Is it possible to retrieve a local variable from a programme-function i've run in matlab? i.e. i want to retrieve a variable from the code, which is not appeared in the outputs.
Thanks in advance
The following describes code to add to the function itself to make the variable available outside the local scope. When you can't change the function, from the outside there is nothing to be done about changing the scope of course (which is intended, correct behaviour!!).
Dirty ways:
global variables
global t
t=2.468;
For scalars, strings, simple values: assign to variables in base workspace using evalin:
t=2.468;
evalin('base', ['var_in_base=' num2str(t) ';']);
Any other variable, use assignin:
A=magic(20);
assignin('base','A',A);
Proper way:
Inspect them during debugging
If you really want them outside the local scope, add them as output variable!!
Look at Declare function. You can access local variables if you return them as return values. If you do not, you can't access them from outside.
So in
function [mean,stdev] = stat(x)
n = length(x);
mean = sum(x)/n;
stdev = sqrt(sum((x-mean).^2/n));
You have access to mean and stdev but there is no way to access n.
I don't know matlab at all, but from programmer's logic
that seems improper and impossible without hacking the code.
That being said, through Google I saw this:
When you call a script from a function, the script uses the function workspace.
Like local functions, nested functions have their own workspaces. However, these workspaces are unique in two significant ways:
Nested functions can access and modify variables in the workspaces of the functions that contain them.
All of the variables in nested functions or the functions that contain them must be explicitly defined. That is, you cannot call a function or script that assigns values to variables unless those variables already exist in the function workspace.
Base and Function Workspace
Not sure if this helps you at all, but it may clarify some points
Related
I have a script with lots of variables (10-30), and in a for loop I do some processing using basically every variable. I have a helper script so that the interior of the loop looks clean.
main script
a=2;b=5;c=8;d=10;%and many many more
for i=1:1000
helper;
end
I want to turn helper into a function, primarily because this would allow me to have the helper script (now a function) at the bottom of the main file. But I definitely do not want to individually pass every variable and every variable that I have yet to need in this project.
What I really need is a goto, but matlab doesnt have it.
There are many solutions to this. In rinkert's comment you've gotten the only one that is good coding practice: put the variables in a struct, then pass the struct into your function. I highly recommend that you do that. Using nested functions is not a terrible solution either, though it could be a lot more difficult to manage (variable visibility rules are funny in nested functions, I find it confusing at times).
Here I'm going to give you the worst possible solution: evalin. You should not do this, I post this here for educational purposes only.
evalin allows a function to retrieve a variable in the caller's workspace:
a=2;b=5;c=8;d=10;%and many many more
for i=1:1000
helper;
end
function helper
% NEVER DO THIS IN PRACTICE
a = evalin('caller','a');
b = evalin('caller','b');
c = evalin('caller','c');
d = evalin('caller','d');
% do computations....
assignin('caller','result',result);
end
Note that a function result = helper(a,b,c,d) is much more efficient and much easier to maintain because it's clear what is going on. evalin makes for surprising results, and assignin even more so. Do not use this.
There are a few legitimate uses of these functions. For example, I've written a GUI that allows a MATLAB user easier access to a set of functions, but is meant to work in conjunction with the interactive MATLAB command prompt. The GUI would fetch variables from the 'base' workspace, evaluate function calls in the 'base' workspace, and write new variables to the 'base' workspace. Running a function within the GUI is just like running it at the command prompt. There is no other way of accomplishing this other than using evalin and assignin.
although it's not good coding conduct, you could use global variables :
for example :
global a;
a= 5;
function out =test()
global a;
out=2*a;
end
Is there any way to define a variable in main function and use it in all sub-function.
I've tried to declare variables as global but it seems I should repeat it in all function again. I'm wonder what's the benefit of global variable at all!
use variable as global:
main program
global x
syms x
subfunc1
subfunc2
...
and
subfunc1
global x
and
subfunc2
global x
(maybe this format remind us to have global variable in function but it was better to cause error if we use same name of variable in function same as Matlab keywords)
I don't want to import the variable as all function argument and don't want to declare that variable in all function again and again.
any help would be appreciated.
If you really want to have access to the same variable, then there are only two ways I know of in Matlab:
nested functions(described by answer from #justthom8) and global variables. Other methods of getting data into functions exist, such as getappdata, guidata and (my personal favorite:) passing function arguments. However, these approaches make copies of variables.
Perhaps you should ask yourself why you want to avoid making copies of variables. If you are worried about performance, you should know that Matlab efficiently use variables as reference to data only, so you can safely send a variable in to a function (thereby copying the variable) without copying the actual data. its first after you modify the data inside of the function that the data is actually copied. All this is totally invisible to us, except as a possible drop in performance during alot of copying. This is called copy-on-write.
Global variables can be used to optimize Matlabs performance, by coding them in such a way as to avoid copying data, but that really requires knowing what you are doing, and it opens up for a whole lot of pitfalls, especially if your projects grow in size.
One thing you can do is define the other functions as subfunctions of the main function. Something like below
Both of the functions subFunc1 and subFunc2 should have access to data you define above it in mainFunc
function mainFunc()
variable1 = 'stuff';
variable2 = 5;
function subFunc1()
%do stuff
end
function subFunc2()
%do more stuff
end
end
Edit 1
Of course you can define global data in the mainFunc that gets used in the subfunctions, but I wouldn't recommend doing that since it can get changed in unexpected ways that you don't intend for to happen.
I would like to have a list of constants readily available to use in any script or function I write. For example, I have been defining constants like hbar (Planck's constant) at the start of any script that will be using it.
Instead of that, should I:
make a list of constants in a script and load that script every time
I want to use it,
or save constants in a workspace and load that,
or is it possible for me to have global variables that will be there even when I close and reopen Octave,
or something else?
If you use GNU Octave I would suggest using the miscellaneous package and the function physical_constant which already has 335 constants. In your case:
[val, uncertainty, unit] = physical_constant ("Planck constant over 2 pi")
val = 1.0546e-34
uncertainty = 4.7000e-42
unit = J s
If you want don't want this, then use functions, not global vars.
As you indicated, there are a few ways to solve this problem. To address your third option, which seems to be closest to the spirit of what you want, you've got at least two ways of handling this.
1.) If the variables need to be mutable. Create a function or script that initializes the variables to what you want them to be. I'm going to reference the MATLAB documentation but it should basically be the same.
function initglobals()
global the_answer
the_answer = 42;
end
Then any time you want to use these globals in a script, you first indicate to Octave that you'll be using the variable as a global:
...
global the_answer
disp(the_answer) %prints 42
...
For this to be more useful, I'd recommend generating a startup script and putting in your .octaverc docs. This startup script can call this function to initialize your globals.
2.) Your other choice, if the globals ought to be immutable (for example, a physics constant) is to define a function that returns the value you want.
function [out] = the_answer()
out = 42;
end
Then you can simply just use the_answer to access your constant.
In both cases you'll want to add these functions to your path. Create your collections of functions and put them somewhere, then add that location to your path. docs
I have built a matlab function and I want to access one of its variables ,say x in the workspace. If I write x in the workspace it says Undefined function or variable "x"
Using the global modifier will definitely make your variable visible to the workspace, but it will also make it visible to any other function you call that happens to use that variable name. So if you insist on doing it this way, make sure that your variable name is unique.
A better way, in my opinion, is to pass back the value of the variable as a return value from the function, though this may require changes to calling functions.
Other options are detailed here:
http://www.mathworks.com/help/matlab/matlab_prog/share-data-between-workspaces.html
When one has a function, which needs many specific variables when called, what is a good way, to pass them?
Should one always pass all variable explicitly? How about storing them in structs (or objects)? But somehow I think, this makes things a bit obscure, as these structs/objects have to be well defined to make sure, they have all the fields, when handed in to the function in question.
Also probably local variables are accessed most quickly, whereas adressing struct fields might be slow in a loop...
Or is it a good thing to run an external script where everything is defined global (but even in this case on has to make everything available to the function using the global keyword)
A good way to pass many variables to a function, is to use varargin. Use it together with nargin.
function varlist2(X,Y,varargin)
fprintf('Total number of inputs = %d\n',nargin);
nVarargs = length(varargin);
fprintf('Inputs in varargin(%d):\n',nVarargs)
for k = 1:nVarargs
fprintf(' %d\n', varargin{k})
end
Varargin can also be used for optional variables.. So it gives a lot of options...