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

% 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!

Answer 1

[David Hill]

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);

[Songbai Jin]

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.