Riccardo Rossi
Riccardo Rossi 2019-1-17
编辑：Riccardo Rossi 2019-1-17
Thank's for answering.
Yes i was wrong, i mean 3D arrays. I need a loop because for each row wise result i have to put an "if" - "else if" statement.
So is it possible write a loop and process one array rowwise?
Thank you!
Jan
Jan 2019-1-17
编辑：Jan 2019-1-17
Yes, of course this is possible. Because I cannot know, what "put an "if" - "else if" statement" exactly means, I cannot post some explicit code. But I suggest to call pdist2 for the matrices. not for single rows, and use logical indexing to examine the results.
% Two 3D arrays, the 2nd dimension must be equal:
A = rand(5, 3, 2);
B = rand(10, 3, 7);
for iA = 1:size(A, 3)
for iB = 1:size(B, 3)
D = pdist2(A(:, :, iA), B(:, :, iB));
match = (D > 0.4); % Or whatever you need
...
end
end
Note: You neither posted the correct input, nor explained, what you want as output, nor asked a question, nor mentioned, what should be computed actually. This makes it hard to help you. Sorry, if my bold guesses are confusing only.

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更多回答（1 个）

Star Strider
Star Strider 2019-1-16
No loop needed, since pdist2 does it all for you:
A = [0.2 0.4 0.5
1.4 0.6 4.5
0.3 0.5 3.1];
B = [0.8 0.7 0.5
1.4 0.9 1.5
0.2 3.5 2.1
0.3 0.7 0.5
2.4 0.3 0.5
0.2 3.5 3.1];
D = pdist2(A,B);
fprintf(1, 'B:\t\t%d\t\t%d\t\t%d\t\t%d\t\t%d\t\t%d\n',1:size(B,1))
fprintf(1, 'A:%d\t%.3f\t%.3f\t%.3f\t%.3f\t%.3f\t%.3f\n', [(1:size(A,1))' D]')
A1B2 = sqrt((A(1,:)-B(2,:))*(A(1,:)-B(2,:))'); % First Row Of A, Second Row Of B
A3B6 = sqrt((A(3,:)-B(6,:))*(A(3,:)-B(6,:))'); % Third Row Of A, Sixth Row Of B
The ‘proof’ is in the last two lines, that show those distances.

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