How to add multiple variables to a structure

I have a cell ('varNames') containing variable names that I want to add to a structure ('s'). I am currently acheiving this using the evaluate command as:

for i=1:length('varNames')
    eval(strcat('s.',varNames{i},'=',varNames{i},';'));
end

However, I want to avoid using the evaluate command, as I will be using this code inside a parfor loop to speed up Simulink simulations. I have so far tried the solution here with a method to extract variable names described here as:

getname = @(x) inputname(1);
makeStruct = @(x) struct(getname(x),x);
s = cellfun(makeStruct, varNames);

But it just adds the variable names to the structure 's', not the actual variables. The variables are structures themseleves generated by the ToWorkspace blocks of Simulink.

[Stephen23]

EVAL is completely unnecessary, the approaches you link to are... very complex.

As Jiri Hajek shows, all you need is INPUTNAME() and this:

https://www.mathworks.com/help/matlab/matlab_prog/generate-field-names-from-variables.html

Answer 1

[Jiri Hajek]

Hi, eval is indeed completely unnecessary for your assignment.To create a new field in a structure, for which you have a name defined by a character vector or a string, use parentheses.

s.(newFieldName) = newData;

The newData may of course by a struct. The newFieldName must be string or character vector, which you may supply in a variable, e.g. varNames{i} as in your example.

[Afzal]

If I remove the evaluate, and run s.varNames{i} = varNames{i}, it just assigns the variable name to s, not the variable.

[Steven Lord]

If I remove the evaluate, and run s.varNames{i} = varNames{i}, it just assigns the variable name to s, not the variable.

That's correct. But if you compare that command and the one posted by @Jiri Hajek you'll see that Jiri's command includes parentheses and the one you wrote does not. Those parentheses are very important to use dynamic field names functionality.

f = 'abc';
s.f = 1 % Write to a field named f

s = struct with fields:
    f: 1

s.(f) = 2 % Write to a field whose name is stored in the variable f

s = struct with fields:
      f: 1
    abc: 2

s.('hello') = 3 % You can also use text literals

s = struct with fields:
        f: 1
      abc: 2
    hello: 3

s.("world") = 4

s = struct with fields:
        f: 1
      abc: 2
    hello: 3
    world: 4

[Afzal]

I want to avoid having to do s.varName{1} = newData1; s.varName{2} = newData2, and so on. I don't want to store the variable name in the structure, but the variable itself.

[Stephen23]

"I want to avoid having to do s.varName{1} = newData1; s.varName{2} = newData2, and so on."

So don't do that. No one here is advising you to do that.

Jiri Hajek and everyone else here is advising you to use dynamic fieldnames, using parentheses.

"I don't want to store the variable name in the structure, but the variable itself."

Jiri Hajek's code does not store the variable name in the structure, their code stores the variable itself.

[Afzal]

Here's how I understand what Jiri Hajek has said to apply to my situation. I've added parenthese as well, but the structure s just contains the strings. 'a', 'b', 'c'. I want it to contain the values of the variables (1, 2, 3). I need to use a for loop, as there are many variables, and don't want to assign each variable to the structure without a loop.

a=1; b=2; c=3;
varNames={'a','b','c'};
for i=1:length(varNames)
    s.(varNames{i})=varNames{i};
end

[Steven Lord]

You're going to need to use eval or save and load to do it this way.

But let's take a step back. You say you have a lot of variables; how did you get those into your workspace in the first place? There may be ways to modify that code to avoid creating those variables.

One example is if you're calling load without an output modify that call to call it with an output. Let's set up some sample data.

cd(tempdir)
x = 1;
y = 'two';
z = magic(3);
save mymatfile.mat x y z

Now let's clear the workspace and confirm that it's clear.

clear all
whos

You could load without an output:

load mymatfile.mat
whos

  Name      Size            Bytes  Class     Attributes

  x         1x1                 8  double              
  y         1x3                 6  char                
  z         3x3                72  double              

Or you could call load with an output. Note that x, y, and z are not loaded as individual variables. They're loaded as fields of the struct named data.

clear all
data = load('mymatfile.mat')

data = struct with fields:
    x: 1
    y: 'two'
    z: [3×3 double]

whos

  Name      Size            Bytes  Class     Attributes

  data      1x1               590  struct              

[Stephen23]

"...as there are many variables, and don't want to assign each variable to the structure without a loop."

That is a sign that your data design probably need to be improved. Instead of creating lots of separate variables, you should probably create one array in the first place. For example, if you are LOADing data from a MAT file, then simply LOAD into an output variable (which is a scalar structure):

S = load(..);

See more tips and advice here: https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

If you only have a handful of variables, then you could use a function like this (untested):

function S = mystruct(varargin)
S = struct();
for k = 1:nargin
    S.(inputname(k)) = varargin{k};
end
end