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How to load in variable if it is present in a matfile?

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K E
K E on 21 Jul 2015
Commented: K E on 24 Jul 2015
I am having trouble with "conditional" loading of a variable from a matfile, depending on whether the matfile contains the variable. Here is what I am trying to do:
  1. Go through many different (large) matfiles, some of which contain mountain height in a variable called topo, and some of which contain mountain height in variable called elevation.
  2. If the matfile contains the variable topo, load that into a workspace variable called mountainHeight. If the matfile contains the variable elevation, load that into a workspace variable called mountainHeight. If neither variable is contained in matfile, do nothing.
  3. Avoid crashing the load command by trying to load variables which are not in the matfile.
Here is what I have tried (R2015a), but I'd bet there is a better/shorter way.
filename = 'topography.mat'; % A matfile that comes with matlab
m = matfile(filename); % Make a matfile object
FileInfo = whos(m); % Structure containing information about matfile including variables it contains
for iVar = 1:length(FileInfo)
if strcmp(FileInfo(iVar).name, 'topo') % Check if topo is a variable in topography.mat
mountainHeight = m.topo; % Load topo
elseif strcmp(FileInfo(iVar).name, 'elevation') % Check if elevation is a variable in topography.mat
mountainHeight = m.elevation; % Load elevation
end
end

Accepted Answer

Steven Lord
Steven Lord on 21 Jul 2015
D = load('mymatfile.mat');
if isfield(D, 'elevation')
elevation = D.elevation;
else
elevation = NaN;
end
Alternately, check if the field exists and if it doesn't, add it with some placeholder data.
D = load('mymatfile.mat');
if ~isfield(D, 'elevation')
D.elevation = NaN;
end
If you want to do this with multiple fields, use dynamic field names.
D = load('mymatfile.mat');
listOfFields = {'elevation', 'material', 'price'};
isPresent = isfield(D, listOfFields);
for fieldnum = 1:length(listOfFields)
if ~isPresent(fieldnum)
F = listOfFields{fieldnum};
D.(F) = NaN;
end
end
  7 Comments
Cedric Wannaz
Cedric Wannaz on 23 Jul 2015
Then just build a small function that does what Steven proposes, and outputs mountainHeight:
function mountainHeight = loadMountainHeight( locator )
data = load( locator ) ;
if isfield( data, 'topo' )
mountainHeight = data.topo ;
elseif isfield( data, 'elevation' )
mountainHeight = data.elevation ;
else
mountainHeight = [] ; % Or NaN or whatever you want, to tell
% the caller "field not found".
end
end
Then in your main script, you do something like the following:
dir_mat = dir( '*.mat' ) ;
for fId = 1 : numel( dir_mat )
mountainHeight = loadMountainHeight( dir_mat(fId).name ) ;
if isempty( mountainHeight )
% Throw warning, or print message, or just skip.
warning( 'File "%s" : no valid field found!', dir_mat(fId).name ) ;
continue ;
end
fprintf( 'File "%s" : processing height..\n', dir_mat(fId).name ) ;
% .. code for further processing mountainHeight ..
end

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More Answers (2)

Sean de Wolski
Sean de Wolski on 21 Jul 2015
Edited: Sean de Wolski on 23 Jul 2015
Use this to determine what's in the MAT file, then use the MAT File class to load that variable in.
  1 Comment
K E
K E on 24 Jul 2015
Useful to avoid loading whole matfile (they are large).

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Cedric Wannaz
Cedric Wannaz on 21 Jul 2015
Edited: Cedric Wannaz on 21 Jul 2015
An alternate solution is to catch the warning (LOAD doesn't throw an error).
Or you can turn off this warning and just test if the output struct is empty (at least on 2015b-pre, I can't test on earlier versions now).

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