Use string to define variable

Hello guys!

I am looking for a way to name a variable with a string name that will be changing according to the T and V values

My code is the following:

-------------------------------

T=15; % input variable T

V=1; % input variable V

load('FRF_T15_V1.mat')

load('FRF_T15_V2.mat')

load('FRF_T15_V3.mat')

formatSpec = 'FRF_T%d_V%d';

filename = sprintf(formatSpec,T,V);

x=eval(filename);

XP=(x{3,3});

save ('XP,'XP')

-----------------------------

I would like for variable XP to be assigned according to the input values of T and V, e.g.,

FRF_T15_V1_XP=(x{3,3});

And then save it as

save (' FRF_T15_V1_XP,' FRF_T15_V1_XP ')

Thanks in advance for your help!

Kind regards,

Hugo

[Stephen23]

https://www.mathworks.com/matlabcentral/discussions/tips/849901-tutorial-why-variables-should-not-be-named-dynamically-eval

[Hugo Policarpo]

Thanks!

Answer 1

[Stephen23]

Storing meta-data in variable names (or fieldnames) is a very bad way to design your data, which invariably forces you into writing slow, complex, inefficient code that is much harder to debug than if you stored that meta-data as data in a variable (because meta-data is data):

https://www.mathworks.com/matlabcentral/discussions/tips/849901-tutorial-why-variables-should-not-be-named-dynamically-eval

But if you insist on this anti-pattern data design, something like this is probably the least-worst way of doing it:

T = 15;
V = 1;
F = sprintf('FRF_T%d_V%d',T,V);
S = load([F,'.mat']);
C = struct2cell(S);
X = C{1};
S = struct([F,'_XP'],{X{3,3}});
save([F,'_XP.mat'],'-struct',S)

However, treating meta-data as data in its own right (which it is) would be much better data design (it would make your code simpler, more robust, and more efficient).

See also:

https://www.mathworks.com/help/matlab/ref/save.html#btox10b-1-variables

https://www.mathworks.com/help/matlab/ref/struct2cell.html

[Hugo Policarpo]

Thanks!

[Hugo Policarpo]

your just missing a " ] " in the last line.

[Stephen23]

"your just missing a " ] " in the last line."

Thank you, I fixed that.

Answer 2

[Steven Lord]

Can you dynamically create variables with numbered names like x1, x2, x3, etc.? Yes.

Should you do this? The general consensus is no. That Discussions post explains why this is generally discouraged and offers several alternative approaches.

[Hugo Policarpo]

Thanks!

Answer 3

[Meg Noah]

There are many ways to do this. Here's one that shows how to make the format of the T and V values have the same width and will sort based on the value of T in the directory. Just change the '%n.n' to the largest width of your number range:

% for the character array version
fname = @(t,v) ['FRF_T' num2str(t,'%3.3d') '_V' num2str(v,'%2.2d') '_XP'];

for T=[1 3 5]
    for V=1:2
        fprintf(1,'%s\n', fname(T,V));
    end
end

FRF_T001_V01_XP
FRF_T001_V02_XP
FRF_T003_V01_XP
FRF_T003_V02_XP
FRF_T005_V01_XP
FRF_T005_V02_XP

% for the string version
strfname = @(t,v) strcat("FRF_T", num2str(t,'%3.3d'), "_V", num2str(v,'%2.2d'), "_XP");

for T=[1 3 5]
    for V=1:2
        fprintf(1,'%s\n', strfname(T,V));
    end
end

FRF_T001_V01_XP
FRF_T001_V02_XP
FRF_T003_V01_XP
FRF_T003_V02_XP
FRF_T005_V01_XP
FRF_T005_V02_XP

[Hugo Policarpo]

Thanks!

Answer 4

[Hugo Policarpo]

Issue solved!

Thanks for the help!