Calculate a varibale matrix for a specific value

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Sa Moha
Sa Moha 2021-12-16
评论： Walter Roberson 2021-12-16
I know this is a beginnary question. But I couldn't find a solution for it.
syms x;
A(x)=[x,x+1,x-1];
A(2)
  1 个评论
Awais Saeed
Awais Saeed 2021-12-16
You did not explain what you are trying to acheive and what is the problem. A(2) gives you [2 3 1] though.

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Walter Roberson
Walter Roberson 2021-12-16
Ran in:
syms x;
A(x)=[x,x+1,x-1];
A(2)
ans = 
I suspect that you are trying to index the formula A(x) to extract the x+1 .
If so, then you cannot do that directly.
When you define a symbolic function like you do on the second line, then MATLAB starts with a sym() object and attaches subsref() behaviour for indexing with () indices. The subsref() behaviour that is attached to the symbolic function object is that () indexing works like
extract the variables from the defined parameter list
find the underlying sym() object representation,
subs() into that symbolic representation replacing the
parameter variables with the values that are pass in through the () notation
After that is done, you cannot index into the formula to extract parts of the formula, because the () behavior has been defined as evaluating the formula according to the parameters passed in. Thus A(2) is subs([x,x+1,x-1], {x}, {2}) giving [2 3 1] .
What you can do, though, is
parts = formula(A)
parts = 
parts(2)
ans = 
  3 个评论
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Sa Moha
Sa Moha 2021-12-16
I found it myself:
A=zeros(2);
for i=1:2
A(1:2,2*i-1:2*i)=[i,i;i,i];
A
end
Walter Roberson
Walter Roberson 2021-12-16
Ran in:
A = cell(2, 1);
for i=1:2
A{i}=[i,i;i,i];
end
A
A = 2×1 cell array
{2×2 double} {2×2 double}
A{2}
ans = 2×2
2 2 2 2

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