I've often found myself doing something like this:
unprocessedData = fetchData(); % returns a vector of structs or objects
processedData = []; % will be full of structs or objects
for dataIdx = 1 : length(unprocessedData)
processedDatum = process(unprocessedData(dataIdx));
processedData = [processedData; processedDatum];
end
Which, whilst functional, isn't optimal - the processedData vector is growing inside the loop. Even mlint warns me that I should consider preallocating for speed.
Were data a vector of int8, I could do this:
% preallocate processed data array to prevent growth in loop
processedData = zeros(length(unprocessedData), 1, 'int8');
and modify the loop to fill vector slots rather than concatenate.
is there a way to preallocate a vector so that it can subsequently hold structs or objects?
Update: inspired by Azim's answer, I've simply reversed the loop order. Processing the last element first forces preallocation of the entire vector in the first hit, as the debugger confirms:
unprocessedData = fetchData();
% note that processedData isn't declared outside the loop - this breaks
% it if it'll later hold non-numeric data. Instead we exploit matlab's
% odd scope rules which mean that processedData will outlive the loop
% inside which it is first referenced:
for dataIdx = length(unprocessedData) : -1 : 1
processedData(dataIdx) = process(unprocessedData(dataIdx));
end
This requires that any objects returned by process() have a valid zero-args constructor since MATLAB initialises processedData on the first write to it with real objects.
mlint still complains about possible array growth, but I think that's because it can't recognise the reversed loop iteration...
In addition to Azim's answer, another way to do this is using repmat:
% Make a single structure element:
processedData = struct('field1',[],'field2',[]);
% Make an object:
processedData = object_constructor(...);
% Replicate data:
processedData = repmat(processedData,1,nElements);
where nElements is the number of elements you will have in the structure or object array.
BEWARE: If the object you are making is derived from the handle class, you won't be replicating the object itself, just handle references to it. Depending on your implementation, you might have to call the object constructor method nElements times.
Since you know the fields of the structure processedData and you know its length, one way would be the following:
unprocessedData = fetchData();
processedData = struct('field1', [], ...
'field2', []) % create the processed data struct
processedData(length(unprocessedData)) = processedData(1); % create an array with the required length
for dataIdx = 1:length(unprocessedData)
processedData(dataIdx) = process(unprocessedData(dataIdx));
end
This assumes that the process function returns a struct with the same fields as processedData.
You can pass in a cell array to struct of the appropriate size:
processedData = struct('field1', cell(nElements, 1), 'field2', []);
This will make a structure array that is the same size as the cell array.
Related
I need to iterate through a large number of LinearModel fit Objects and have them stored in a logical way, preferably through an indexing method, e.g. model{x,y}. Less preferable is eval(), which I have tried and in any case is not working. I get the error Index exceeds matrix dimensions. - although the string expression works just fine outside of eval.
counter = 48;
str=strcat('model',+num2str(counter)); % Dynamic variable name
str1 = strcat(str,'=fitlm(tbl,modelspec)'); % Full string to be evaluated
eval(str1)
Ideally I wanted to do, while iterating through x
model{x,y} = fitlm(tbl,modelspec) % This is the equivalent expression
But the error I get is
"Assignment using {} is not allowed for a FitObject."
I think this is similar to this question - with no answer:
Dynamic Objects in Matlab
Managed to solve this by assigning to a cell structure.
e.g.
model = cell(3,3) % Pre-assign a cell structure
for x=1:3
for y=1:3
model{x,y} = fitlm(tbl,modelspec);
end
end
Suppose I have a struct array arr, where each element has a bunch of fields, including one called val. I'd like to increment each element's val field by some constant amount, like so:
for i = 1:length(arr)
arr(i).val = arr(i).val + 3;
end
This obviously works, but I feel there should be a way to do this in just one line of code (and no for loop). The best I've come up with is two lines and requires a temp variable:
newVals = num2cell([arr.val] + 3);
[arr.val] = deal(newVals{:});
Any ideas? Thanks.
Just a note, the deal isn't necessary there:
[arr.val] = newVals{:}; % achieves the same as deal(newVals{:})
The only other way I know how to do this (without the foor loop) is using arrayfun to iterate over each struct in the array:
% make a struct array
arr = [ struct('val',0,'id',1), struct('val',0,'id',2), struct('val',0,'id',3) ]
% some attempts
[arr.val]=arr.val; % fine
[arr.val]=arr.val+3; % NOT fine :(
% works !
arr2 = arrayfun(#(s) setfield(s,'val',s.val+3),arr)
That last command loops over each struct in arr and returns a new one where s.val has been set to s.val=3.
I think this is actually less efficient than your previous two-liner and the for loop though, because it returns a copy of arr as opposed to operating in-place.
(It's a shame Matlab doesn't support layered indexing like [arr.val]=num2cell([arr.val]+3){:}).
I like Carl's and mathematical.coffee's original ideas.
I have multiple similar lines to express, so for concision of my mainline code,
I went ahead and made the generic subfunction
function varargout = clist(in)
varargout = {in{:}};
end
then I could express each such line in a fairly readable way
[arr.var] = clist(num2cell([arr.var]+3));
[arr.var2] = clist(num2cell([arr2.var]/5+33));
Are all the fields in that struct scalar, or the same size? If so, the idiomatic Matlab way to do this is to rearrange your struct to be a scalar struct with arrays in each of its fields, instead of an array of structs with scalar values in the fields. Then you can do vectorized operations on the fields, like arr.val = arr.val + 3;. See if you can rearrange your data. Doing it this way is much more efficient in both time and memory; that's probably why Matlab doesn't provide convenient syntax for operating over fields of arrays of structs.
if the struct array you are trying to set is a set of graphics objects (line handles, figure handles, axes handles, etc), then you need to use the function set:
x = (1:10)';
Y = rand(10,5);
l = plot(x,Y,'-k'); % returns an array of line handles in l
set(l,'Color','r'); % sets the property 'Color' for all the five lines in l
I am working in MATLAB for my image processing project.
I am using a for loop to generate some kind of image data (size of image varies) with each loop iteration. My problem is how do stop it from overwriting the image in next iteration.
Img(i,j)=data
Ideally I would like it to have
Img_1 = data (for 1st iteration)
Img_2 = data (for 2nd iteration)
Img_3 = data (for 3rd iteration)
and so on...
Is there any way, it can be acheived?
Yes, you can use dynamic field names with structures. I wouldn't recommend using separate variable names because your workspace will become unwieldy. Do something like this:
img_struct = struct(); %// Create empty structure
for ii = 1 : num_iterations
%// Do your processing on data
%...
%...
img_struct.(['Img_' num2str(ii)]) = data; %// After iteration
end
This will create a structure called img_struct where it will have fields that are named Img_1, Img_2, etc. To access a particular data from an iteration... say... iteration 1, do:
data = img_struct.Img_1;
Change the _1 to whatever iteration you choose.
Alternatively, you can use cell arrays... same line of thinking:
%// Create empty cell array
img_cell = cell(num_iterations, 1);
for ii = 1 : num_iterations
%// Do your processing on data
%...
%...
img_cell{ii} = data; %// After iteration
end
Cell arrays are arrays that take on any type per element - or they're non-homogeneous arrays. This means that each element can be whatever you want. As such, because your image data varies in size at each iteration, this will do very nicely. To access data at any iteration, simply do:
data = img_cell{ii};
ii is the index of the iteration you want to access.
If you want to literally obtain what you are asking for, you can use the eval() function, which takes a string as input that it will evaluate as if it were a line of code. Example:
for i=1:3
data=ones(i); % assign data, 'ones(i)' used as dummy for test
eval(['Img_' num2str(i) '=data;'])
end
However, I would recommend using cell arrays {}, or alternatively the struct function that rayryeng both suggested.
I feel dumb even having to ask this, it really should be dead simple, but being new to MatLab I'd like to know how a more experienced person would do it.
Simple problem; I need to find some regions in multiple images, correlate them by position, save those regions of interest, and use them later. One way to do that would be to store the regions in a vector.
%% pseudo code
regions = [];
for i = some_vector_of_images
% segment, get mask
% find regions
cc = bwconncomp(mask);
stats = regionprops(cc, 'all');
% correlate against known x/y
% save for later
regions[index++] = stats;
end
% use 'regions'
However, the array declaration is problematic. Its default type is double, so that won't work (can't assign a struct to an element). I've tried struct.empty, but the array is not resizable. I've tried a cell array, but I get a similar error (Conversion to cell from struct is not possible.)
Really, I just need a way to have some collection declared prior to the loop that will hold instances of these structures. Again, pretty noobish question and slightly embarrassed here... please take pity.
See if using struct2cell helps you with this. Give this pseudo-code a try -
regions = cell(num_of_images,1) %// This will be before the loop starts
...
regions[index++] = {struct2cell(stats)} %// Inside the loop
Please not that this a pseudo-code, so square brackets and ++ won't work.
Thus, the complete version of pseudo-code would be -
%%// --- pseudo code
%// Without this pre-allocation you would get the error -
%%// "Conversion to cell from struct is not possible"
regions = cell(numel(some_vector_of_images),1)
for i = some_vector_of_images
% segment, get mask
% find regions
cc = bwconncomp(mask);
stats = regionprops(cc, 'all');
% correlate against known x/y
% save for later
regions(i) = {struct2cell(stats)}
end
You can cast your empty array to a structure array by appending a structure. Replace regions[index++] = stats; with
regions = [regions, stats];
This line will continue to build the array within the loop. This idiom is generally frowned on in MATLAB because a new array needs to be created each loop.
Another method is to preallocate the array with a template structure, using repmat.
stats = some_operations_on(some_vector_of_images(1));
regions = repmat(stats, numel(some_vector_of_images), 1);
and within the loop, assign with
regions(i) = stats;
In this scenario, typically I just don't preallocate at all, or use a cell-cat pattern.
Not initializing
This one doesn't initialize the struct array, but works fine. Make sure i is an index of each element in this case.
for i = 1:numel(some_vector_of_images)
% mask = outcome of some_vector_of_images(i)
cc = bwconncomp(mask);
regions(i) = regionprops(cc, 'all');
end
cell-cat pattern
This one catches results in a cell array, and concatenates all elements at the end.
regions = cell(numel(some_vector_of_images), 1);
index = 1;
for i = some_vector_of_images
% mask = outcome of i
cc = bwconncomp(mask);
regions{index} = regionprops(cc, 'all');
index = index + 1;
end
regions = cat(1, regions{:});
Suppose I have n .mat files and each are named as follows: a1, a2, ..., an
And within each of these mat files there is a variable called: var (nxn matrix)
I would like to create a matrix: A = [a1.var a2.var, ..., an.var] without writing it all out because there are many .mat files
A for-loop comes to mind, something like this:
A = []
for i = 1:n
[B] = ['a',num2str(i),'.mat',var];
A = [A B]
end
but this doesn't seem to work or even for the most simple case where I have variables that aren't stored as a(i) but rather 'a1', 'a2' etc.
Thank you very much!
load and concatenate 'var' from each of 'a(#).mat':
n = 10;
for i = n:-1:1 % 1
file_i = sprintf('a%d.mat', i); % 2
t = load(file_i, 'var');
varsCell{i} = t.var; % 3
end
A = [varsCell{:}]; % concatenate each 'var' in one step.
Here are some comment on the above code. All the memory-related stuff isn't very important here, but it's good to keep in mind during larger projects.
1)
In MATLAB, it is rarely a good idea or necessary to grow variables during a for loop. Each time an element is added, MATLAB must find and allocate a new block of RAM. This can really slow things down, especially for long loops or large variables. When possible, pre-allocate your variables (A = zeros(n,n*n)). Alternatively, it sometimes works to count backwards in the loop. MATLAB pre-allocates the whole array, since you're effectively telling it the final size.
2)
Equivalent to file_i = ['a',num2str(i),'.mat'] in this case, sprintf can be clearer and more powerful.
3)
Store each 'var' in a cell array. This is a balance between allocating all the needed memory and the complication of indexing into the correct places of a preallocated array. Internally, the cell array is a list of pointers to the location of each loaded 'var' matrix.
to create a test set...
generate 'n' matrices of n*n random doubles
save each as 'a(#).mat' in current directory
for i = 1:n
var = rand(n);
save(sprintf('a%d.mat',i), 'var');
end
Code
%%// The final result, A would have size nX(nXn)
A = zeros(n,n*n); %%// Pre-allocation for better performance
for k =1:n
load(strcat('a',num2str(k),'.mat'))
A(1:n,(k-1)*n+1:(k-1)*n+n) = var;
end