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stringmatlabfor-loopsymbolic-math

Use a custom string to define symbolic variable

I am wondering how to make symbolic variables out of already defined strings. It actually will help us to make this code run:

N=2
for i=1:N
    syms sprintf('r%g',i)
end

This error is appeared after running it:

Error using symfun.parseString (line 50)
Not a valid variable name.

Error in syms (line 166)
[name, vars] = symfun.parseString(x);

I want it to create r1 and r2 as two symbolic variables.

Solution

  • The issue is because if you want to pass a string to syms you need to use the function syntax rather than the command syntax. With your current syntax (command syntax), MATLAB is trying to create a symbolic variable named 'sprintf('r%g',i)' and doesn't actually evaluate the sprintf.

    Instead the function syntax should look like the following. Also, you should use %d in your format string to ensure that i is an integer otherwise it won't be a valid variable name.

    syms(sprintf('r%d', i))