Matlab array of struct : Fast assignment

Question

Is there any way to "vector" assign an array of struct.

Currently I can

edges(1000000) = struct('weight',1.0); //This really does not assign the value, I checked on 2009A.
for i=1:1000000; edges(i).weight=1.0; end; 

But that is slow, I want to do something more like

edges(:).weight=[rand(1000000,1)]; //with or without the square brackets. 

Any ideas/suggestions to vectorize this assignment, so that it will be faster.

Thanks in advance.

Answer 1

This is much faster than deal or a loop (at least on my system):

N=10000;
edge(N) = struct('weight',1.0); % initialize the array
values = rand(1,N);  % set the values as a vector

W = mat2cell(values, 1,ones(1,N)); % convert values to a cell
[edge(:).weight] = W{:};

Using curly braces on the right gives a comma separated value list of all the values in W (i.e. N outputs) and using square braces on the right assigns those N outputs to the N values in edge(:).weight.

Answer 2

You can try using the Matlab function deal, but I found it requires to tweak the input a little (using this question: In Matlab, for a multiple input function, how to use a single input as multiple inputs?), maybe there is something simpler.

n=100000;
edges(n)=struct('weight',1.0);
m=mat2cell(rand(n,1),ones(n,1),1);
[edges(:).weight]=deal(m{:});

Also I found that this is not nearly as fast as the for loop on my computer (~0.35s for deal versus ~0.05s for the loop) presumably because of the call to mat2cell. The difference in speed is reduced if you use this more than once but it stays in favor of the for loop.

Answer 3

You could simply write:

edges = struct('weight', num2cell(rand(1000000,1)));

Answer 4

Is there something requiring you to particularly use a struct in this way?

Consider replacing your array of structs with simply a separate array for each member of the struct.

weights = rand(1, 1000);

If you have a struct member which is an array, you can make an extra dimension:

matrices = rand(3, 3, 1000);

If you just want to keep things neat, you could put these arrays into a struct:

edges.weights = weights;
edges.matrices = matrices;

But if you need to keep an array of structs, I think you can do

[edges.weight] = rand(1, 1000);

Answer 5

The reason that the structs in your example don't get initialized properly is that the syntax you're using only addresses the very last element in the struct array. For a nonexistent array, the rest of them get implicitly filled in with structs that have the default value [] in all their fields.

To make this behavior clear, try doing a short array with clear edges; edges(1:3) = struct('weight',1.0) and looking at each of edges(1), edges(2), and edges(3). The edges(3) element has 1.0 in its weight like you want; the others have [].

The syntax for efficiently initializing an array of structs is one of these.

% Using repmat and full assignment
edges = repmat(struct('weight', 1.0), [1 1000]);

% Using indexing
% NOTE: Only correct if variable is uninitialized!!!
edges(1:1000) = struct('weight', 1.0);  % QUESTIONABLE

Note the 1:1000 instead of just 1000 when indexing in to the uninitialized edges array.

There's a problem with the edges(1:1000) form: if edges is already initialized, this syntax will just update the values of selected elements. If edges has more than 1000 elements, the others will be left unchanged, and your code will be buggy. Or if edges is a different type, you could get an error or weird behavior depending on its existing datatype. To be safe, you need to do clear edges before initializing using the indexing syntax. So it's better to just do full assignment with the repmat form.

BUT: Regardless of how you initialize it, an array-of-structs like this is always going to be inherently slow to work with for larger data sets. You can't do real "vectorized" operations on it because your primitive arrays are all broken up in to separate mxArrays inside each struct element. That includes the field assignment in your question – it is not possible to vectorize that. Instead, you should switch a struct-of-arrays like Brian L's answer suggests.

Answer 6

You can use a reverse struct and then do all operations without any errors like this

x.E(1)=1;
x.E(2)=3;
x.E(2)=8;
x.E(3)=5;

and then the operation like the following

x.E

ans =

    3     8     5

or like this

x.E(1:2)=2

x = 

    E: [2 2 5]

or maybe this

x.E(1:3)=[2,3,4]*5

x = 

    E: [10 15 20]

It is really faster than for_loop and you do not need other big functions to slow your program.