Is it possible to use zero padding in the variable names when creating a symbolic array of automatically generated elements.

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Armin Römer 2020-9-14

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https://ww2.mathworks.cn/matlabcentral/answers/593509-is-it-possible-to-use-zero-padding-in-the-variable-names-when-creating-a-symbolic-array-of-automatic

编辑： Walter Roberson 2020-9-17

I am currently trying to implement an algorithm that generates and solves a set of polynomial equations. I generally need to create R symbolic variables for the polynomials .I wanted to use the sym('a',n) functionality for this. For example:

x_var_list = sym('x%d',[1 R])

Since R can sometimes be 10 or higher, I wanted to introduce zero padding, such that the variables will usually be displayed in sorted order, when printing.

n_digits_R = num2str(ceil(log10(abs(R))))
x_var_list = sym(strcat('x%0',n_digits_R,'d'),[1 R])

This code snippet, however, yields the following error message:

Error using sym>createCharMatrix (line 1561)

If the second argument specifies the dimensions of the generated symbolic vector or matrix, then the first argument must be a character vector specifying the base name for vector or matrix elements. It must be a valid variable name.

Is padding generally incompatible with the sym function?

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Vinai Datta Thatiparthi 2020-9-17

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Hey Armin,

Is there any specific reason why you use "%0"?

Instead,

sym(strcat('x0',n_digits_R,'d'),[1 R])
% Does this fetch the expected results?

Also, could you mention what are the expected output variable names?

Armin Römer 2020-9-17

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Hello Vinai Datta,

it still throws the same error as posted in the original question. I am using % according to the documentation:

https://de.mathworks.com/help/symbolic/sym.html

It says there:

Format the names of elements of a by using a format character vector as the first argument. sym replaces %d in the format character vector with the index of the element to generate the element names.

a = sym('x_%d',[1 4])

The output then is

a =

[x_1, x_2, x_3, x_4]

I would like to be able to use zero padding in the variable names. For example. Say that I need to create 10 variables. Then I would like to create the 10 symbolic variables in the following way:

x_01, x_02, x_03, x_04, x_05, x_06, x_07, x_08, x_09, x_10,

Currently I have to use explicit loops and use the eval function. It would just be nice to get the same result in one line of code.

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Answer 1

Walter Roberson 2020-9-17

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https://ww2.mathworks.cn/matlabcentral/answers/593509-is-it-possible-to-use-zero-padding-in-the-variable-names-when-creating-a-symbolic-array-of-automatic#answer_496075

编辑：Walter Roberson 2020-9-17

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No, the code checks that what you pass in is a variable name itself, and if not then it deletes all '%d' and tries again to see if the result is a variable name. There is no possibility in the code for anything other than '%d'

To get around this problem,

syms(compose("x_%02d", 1:R))

or similar variations like

a = sym(compose("x_%02d", 1:R))