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How to read elements from a computed matrix without define a new variable?

Asked by Yize Wang on 10 May 2019
Latest activity Edited by Stephen Cobeldick on 10 May 2019
Dear all,
I am having a question about matrix computation. For example, a matrix A is calculated by an expression, . I am only interested in the element . Simply I can do and . I am wondering whether there is a way to do this without defining A. This line of code is not legal in MATLAB but it shows my goal clearly, .
Thanks!

  7 Comments

The matrix exponential of a matrix is not equal to the matrix where all of its elements are "exponentialized".
Yize Wang, this is the easiest way and fastest to do it as I know. If you are concern about legibility, you can encapsulate this piece of code in a function called get_1_comp_system or some cool name (I'm not good at all naming).
"Eeeeeemmmm, Torsten... this equation is true always..."
Except that you did not take into account the matrix exponential shown in the original question.

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1 Answer

Answer by Stephen Cobeldick on 10 May 2019
Edited by Stephen Cobeldick on 10 May 2019
 Accepted Answer

MATLAB does not allow indexing into the results of operations. You can either use a temporary variable (which is probably the most efficient solution), or you could call subsref directly:
>> B = rand(3);
>> C = rand(3);
>> D = rand(3);
>> a = subsref(expm(B*C+D),substruct('()',{1,1}))
a = 8.6593
>> A = expm(B*C+D) % for comparison:
A =
8.6593 7.3898 6.2134
13.1945 16.0237 11.6736
13.1316 13.8705 11.8273

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