array of symbolic variables which are function of time

Question

Jai Tushar 2017-10-25

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https://ww2.mathworks.cn/matlabcentral/answers/363250-array-of-symbolic-variables-which-are-function-of-time

编辑： Juraj Lieskovský 2021-3-4

p = sym('p', [m 1]);

the above code gives me an array of symbolic variables, where m is the size of the matrix. However i want to create an array of symbolic variables which are function of time i.e. p1(t),p2(t),.... .

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Answer 1

Walter Roberson 2017-10-26

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There is no facility to do this. You will need to loop (or equivalent). For example in R2017b or later,

p = arrayfun(@(N) str2sym(sprintf('p%d(t)',N)), 1:5).'

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Walter Roberson 2017-10-26

在 MATLAB Online 中打开

ptemp = arrayfun(@(N) sym(sprintf('p%d(t)',N)), (1:m).', 'uniform', 0);
p = vertcat(ptemp{:});

You might get m warnings along the line of

Warning: Support of character vectors that are not valid variable names or define a number will be removed in a future release. To create symbolic
expressions, first create symbolic variables and then use operations on them.

Jai Tushar 2017-10-28

编辑：Jai Tushar 2017-10-28

在 MATLAB Online 中打开

it worked!!

what does uniform and 0 do in the code?

Also,

ptemp = arrayfun(@(N) sym(sprintf('p%d(t)',N)), (1:2), 'uniform', 0); p = vertcat(ptemp{:});

A = [1 2; -1 1]; odes = diff(p) == A*p

Gives me the output as

odes =

 diff(p1(t), t) == p1(t) + 2*p2(t)
   diff(p2(t), t) == p2(t) - p1(t)

Why is odes not the funtion of time in the output. When i define p1(t),... and so on manually and define P = [p1; p2] then odes == diff(P) = A*p gives odes as a function of time i.e. odes(t) in the output. W

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Answer 2

Juraj Lieskovský 2021-3-4

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https://ww2.mathworks.cn/matlabcentral/answers/363250-array-of-symbolic-variables-which-are-function-of-time#answer_638821

编辑：Juraj Lieskovský 2021-3-4

在 MATLAB Online 中打开

Simple solution (using R2020b)

syms t
syms p(t) [m,1]

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Answer 3

KSSV 2017-10-26

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https://ww2.mathworks.cn/matlabcentral/answers/363250-array-of-symbolic-variables-which-are-function-of-time#answer_287789

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syms p1(t) p2(t) p3(t)
p = [p1 p2 p3]

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Jai Tushar 2017-10-26

编辑：Jai Tushar 2017-10-26

you are defining p1(t),... manually here.

I want my script to define them according to the size of the square matrix i enter. For example if i have a (user input) nxn matrix, then the script should define n such symbolic variables which are functions of time.

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