Remove all fields from a struct except if

Hi,

I have a struct mk with fields mk.id, mk.x and mx.z. There are 800 of each, so I have mk(1).id, ..., mk(800).id, and the same for x and z.

I need to do several calculations that require loading only one of these at the time, for a given parameter. This is, I have a function that takes one argument, say i, and I would like to load the fields that correspond to that i. Something such as load('input','mk(i)'), to get mk(i).id, mk(i).x and mk(i).z. This doesn't work, and load(fullfile('input.mat),'mk(i)') doesn't work either (with i replaced by a number). I receive a message saying "Warning: Variable 'mk(1)' not found".

Any suggestions? An alternative would be to load everything and delete all but mk(i).id, mk(i).x and mk(i).z, but I have no idea how to remove all but those fields.

Thanks,

Fernando

Answer 1

[per isakson]

MATFILE is the closest there is to your requirement. And it is a rather new feature. I use R20012a.

N   = 8;
mk  = struct(   'id'    , num2cell(transpose(1:N))  ...
            ,   'x'     , num2cell(rand(N,1))       ...
            ,   'y'     , num2cell(randn(N,1))      );
save( 'mk.mat', 'mk' )
mo = matfile( 'mk.mat' ); % creates an object
mo.mk(7,1)
ans = 
    id: 7
     x: 0.8407
     y: 0.4882

.

However, there are problems with MATFILE, see: Writing to matfile in a loop

[Fernando]

Thanks! I didn't know about this.