Sorting array of sym

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Tanmaya
Tanmaya 2024-7-15
评论： Tanmaya 2024-7-18
I get wrong answers in this case. Am I doing anything wrong ?
>> a = sym.empty();
>> a = [a,0,pi,pi/2,-pi/2,pi/4]
a =
[0, pi, pi/2, -pi/2, pi/4]
>> sort(a)
ans =
[0, pi, -pi/2, pi/2, pi/4]
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Walter Roberson
Walter Roberson 2024-7-15
Ran in:
Pi = sym(pi);
a = [-Pi/4 -Pi/3 -Pi/2 -Pi -Pi*3/4 -Pi*2/3 Pi/4 Pi/3 Pi/2 Pi Pi*3/4 Pi*2/3 -Pi*1/5 -Pi*2/5 -Pi*3/5 -Pi*4/5 Pi*1/5 Pi*2/5 Pi*3/5 Pi*4/5]
a = 
sort(a)
ans = 
a = a(randperm(numel(a)))
a = 
sort(a)
ans = 
So the sorting results are stable, but they are not obvious.

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回答（1 个）

Milan Bansal
Milan Bansal 2024-7-15
Ran in:
Hi Tanmaya
To sort the symbolic array numerically, you can convert the symbolic expressions to their numerical equivalents before sorting. Here's how you can do it:
  1. Convert the symbolic expressions to numerical values.
  2. Sort the numerical values.
  3. Recreate the sorted symbolic array using the indices from the sorted numerical values.
Please refer to the following code snippet to implement the same:
a = sym.empty();
a = [a, 0, pi, pi/2, -pi/2, pi/4];
a = sym(a); % Ensure the array is symbolic
% Convert to numerical values
num_a = double(a);
% Sort the numerical values and get the sorting indices
[sorted_num_a, sort_idx] = sort(num_a);
% Recreate the sorted symbolic array using the indices
sorted_a = a(sort_idx);
disp(sorted_a);
Hope this helps!
  2 个评论
John D'Errico
John D'Errico 2024-7-15
Ran in:
This is pretty much all you can do, to convert the elements to actual numbers in some form. But there is no value in creating a as a sym.empty first.
a = sym([0, pi, pi/2, -pi/2, pi/4]);
a = sym(a)
a = 
You don't need ot go all the way to doubles though. vpa would seem more accurate, and still works.
[~,idx] = sort(vpa(a));
a = a(idx)
a = 

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