Problems when sort 'sym' variables and 'vpa' double-precision number.

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Songbai Jin
Songbai Jin 2022-4-27
评论： Songbai Jin 2022-5-6
% this is my data
X = [-7.82 -4.58 -3.97
-6.68 3.16 2.71
4.36 -2.19 2.09
6.72 0.88 2.80
-8.64 3.06 3.50
-6.87 0.57 -5.45
4.47 -2.62 5.76
6.73 -2.01 4.18
-7.71 2.34 -6.33
-6.91 -0.49 -5.68
6.18 2.81 5.82
6.72 -0.93 -4.04
-6.25 -0.26 0.56
-6.94 -1.22 1.13
8.09 0.20 2.25
6.81 0.17 -4.15
-5.19 4.24 4.04
-6.38 -1.74 1.43
4.08 1.30 5.33
6.27 0.93 -2.78];
beta = 0.01;
u1 = [1,1,1];
u2 = [-1,-1,-1];
u = [u1; u2];
% method 1
D1 = vpa(1 - exp(sym(-beta*pdist2(X,u).^2)));
[~, I1] = sort(D1, 2);
I1
% method 2
D2 = 1 - exp(sym(-beta*pdist2(X,u).^2));
[~, I2] = sort(D2, 2);
I2
I1 and I2 are different. The only difference is to use "vpa" or not.
How to solve this problem?
Thank you very much!

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回答（1 个）

David Hill
David Hill 2022-5-6
Why are you using symbolics? Double should be sufficient (these are not large numbers).
D1 = (1 - exp(-beta*pdist2(X,u).^2));
[~, I1] = sort(D1, 2);
  1 个评论
Songbai Jin
Songbai Jin 2022-5-6
Thank you for your reply!
In fact, initially I set beta=100, and I noticed that all numbers in my distance matrix D is 1.
I tried to use 'sym' to improve accuracy, however, I noticed even a small beta (like 0.01) led to a wrong sort result.
I'd really appreciate it if you could help solve my confusion.

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