manipulating through sequential variables using for loop

Hi,

(I tried to search on the forum and found many similar questions and answers, but none seems to be exactly the same as my question, so here we go...)

I have successfully loaded 32 sequential variables from a MAT file.

A_001, A_002, ..., A_032

Each variable is an array of different length (1 x N). I want to manipulate these variables (e.g. plot all 32 of them), and I want to use a for loop to do this. How can I do this? I imagine things will be a lot easier to handle once I create a structure containing all of these variables, but I have no idea how to do that.

It looks like eval can help me do that, but many suggest not to use eval.

Many thanks in advance!

Hemmings

[Stephen23]

Both creating and accessing of dynamically named variables (such as sequentially numbered ones) is a really bad idea, leading to slow buggy code. Avoid creating numbered variables, and use simple numeric arrays, cell arrays or non-scalar structures instead:

http://www.mathworks.com/matlabcentral/answers/196952-how-to-create-new-variables-in-batches-with-strcat.

Here is a discussion of why it is a bad idea to include meta-data (such as an index) in a variable name:

http://www.mathworks.com/matlabcentral/answers/225435-save-variable-as-string-from-user-input

Answer 1

[Walter Roberson]

http://matlab.wikia.com/wiki/FAQ#How_can_I_create_variables_A1.2C_A2.2C....2CA10_in_a_loop.3F

[Hemmings Wu]

oh wow...

I've seen that post but thought it was more for creating variables.

thanks...

So, just to answer my own question:

First, create a structure while you load the MAT file.

% Assume the MAT-file example1.mat contains 2000 variables, A1 through A2000
S = load('example1.mat');
% S is now a struct array with 2000 fields, S.A1 through S.A2000.
% To access A1428, use:
x1 = S.A1428;
% If the "index" of the variable you want to access is stored in a variable:
k = 1428;
x2 = S.(sprintf('A%d', k));
x3 = S.(['A', num2str(k)]);

[Walter Roberson]

If you need to access by index a lot, struct2cell and index the cell.

If the struct does not have its fields in the order you want to index, you can create a index vector:

fn = fieldnames(S);
fnidx = str2double( regexprep(fn, '^A','') );
[~, indexvec] = sort(fnidx);
Sc = struct2cell(S);

Sc{indexvec(k))

[Stephen23]

This whole situation could have been avoided by planning the data better: use of a multidimensional numeric array, a cell array or a non-scalar structure would avoid the need to create dynamic fieldnames and allow the data to be accessed simply using indeices. For example if the matrices were in a cell array named data:

S = load(filename);
X = S.data;
X{1} % first matrix
X{2} % second matrix
... etc

Another neat solution would be to use a non-scalar structure:

S = load(filename);
X = S.data;
X(1).fieldname % first matrix
X(2).fieldname % second matrix

this has the advantage that the matrices can be easily manipulated as a Comma-Separated List:

Y = [X.fieldname] % concatenate into one numeric array
Z = {X.fieldname} % convert into cell array of numeric arrays