How to ensure that dynamically create symbolically variables are real?

Hello,

I'm creating symbolic variables dynamically and I wanted to ensure that the symbolic variables are real.

I have the code

clear all
index = {'1'};
List = {'A','B','C'};
A=sym(strcat(List,index))

How can I modify this to make sure that the symbolic variables stored in A are real? I have tried changing to A=sym(strcat(List,index),'real'), but this doesn't work.

Thank you, Kevin

[Kevin Bachovchin]

Al, B1, and C1 can be declared to be real syms using the following syntax.

syms A1 B1 C1 real
A = [A1 B1 C1]

It seems to be there should be some equivalent way when declaring the syms dynamically.

Answer 1

[Azzi Abdelmalek]

I don't think you can, for example force the variable A1 to be real. If

A1=5+j

Then

index = {'1'};
List = {'A','B','C'};
A=sym(strcat(List,index))
out=subs(A(1))

The result is a complex number

[Kevin Bachovchin]

I'm not setting a value for A1, just solving for things symbolically in terms of A1, B1, and C1. Since the symbolic variables are not declared as real, I'm getting unwanted conjugates in my solution.

You can declare

syms A1 B1 C1 real
A = [A1 B1 C1]

and all syms are real. There should be some way to do the same thing with declaring syms dynamically, right? (My actual code is much more complex and involves many more variables than this example so declaring symobolic variable names dynamically saves a ton of time for the user.)

[Azzi Abdelmalek]

n=3
a=sprintf('A%d ',1:n)
eval(['syms ' a ' real'])
eval(['A=[' a ']'])

[Kevin Bachovchin]

Is there a way to make this work for A1, B1, and C1? I can't assume the list entered by the user is going to be a numerically increasing string like that.

[Azzi Abdelmalek]

s=regexp('A':'Z','.','match') 
n=3
a=strjoin(strcat(s(1:n),'1'))
eval(['syms ' a ' real'])
eval(['A=[' a ']'])

[Kevin Bachovchin]

I can't assume that the list is going to be alphabetic either. Basically the list can be whatever cell array the user inputs. Is there a way to do something like

index = {'1'};
List = {'A','B','C'};
A=sym(strcat(List,index))
out=subs(A(1))

and just ensure the symbolic variables are real for the purposes of symbolic math that comes later.

Answer 2

[Kevin Bachovchin]

clear all
index = {'1'};
List = {'A','B','C'};
A=sym(sym(strcat(List,index)),'real')

[Walter Roberson]

Not if the variable names are not all the same number of characters.

[Kevin Bachovchin]

It seems to work fine even when the variables names in List have different number of characters.

clear all
index = {'1'};
List = {'AR','BRR','C'};
A=sym(sym(strcat(List,index)),'real')