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

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Yize Wang 2019-5-10

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编辑： Stephen23 2019-5-10

采纳的回答： Stephen23

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!

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gonzalo Mier 2019-5-10

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).

Stephen23 2019-5-10

编辑：Stephen23 2019-5-10

"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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Stephen23 2019-5-10

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编辑：Stephen23 2019-5-10

在 MATLAB Online 中打开

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