in a matlab script I have a vector which contains different instances of the same class. Now I would like to call the same function of each instance at the same time.
Is it, with tricky indexing, possible to do this WITHOUT a loop? For Example:
runner(1:100) = Human();
runner(:).run('fast'); % causes no error and no function call
In my project there is a vector with different neuronal networks (as objects). I tried to propagate all forward with one call.
Thanks for your time!
Well, there's one way if you're willing to modify your methods. What happens when you call something like runner(:).run('fast');? Rather than pass an instance of Human() to the run method 100 times it instead passes the entire vector of objects (an object array) to the method one time. Print out the object input from inside the method and see for yourself. Because nothing happens in your case, your run method must be written in such a way to ignore non-scalar objects. What you need to do is vectorize your methods. Depending on the nature of your methods, this may mean using for loops in some cases, but it will likely be the faster and more elegant option as it minimize the number of function calls.
Of course another option is to just call the run method 100 times using a for loop. If you really want "one call" then just hide the loop in a function or if you don't mind things to be a bit slower, use arrayfun which is nothing more than a function that hides a for loop:
arrayfun(#(obj)run(obj,'fast'),runner(:));
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Are Matlab Matrices transferred pass-by-value or pass-by-reference?
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I have several large matrices that should be passed to different functions many times in an iterative algorithm. How can I avoid the unnecessary copy of variables to the function to speed up the program? Is there any way to group these matrices in a structure or class and pass them by reference or pointer?
a=zeros(1000,1000);
b=zeros(1000,1000);
d=myfunction(a,b);
a=myfunction2(b,d);
....
Thanks.
Update: I should have provided more details as I see in the comments. I have several large matrices. Few of them remain constant in the program and others change in each function at each iteration. So if I pass these matrices by value, Matlab makes a copy of that, changes the value and again copy them as the output of the function and then they go out of scope and get destroyed and at each iteration all these unnecessary copying make the program awfully slow. If I were to program it in C++, I would use object oriented programming for these matrices and I would pass them as reference to the function but I don't know if that's possible in MATLAB.
Ander's comment is absolutely true. That said, if you do need the functions to modify the variable contents, then you might want to think about refactoring the code to try to avoid this scenario. I know the code that you posted is just a simplified example, but you could look at that and ask questions like "do a and b need to be initialized outside of the function? Could the function initialize them and return them instead?".
If you really can't get around it, then you could encapsulate the data in a handle class and pass the handle class to the functions instead.
I need to define a variable in matlab using a very long string lets call it S. S contains the result of a symbolic calculation and I want to use that result in my matlab code. S is too long to copy directly into the code so instead I read it from a file. I can evaluate the expression in S using eval and it works alright but it is really slow (It takes about 15 sec). I need to get the value of S for different values of the variables within it 1000s of times so I need a faster way to do this. Any ideas would be great.
Depending on what is your main goal you can do several things.
convert the expression to a script and call it from your function (speed improvement will depend on the expression)
if the expression is relatively smooth function without singular points then you can precalculate it for several values of input parameters. Then you interpolate that table for the remaining points. Should be faster than 15 sec anyway.
you can combine both ways with a draft/final switch in your code using interpolation for draft calculations and exact code for "final" run (possibly overnight ;o)
I have an analog input object (winsound) taking samples and performing a task on audio in MATLAB.
set(AI, 'SamplesAcquiredFcnCount',num_samples)
set(AI, 'SamplesAcquiredFcn',{#function1,AI,num_samples})
My understanding is that the analog input object just keeps taking samples, and hence function1 keeps being called when samples reach the set number. So it has 'priority'.
Now I have another function (function2), that I want to continuously process the (global) variables that the function1 updates. My problem is that in my current implementation function2 won't "come back" and look at the updated variables from the analog input.
I want to call function2 continuously up to a certain duration or condition, but how do I make it see the updated variables from the analog input function1? (they are already global, but once function2 has begun it doesn't get the updated variables, only as they were when it started).
I've tried to look into stuff in parallel computing toolbox to help me here, but haven't found anything. I really appreciate any help! This will really piece my project together
In the end the best way to ensure good side-by-side operation is just to call function2 inside function1. Since function1 is nicely controlled as a callback in the analog input object, this is the safest way to go about it.
I have a two-dimensional and 1-D arrays of different variables of a system which characterize an Equilibrium state of the system from where the next configurations going to evolve from. Now i need to save the data in these arrays at the specific situation and proceed, so how should i write on the declaration (the syntax) of the main program and also in the subroutine which use these arrays to be delivered to them by adjustable-size array method of passing arrays?
Uuugh, sorry, but I'm not quite clear on what you are asking.
Are you interested in the SAVE statement, which enables a variable to retain its previous state, once for example, a subroutine is called again?
MATLAB is a pass by value language. I have a recursive function that processes pixel's neighbors. It is very expensive to make the copy of the image (in my case two images) each time the function is called.
I used global variables to solve the problem. Is there any other way to make a recursive function modify an array?
You have three options here, but maybe you don't need any of them, since Matlab used 'copy-on-write', i.e. variables are only copied if you modify them.
As #gnovice mentions, you can use a nested function. Variables used inside the nested function are shared between the nested function and the enclosing function. Nested functions are somewhat tricky to debug, and a bit more complicated to write/understand.
You can store your images as properties of a handle object, which is passed by reference.
You can write code differently in order to not use a recursive function, since Matlab isn't the best language for using those. If you have access to the image processing toolbox, you may be able to use functions like blockproc, or im2col to rewrite the function.
Finally, if you want to stay with your current scheme, I strongly suggest using persistent variables instead of globals.
MATLAB is not always pass-by-value, newer versions of MATLAB do pass-by-reference under some circumstances, see in-place operations and a more general discussion about MATLAB memory management in this SO post.
Without tail-call optimization it is inefficient to use recursion and MATLAB does not have it as far I know, but every recursion can be transformed into a loop.
If you make your recursive function a nested function within another function where the image data is stored, then the recursive function can modify the image data without needing to have it passed to it.
This is a common misconception. Although the sytanx of MATLAB is pass by value, it does not actually pass by value as in C. The interpreter is smart enough to only make copies when necessary. So you should just go ahead and pass by value and see if you run into memory problems.
As other posters have noted, you should try to avoid recursion in MATLAB anyway.