Better code for accessing fields in a matlab structure array?

Question

I have a matlab structure array Modles1 of size (1x180) that has fields a, b, c, ..., z.

I want to understand how many distinct values there are in each of the fields. i.e.

max(grp2idx([foo(:).a]))

The above works if the field a is a double. {foo(:).a} needs to be used in the case where the field a is a string/char.

Here's my current code for doing this. I hate having to use the eval, and what is essentially a switch statement. Is there a better way?

names = fieldnames(Models1);
for ix = 1 : numel(names)
    className = eval(['class(Models1(1).',names{ix},')']);
    if strcmp('double', className) || strcmp('logical',className)
        eval([' values = [Models1(:).',names{ix},'];']);
    elseif strcmp('char', className)
        eval([' values = {Models1(:).',names{ix},'};']);
    else
        disp(['Unrecognized class: ', className]);
    end
    % this line requires the statistics toolbox.
    [g, gn, gl] = grp2idx(values);
    fprintf('%30s : %4d\n',names{ix},max(g));
end

The use of eval is indeed best minimized, but a switch is not bad practice. If you want it to look nicer you can use the actual switch case syntax. — Dennis Jaheruddin
– Dennis Jaheruddin, Commented Nov 9, 2012 at 21:47
@Dennis It isn't a switch as such that I was not liking, it was that I had to switch to handle different class types. That strikes me as just ugly. — John
– John, Commented Nov 9, 2012 at 22:52

Eitan T · Accepted Answer · 2012-11-09 21:21:50Z

5

Indeed, there is a better way. Surprisingly, MATLAB allows you to access the struct fields using a key string without eval, for instance:

Models1(1).(names{ix})

so instead, you can write this:

className = class(Models1(1).(names{ix});
...
values = [Models1(:).(names{ix})];
...
values = {Models1(:).(names{ix})};

Also, instead of using class and strcmp, you can just test the same conditions with isa:

v1 = Models1(1).(names{ix});
if (isa(v1, 'double') || isa(v1, 'logical'))
    values = [Models1(:).(names{ix})];
    % # ...
elseif (isa(v1, 'char'))
    values = {Models1(:).(names{ix})};
    % # ...
else
    disp(['Unrecognized class: ', class(v1)]);
end

It should be much faster.

edited Nov 9, 2012 at 21:21

answered Nov 9, 2012 at 21:11

Eitan T

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1 Comment

John Over a year ago

I am very surprised I didn't see that in the documentation. Sweet!

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