For classification I'm building a number of models for a classifier in MATLAB. I use the class ClassificationKNN for this.
I would very much like to store multiple models (or objects of this class) inside a matrix.
Normally you could access and create matrices inside a matrix with the curly braces ({}).
My loop looks like this:
models = []
for i = 1:length(x)
models = [models, {ClassificationKNN.fit(x,y)}]
end
Unfortunately this returns a matrix models of size (1,3) but all cells are empty which means the models are lost...
How can I make sure every model is stored in a matrix? I need to do this because I need all models later in my calculations and the position in the matrix is important...
Any ideas?
You want a cell array of models, right? It sure looks that way, if that will work try this:
models = {}
for ii = 1:length(x)
models = [models, {ClassificationKNN.fit(x,y)}]
end
Also, you loop through calling ClassificationKNN.fit(x,y) with the same arguments every time, is this just a test, or pseudo-code for an example. Like the comment says, it's best to preallocate like:
models = cell(length(x),1);
for ii = 1:length(x)
models{ii} = ClassificationKNN.fit(x,y);
end
But, either way is likely fine.
Thanks to macduffs post I finally figured out what was going on. Whilest reading his proposition I realised that that indeed should be the correct way if getting a cell array of objects.
After trying it, the array again seemed empty when opening it in the variable editor. I tried calling the first cell in the array to see if it was indeed empty and it was not. It returned the object I had stored in it. This means the question was answered.
I then reverted back to my own method to see if that worked as well and it did. When calling a cell it also returned an object.
Bottom line:
Do not trust the variable editor ^^.
Related
I have the following problem:
I have two arrays called Variable_1 and Variable_2 (same size 8x3, only different values). I need to make calculations with the values inside the array and store the result in a new array. The calculation is the same for both arrays.
Right now, I solved it with 2 for-loops.
for i = 1:size(Variable_1,1)
Calculation_1 = 5 * Variable_1(i,1);
Result_1(i,:) = Calculation_1;
end
for i = 1:size(Variable_2,1)
Calculation_2 = 5 * Variable_2(i,1);
Result_2(i,:) = Calculation_2;
end
I want to get rid of two separate for-loops and do it with one loop or a loop in a loop. The name "Variable_x" needs to be kinda dynamical. It is different for every run in the outer for-loop. First, it is Variable_1 and matlab has to look in the array "Variable_1" at position i. Later, it is Variable_2 and it has to look in another array, here called "Variable_2".
I know that I shouldn't use dynamic variables and however I didn't found a solution anyway. Maybe it works with Cellarrays but I dont't know exactly how I use them in this specific case.
It is kinda hard to explain, so feel free to ask questions if you have one. I am really looking forward for any suggestions.
I'm going to create an adjacency matrix from a cell array, but faced two main problems:
I don't know how to access to cell array's elements; so an adhoc method was used.
(and the most important) The code produces an error and the partial result is also a weird one!
The cell array is as the following image:
The code is as follows:
for i=1:N
L=size(Al{i});
Len=L(1,2);
for j=1:Len
elm=Al{i};
D=elm(i,j);
Adjm(i,D)=1;
end
end
The code produces this error:
and the output is as follows:
P.S.: The code is part of a program to construct adjacency matrix to represent superpixels adjacency within an image. There may be a specific solution for that!
There are lots of ways your code could be made better, but the specific error you are seeing is because you want D=elm(1,j); instead of D=elm(i,j);. Note the 1 instead of i.
A somewhat more efficient approach would be to do,
for i=1:numel(Al)
Adjm(i,Al{i})=1;
end
As with your code, this assumes that there are no empty elements in Al.
I am trying to loop through multiple structures at once, extract variables of interest, and combine them into a single cell array. The problem: all of the variables have the same names. I have a working pseudocode--here it is:
Let's say I load i structures in my workspace. Now I want to loop through each structure, and extract time and position data from each structure.
First, I load my structures. Something like...
data_1
data_2
data_3
Then, I create appropriately sized cell arrays.
time{i,:} = zeros(size(structures));
position{i,:} = zeros(size(structures));
Finally, I loop through my structures to extract cell arrays and create a single array.
for i = 1:size(structures)
time_i= data_i.numbers.time;
position_i= data_i.numbers.position;
time {i,:} = time_i;
position{i,:} = position_i;
end
I want to end with a cell array containing a concatenation of all the variables in a single cell structure.
Could you please help convert my pseudo code/ideas into a script, or point me to resources that might help?
Thanks!
You're likely going to be better off loading your data internal to the loop and storing it into a cell or structure rather than trying to deal with iteratively named variables in your workspace. eval is, in nearly all cases, a significant code smell, not least of which because MATLAB's JIT compiler ignores eval statements so you get none of the engine's optimizations. eval statements are also difficult to parse, debug, and maintain.
An example of a stronger approach:
for ii = 1:nfiles
tmp = load(thefilenames{ii}); % Or use output of dir
trialstr = sprintf('trial_%u', ii); % Generate trial string
data.(trialstr).time = tmp.numbers.time;
data.(trialstr).position = tmp.numbers.position;
end
Which leaves you with a final data structure of:
data
trial_n
time
position
Which is far easier to iterate through later.
My final script for anyone interested:
for i = 1:4 %for 4 structures that I am looping through
eval(['time_',num2str(i),'= data_',num2str(i),'.numbers.time;']);
eval(['position_',num2str(i),'= data_',num2str(i),'.numbers.position;']);
%concatenate data into a single cell array here
time{i} = {eval(['time_',num2str(i)])};
position{i} = {eval(['position_',num2str(i)])};
end
...
eval(['time_',num2str(i),'= data_',num2str(i),'.numbers.time;'])
eval(['position_',num2str(i),'= data_',num2str(i),'.numbers.position;'])
...
I am going to use an algorithm in a for loop as an iteration loop. I know that some of the calculations can be done once and the their results can be used as necessary inputs for the algorithm in the for loop so in this way there is no need to calculate the same things in each iteration. To do so, I calculate them once and put them in a structure.
I use structure since I have many variables to be kept to be used in the for loop and their name and size are different. I put them in the structure with the same name for example:
out.A = A;
out.myvector = myvector;
out.s = s;
out.Hx_l = Hx_l;
and so on. some of them are matrices, some of them cubes or variables with a forth dimension and some of them are cells.
Is there any way to preallocate this kind of structure?
You could initialise the structure the following way:
out = struct('A',[],'myvector',[],'s',[],'Hx',[]);
When you assign the variables afterwards, the structure fields will be already created. Usually the contents are not initialised upfront. Quoting Loren:
How important is it to initialize the contents of the struct. Of
course it depends on your specifics, but since each field is its own
MATLAB array, there is not necessarily a need to initialize them all
up front. The key however is to try to not grow either the struct
itself or any of its contents incrementally.
For debugging, I have some plots of vectors in an embedded matlab function in Simulink. Up to Matlab R2013b, everything works just fine with the following minimum example code:
function fcn
%#minimum example for plot within for-loop of embedded matlab function
coder.extrinsic('delete');
coder.extrinsic('quiver');
coder.extrinsic('gobjects');
numSteps=4;
persistent hFig;
persistent hVector;
if isempty(hFig)
hFig = figure;
hold on;
hVector=zeros(numSteps,1);
hVector=gobjects(numSteps,1);
for i=1:numSteps
hVector(i) = quiver(0,0,0,0);
end
end
set(0, 'CurrentFigure', hFig);
delete(hVector);
for i=1:numSteps
startInc=[1 1;1 1].*i;
endInc=[2 3;2 -3].*i;
hVector(i) = quiver(startInc(1,:),startInc(2,:),endInc(1,:),endInc(2,:));
end
For the handle array hVector, an initialization is necessary due to its use in the for-loop. However, for an Initialization of a graphics handle object, the function gobjects is needed and in turn the initialization as double with zeros(numSteps,1) becomes necessary, since matlab cannot correctly determine the data type of an output of an extrinsic function.
As I said, this code snippet works just fine if copied to an embedded matlab function block in simulink without anything else in the model.
My problem: Mathworks changed a lot of the plotting functions in R2014a, one of the changes being the datatype of the graphics handles which are no of type quiver for my quiver plot. Thus, the initialization with zeros(numSteps,1) initializes the wrong data type for the handle array. However leaving it out still does not work, because of the problem mentioned above. Neither does a init loop or anything similar compile without errors.
I'd greatly appreciate any help on that issue.
You can try to remove the gobject initialisation and use double() to wrap your call to any matlab graphic object. Ex:
hVector(i) = double( quiver(startInc(1,:),startInc(2,:),endInc(1,:),endInc(2,:)) ) ;
I suggest reading Loren's article about the compatibility issues which can arise when switching to HG2 versions of Matlab.
A quick quote from it which apply more to your issue:
Graphics Functions Return Objects, not Numeric Handles
Prior to R2014b, you could store a set of handles to graphics objects
in an array and then add some numeric data to that array. In R2014b,
that will cause an error.
[...]
If you find yourself really stuck, it is possible to cast object
handles to numeric handles using the double function. You can then
cast the number back to an object handle using the handle function. We
don't recommend this as a long term solution. Be aware that we may
choose to remove this feature in a future version of MATLAB. If we do,
we'll let you know in advance.
Now if you really have to use this solution, note that both functions works on single elements but also on arrays. So
hVector_HG = handle( hVector_Num ) ; %// works if "hVector_Num" is an array of numeric handles
%// and ...
hVector_Num = double( hVector_HG ) ; %// works if "hVector_HG" is an array of HG2 graphic handles
That may simplify the round trips between a format or another if they often become necessary.
Edit:
I put this at the bottom of the post for the moment because the beginning is already accepted answer but please try the following and let me know if it works. It may solve your problem in a better (more future-proof) way.
Another way to initialise an array of handle of a given graphic object is to create one (empty is good enough) and replicate it. For example:
hqNaN = quiver(NaN,NaN) ; %// create an empty quiver
hVector = repmat( hqNaN , numSteps , 1 ) ; %// replicate it in an array
will give you an array hVector containing numSteps HG2 graphic object handles. At this point they all point to the very same object but it is perfectly legal to overwrite each element with a handle of the same type. So a later :
hVector(i) = quiver( ... , etc ... ) ; %// overwrite handle "i" with a new quiver plot handle
will (should) work without problem.
A few things to take care with this approach:
where will the empty quiver be created ?
you may already have a "current" figure and you don't want it to be messed up. If not a new empty plot will be created. So to make sure the empty quiver do not cause problem (just a flicker on the screen), you could wrap it this way:
figure ; hqNaN = quiver(NaN,NaN) ; close(gcf) ;
or you can also park it in a figure (it will be invisible anyway) in case you need to re-use a handle of this type for other array initialisation. Just don't forget that once you close the figure it is on, or you delete the graphic object, the hqNaN variable is still there but it is not the same type of handle any more (so not useful to replicate if you want this type exactly).
What if you do not overwrite all you initial handle array ?
Remember all the initial handles of the array point to the same graphic object. So in case your array contains 12 elements but let's say by mistake you only overwrite 10 of them, then 2 elements will be handles pointing to the same object (which you may already have deleted)). This means that calling:
delete(hVector)
will throw you the annoying:
Error using handle.handle/delete. Invalid or deleted object.
gna gna gna ... luckily you can easily prepare for that by programming defensively and using instead:
delete( hVector(ishandle(hVector)) )
Matlab will do the check automatically and only delete the right handles.