Fast way to check if variable is in .mat file without loading .mat file? 'who'/'whos' is not faster than loading.. Better options than 'who'?

Question

I have a .mat file named "myfile.mat" that contains a huge varible data and, in some cases, another variable data_info. What is the fastest way to check if that .mat file contains the `data_info' variable?

the who or whos commands are not faster than simply loading and testing for the existens of varible.

nRuns=10;
%simply loading the complete file
tic
for p=1:nRuns
    load('myfile.mat');
    % do something with variable
    if exist('data_info','var')
        %do something
    end
end
toc

% check with who
tic
for p=1:nRuns
   variables=who('-file','myfile.mat');
   if ismember('data_info', variables)
       % do something
   end
end
toc

% check with whose
tic
for p=1:nRuns
   info=whos('-file','myfile.mat');
   if ismember('data_info', {info.name})
       %do something
   end
end
toc

All methods roughly take the same time (which is way to slow, since data is huge.

However, this is very fast:

tic
for p=1:nRuns
    load('myfile.mat','data_info');
    if exist('data_info', 'var')
        %do something
    end
end
toc

But it issues a warning, if data_info does not exist. I could suppress the warning, but that doesn't seem like the best way to do this.. What other options are there?

Edit using who('-file', 'myfile.mat', 'data_info') is also not faster:

tic
for p=1:nRuns
    if ~isempty(who('-file', 'myfile.mat', 'data_info'))
      % do something
    end
end
toc    % this takes 7 seconds, roughly the same like simply loading complete .mat file

Answer 1

Try using who restricting it to only the specific variable:

...
if ~isempty(who('-file', 'myfile.mat', 'data_info'))
  %do something
end

Timing the solutions:

Using timeit on the different solutions (code included below, running on Windows 7 and MATLAB version R2016b) shows that the who-based ones appear fastest, with the one I suggested above having a slight edge in speed. Here's the timing, from slowest to fastest:

Load whole file:        0.368235871921381 sec
Using matfile:          0.001973860748417 sec
Load only `data_info`:  0.000316989486384 sec
Using whos + ismember:  0.000174207817967 sec
Using who + ismember:   0.000151289605527 sec
Using who + isempty:    0.000137261391331 sec

I used a sample MAT file containing the following variables:

data = ones(10000);
data_info = 'hello';

Here's the test code:

function T = infotest

  T = zeros(6, 1);
  T(1) = timeit(@use_load_exist_1);
  T(2) = timeit(@use_load_exist_2);
  T(3) = timeit(@use_matfile);
  T(4) = timeit(@use_whos_ismember);
  T(5) = timeit(@use_who_ismember);
  T(6) = timeit(@use_who_isempty);

end

function isThere = use_load_exist_1
  load('infotest.mat');
  isThere = exist('data_info', 'var');
end

function isThere = use_load_exist_2
  load('infotest.mat', 'data_info');
  isThere = exist('data_info', 'var');
end

function isThere = use_matfile
  isThere = isprop(matfile('infotest.mat'), 'data_info');
end

function isThere = use_whos_ismember
  info = whos('-file', 'infotest.mat');
  isThere = ismember('data_info', {info.name});
end

function isThere = use_who_ismember
  variables = who('-file', 'infotest.mat');
  isThere = ismember('data_info', variables);
end

function isThere = use_who_isempty
  isThere = ~isempty(who('-file', 'infotest.mat', 'data_info'));
end

Answer 2

You can use the who command https://www.mathworks.com/help/matlab/ref/who.html

The syntax for this is to call who with the indicator of the file and then the variable you are looking for. You do not need to look for all the variables in the file

Dummy syntax is as follows

variable = who('-file','yourfilenamehere','data_info')

From there you can call

if ~isempty(variable)
%do something
end

This searches for only that variable within the file. In your versions of the who command you looked for all variables whereas this just looks for one.