Hello :) ,
I have problems of runing time with the MatLab program I am working on, I hope someone could be able to give me a hand !
I am working with several classes and trying to use the power of OOP programming. But the size of the data I am working on is quite significant and I create huge arrays of classes object.
Before I show you part of the code in details, I explain that I am modestly familiar with the notion of MatLab code optimizing: I use the profiler to spot the slowness of my code, I pre-allocate my memory and I know what vectorization is.
I have a class called Vertix that has 6 properties, and I create an array V containing N "Vertix". N is big (around 100.000) ... Creating it and filling it is pretty fast (a few seconds), the problem is when I want to modify the following way.
I have :
vertices2change = ... ; % something small of 1xn size, like [ 3 5 7 ]
I want to access V(3), V(5) and V(7) to modify one of their properties. So I do :
for i=1:size(vertices2change,2) V(vertices2change(i)).property = smthg ; end
But this loop, of course, is VERY slow. Since I use it a lot in my code, this make my code run in hours, and according to the profiler, this is 99% the fault of this line...
The question is : how do I vectorize this line to make it faster ?
I already try the following but MatLab doesn't like it :
>> V(vertices2change).property = smthg Insufficient number of outputs from function on right hand side of equal sign to satisfy overloaded assignment.
Actually, the answer of
correspond to V(vertices2change(1)).property. Weird right ? At least there is something I didn't get !
I hope I was clear in my explainations and that my question wasn't stupid.
Thanks in advance ! :)
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But this loop, of course, is VERY slow.
Its not clear to me why it should be super-slow. Not if vertices2change consists of only 3 elements. Is 'Vertix' a handle class?
Anyway, it is a bad idea to work with object arrays of lengths like 100000. Same thing with structs. It scatters your data discontiguously across RAM, making it slow to access. It is better to combine your property data from the different V(i) into one matrix and make that a property of a scalar object V.
See also this recent, related thread,
Ok I made the modification and in fact it is way faster !
But by consequence of this modification I have an other problem that comes out and I don't know how to solve it without going in circles...
The property4 of my class Vertix is a property that can change of sizes in my work, and that I modify in a way that I add elements by concatenation. When I worked with an array of objects, I simply did :
V(i).property4 = cat(2,V(i).property4,smthg) ;
With originaly V(i).property4 of size 1xn and smthg of size 1xm, I get V(i).property4 of size 1x(n+m).
But now that I work with properties that are matrices, I don't know how to initialize V.property4. If I initialize it like 1xN and then use cat(1,..,..), it fill my matrices with 0 (I don't like it for my work), it change dynamicly the dimension of V.property4 and it leads me to a big matrix...
If I initialize it in the maximum dimension and fill it by dealing with zero (twisted), I get Out of memory because NxN is too big for my RAM (to find a contiguous block).
Any idea please :) ?
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