MATLAB: Advise on accessing cell array containing structures

evalnested cell arrays

I have a cell array "wrkspcs" containing names of cell arrays as shown below (some entries omitted for brevity)

    wrkspcs =
      13×1 cell array
        {'ZLH_151210_WrkSpc'}
        {'MXG_151210_WrkSpc'}
        {'LF_151223_WrkSpc' }

each entry refers to a 6×6 cell array in the workspace. Using eval, I can see the referenced cell array

>> eval(wrkspcs{1})
ZLH_151210_WrkSpc =
  6×6 cell array
    {1×1 struct}    {[1]}    {'16557'}    {'1210'}    {'zlh_1a'}    {'ZLH_151210'}
    {1×1 struct}    {[2]}    {'16557'}    {'1213'}    {'zlh_2a'}    {'ZLH_151210'}
    {1×1 struct}    {[3]}    {'16557'}    {'1216'}    {'zlh_3a'}    {'ZLH_151210'}
    {1×1 struct}    {[1]}    {'16676'}    {'1210'}    {'zlh_1b'}    {'ZLH_151210'}
    {1×1 struct}    {[2]}    {'16676'}    {'1213'}    {'zlh_2b'}    {'ZLH_151210'}
    {1×1 struct}    {[3]}    {'16676'}    {'1216'}    {'zlh_3b'}    {'ZLH_151210'}

If I want to get the number of entries in the first workspace "ZLH_151210_WrkSpc", I can do

>> tmp=eval(wrkspcs{1});n = length(tmp(:,1))
n =
     6

but if I try to eliminate the creation of the temporary variable "tmp" and access the length directly, I get the following error:

>> n = length(eval(wrkspcs{1})(:,1))
Error: ()-indexing must appear last in an index expression.

However, if I try the following everything is fine.

>> eval(['n = length(',wrkspcs{iloop},'(:,1))'])
n =
     6

So, I am trying to understand which syntax rule I am violating in the second case and what is the "proper" way of obtaining the length without either creating the variable "tmp" or including the assignment in the 'eval' statement (which the Matlab documentation states I should try to avoid).

Any comments, insights, or suggested alternatives would be appreciated.

Best Answer

Unlike C++ or python, in MATLAB you can't directly index the output of a function. You firstly need to store the data in a separate variable and then index the variable that variable to access the required data. So what is happening here:

1) If first case: eval() is a MATLAB function and you are trying to further index its output. Which as already stated is not supported in MATLAB.

2) In the second case: you are effectively running the following command

n = length(ZLH_151210_WrkSpc(:,1))

i.e. indexing into a cell array. This a perfectly supported MATLAB syntax. Even in the first case, the following line will work

n = length(eval([wrkspcs{1}, '(:,1)']))

as you can see that again I am trying to index in the cell array, not the output of a function.

Note: Accessing variables using eval is a very bad idea. It makes your code slow and difficult to debug. For better coding practice, you should look into storing all the variables in a struct and then access the required data using field names.

Related Solutions

MATLAB: Loading from *.mat file directly into a structure

Copied from https://www.mathworks.com/matlabcentral/answers/398961-advise-on-accessing-cell-array-containing-structures:

"But how can I do the load directly into 'myStruct' struct?"

You can't. Not in the way that you are trying to do it, without a temporary variable. load returns a scalar structure and you will have to allocate that to a temporary variable and then access its fields, as Ameer Hamza already explained. This is quite efficient and does not waste memory, so there is no reason to avoid it.

"...any workaround to avoid going thru loading to a temporary structure..."

as Ameer Hamza wrote, MATLAB does not allow arbitrary indexing/fieldname access to be suffixed onto function calls, so it is quite normal in MATLAB to allocate data to a temporary variable before doing some simple indexing, or accessing fields. This is standard MATLAB practice, wastes no memory whatsoever, and you have not explained why you need to avoid it.

Alternative 1: using a non-scalar structure has advantages also, when you try to process/access the data. You might like to consider doing something like this:

tmp = load([wrkspcs{k},'.mat']);
myStruct(k).data = tmp.(wrkspcs{1});
myStruct(k).name = wrkspcs{k};

The trick is to think of meta-data as data in their own right. Storing data in this way will make your code much simpler, more robust, and more generalized, which means that you can spend more time on actually processing your data rather than worrying about fieldnames and variable names and mat files and ...

Alternative 2: if each .mat file contains exactly one field/variable, then there is no real advantage to using a structure anyway, and you could easily use a cell array for all of your data. If the fields are the same size then it could even be a 3D array and then there would be no nesting of cell arrays:

out = cell(6,6,numel(wrkspcs));
for k = 1:numel(wrkspcs)
    tmp = load([wrkspcs{k},'.mat']);
    out(:,:,k) = tmp.([wrkspcs{k})
end

Alternative 3: Note that most of the complication here come from bad data design anyway: contrary to what some beginners think, it is much easier to process data when the variable names do not change (yes, even the ones inside .mat files). If each .mat file simply had the exactly same variables, e.g. data and name, then you really could import the files in exactly the way that you requested, without any temporary variable:

for k = numel(wrkspcs{k}):-1:1
    S(k) = load([wrkspcs{k},'.mat']);
end

and you would get one non-scalar structure containing all of your data, without any nesting:

S(1).data
S(1).name

or all of the names in a cell array:

{S.name}

etc

MATLAB: Getting cell array of structs to a usable form in R

destination_folder = 'C:\Somewhere\somefolder';  %change as appropriate
filepattern = 'Drive%02d.csv';  %change pattern if needed. Uses sprintf format string. Need a %d for file index
for iter = 1:numel(mydata)
    %convert the scalar structure in each cell of mydata into a table and write to file
    writetable(struct2table(mydata{iter}), fullfile(destination_folder, sprintf(filepattern, iter)));
end

Change the extension in filepattern to .xslx if you want to export as excel file instead.

Best Answer

Related Solutions

MATLAB: Loading from *.mat file directly into a structure

MATLAB: Getting cell array of structs to a usable form in R

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