I have a function that goes something like this:
function [] = function1
-variable1= value1;
-variable2= value2;
-matrix1=[]
-matrix2=[]
-loops that fill in matrix1 and matrix2
-more declarations
-final function1 end
function2[]
takes values from function 1 and does stuff, creates new variables
funcion2 end
function3[] that uses values from function 2 and function4[] that uses function 3.
(Sorry for all of that by the way). Now, my question is, is there anyway to save variables, arrays, etc. in the workspace for later analysis? All 4 functions are on the same tab in the MATLAB editor.
To save any particular variable from workspace in Matlab you should execute the following
save('Name of the mat file','Name of the variable')
To save multiple variable the command shall be
save('name of the mat file','var1','var2'...)
variable can be array, matrix, structure, double anything. For any other use of the function save please docsearch it in the command prompt
Related
I am trying to understand the following commands of a MATLAB script :
global operatorObj
calcEVR_handles = operatorObj.calcEVR_handles;
m = operatorObj.nInputs
E = zeros(m,1);
V = zeros(m,1);
R = zeros(m,m);
for i=1:m
[E(i), V(i), R(i,i)] = calcEVR_handles{i}(t,x);
end
What can calcEVR_handles be, if t is a float and x is a vector?
calcEVR_handles (to me) looks like a cell array where each element is a handle to a function. Each element in calcEVR_handles is an anonymous function that takes in a single value t and a single vector x. As such, by doing calcEVR_handles{i}, you would access the corresponding function stored at the ith element in the cell array. Once you have access, you then pass your parameters to this function and it gives you those three outputs.
To show you an example of this working, consider the following cell array that works similarly to calcEVR_handles.
calcCellFunc = {#sin, #cos, #tan};
This is a three element cell array, where each element is a handle to a function. The # is a special character in MATLAB that denotes that you are creating a handle to a function. It's also used to create anonymous functions, but let's shelve that for this answer. You can read more about it here if you want to delve into more detail regarding this.
Back to our cell array of handles, we will make handles for sin, cos and tan. You can then iterate over your cell array by accessing the function you want by calcCellFunc{idx} where idx is the element you want in the cell array. This will ultimately give you the function stored at index idx. Once you do that, you can then call the function and specify whatever inputs you want (or none if it doesn't take any inputs). Here's a quick example for you. Let's create a random 5 x 5 matrix, and run through each function with this matrix serving as the input. We then take each of these outputs and store them into a corresponding slot in an output cell array. As such:
rng(123); %// Set seed for reproducibility
M = rand(5);
calcCellFunc = {#sin, #cos, #tan};
out = cell(1, numel(calcCellFunc)); %// To store the results for each function
for idx = 1 : numel(calcCellFunc)
out{idx} = calcCellFunc{idx}(M); %// Get the function, then pass
%// the matrix M to it
end
If you want to make things clear, you could split up the out statement to this instead:
func = calcCellFunc{idx}; %// Get access to the function
out{idx} = func(M); %// Pass M to this function
If you're new to handles / anonymous functions, you should probably use the above code first to make it explicitly clear on what MATLAB is doing. You are first getting access to the function you want that is stored in the cell array, and then you pass your arguments to this function.
If we display the output, we get:
>> celldisp(out)
out{1} =
0.6415 0.4106 0.3365 0.6728 0.5927
0.2823 0.8309 0.6662 0.1815 0.7509
0.2249 0.6325 0.4246 0.1746 0.6627
0.5238 0.4626 0.0596 0.5069 0.5737
0.6590 0.3821 0.3876 0.5071 0.6612
out{2} =
0.7671 0.9118 0.9417 0.7398 0.8054
0.9593 0.5564 0.7458 0.9834 0.6604
0.9744 0.7745 0.9054 0.9846 0.7489
0.8518 0.8866 0.9982 0.8620 0.8191
0.7522 0.9241 0.9218 0.8619 0.7502
out{3} =
0.8363 0.4503 0.3573 0.9094 0.7359
0.2942 1.4934 0.8932 0.1845 1.1370
0.2308 0.8167 0.4690 0.1773 0.8850
0.6149 0.5218 0.0597 0.5880 0.7004
0.8761 0.4135 0.4205 0.5884 0.8814
The first element of the output cell array has the output when you pass M to sin, the second when you pass M to cos, and the third when you pass M to tan.
So the next question you're asking... why is this useful?
Point #1 - Nix the copying and pasting
This kind of code writing is very useful because if you want to use the same inputs and supply them to many different functions, we would naturally be inclined to do some copying and pasting. Take each of your function names, and create a single line for each. Each line would call the corresponding function you want, followed by the input arguments. This can become quite tedious, and so one smart way to do it would be to place your function name as a handle into a cell array, and to write one for loop that goes over all of the functions dynamically. You could even explore cellfun and escape using the for loop to iterate over all of the function handles too, but I'll leave that for you to read up on.
In this way, you have very maintainable code and if you want to remove functions that don't need to be run, just remove the handles from the cell array rather than scrolling down to where the line that invokes this function is located and removing that.
This is actually a very common technique in computer science / software engineering in general. In fact, this is actually quite close to what are known as function pointers. This is MATLAB's cheap way of doing it, but the logic behind this is essentially the same.
Point #2 - Higher Order Functions
Another way this is useful is if you have a function where one (or more than one!) of the inputs is a function, and you also specify inputs into this function as additional parameters to this function. This is what is known as a higher order function. The outputs would be based on using this input function, and the additional inputs you specify to it and the outputs are based on using this input function and the inputs you specify for this function.
One very good example is the fzero function in MATLAB. The goal is to find the root of a non-linear function, and the first parameter is a handle to a function that you specify. The base behaviour behind how fzero works is the same no matter what the function is. All you have to do is specify the function you want to solve and the initial guess of where you think this root is.
All in all, anonymous functions are very useful.
I have a MATLAB function to solve a Inertia Tensor , and I have a nested function in my program . All the variables in it are symbolics but it told me
“Error using assignin: Attempt to add ”x“ to a static workspace”
and I don't understand why this happens . Here is my test.m code:
function test
syms x y z
f=x
f1=f+1
f2=f1^2
function r=test2
r=f2^3;
end
f3=test2
end
After searching this web-forum I have found some answers . But at the same time I just don't understand it
Andrew Janke explianed it like this : While syms A may look like a static variable declaration, it isn't. It's just a regular function call. It's using Matlab's "command" invocation style to look like syntax, but it's really equivalent to syms('a', 'b', 'c').
on this page : Matlab: "Error using assignin: Attempt to add "c" to a static workspace"
what does static variable mean ?
I also search the HELP doc and it said :In functions and scripts, do not use syms to create symbolic variables with the same names as MATLAB® functions. For these names MATLAB does not create symbolic variables, but keeps the names assigned to the functions.
I only know syms x to create a symbolic variable in the workspace but why does the documentation say MATLAB does not create ?
'Static' means fixed, 'workspace' is what Matlab calls the places where all of its variables are stored. For non-nested functions the workspace starts off as empty when Matlab is at the beginning of the function; as Matlab continues through function's lines of code it continuously add more variables to the workspace.
For functions with a nested function, Matlab first parses the function to see what variable will be created (it specifically looks for x = type lines), then it creates all of these variables (with value as 'unassigned'), and then only does it start to run through the code; but while running through the code, it can never create a new variable.
This is why the code
function TestNestedFunction
syms x;
function Nested()
end
end
generates an error, there is no x = to tell it to pre-create the unassigned variable x at the start of the code. It fails at syms x;, as that line tries to create a new variable x, which fails as it may not.
This is also why the following code runs
function TestNestedFunction
syms x;
x = x;
function Nested()
end
end
it sees the x = and then pre-creates x. (This is why your example of adding [x, y, z] = deal([]); also works).
You can test this with a break point at the beginning of simple non-nested function and a simple nested function. Just run it step by step.
This code works:
function test
x=sym('x')
y=sym('y')
z=sym('z')
f=x
f1=f+1
f2=f1^2
function r=test2
r=f2^3;
end
f3=test2
end
I think the pages you found are quite clear.
You need to declare the variables one by one and use:
x = sym('x')
Otherwise syms will try to assign the values into a workspace where this is not allowed.
I'm looking for a way to cast the entire workspace in matlab to some class (say double).
For simplicity, lets just assume that only "simple" classes are present in the workspace (no cells or structs). Of course I can go line by line and change each variable, x=double(x) , but that's not practical if I have several 100's of variables. This is what I've wrote so far:
% # generate some variables of different classes
a1=int32(120);
a2=single(rand(10));
a3=double(rand(20));
a4=(rand(5)>0.5); %# logical
% # collect workspace variables using `whos`
ws=whos;
for ii=1:size(ws,1)
[ ? ] = double(eval(ws(ii).name))
end
The last line double(eval(ws{1,ii})) performs the casting, but how should I assign it's output automatically to the original variables names?
You are also welcome to suggest an alternative way to cast all variables of the workspace, if you can think of one...
Interesting question (+1). What about this?
ws = whos; %# Obtain workspace
for n = 1:size(ws,1)
eval([ws(n).name, ' = double(', ws(n).name, ');']); %# Assign to double
end
This worked for me on R2012b. The trick is to alter the variable type and assign it with one call to eval.
For example in testinit.m I have the following
function [x, y, m] = testinit
x=4
y=3
m=2
When I run testinit in the console it correctly displays the value. However when I type x it says
error: 'x' undefined...
Just to add to the above answer, the reason you're getting this is because variables in a MatLab function are local variables, they are not passed to the workspace unless you use one of the functions in the above answer. You can read more about global and local variables here.
P.S If you wrote an m-file that is not a function, then the variables are global.
There's the assignin function (evalin is related). And also global.
The other answers are all possible solutions, but potentially more complicated than what you may be looking for. I think the first part of yuk's answer addresses the real problem you are having, but I think it deserves a more detailed explanation...
If you have a function that has output arguments, you need to actually capture those arguments in variables when you call the function. For example, if you typed this in the Command Window:
[x, y, m] = testinit;
Then you would have your three output values present for you to use. What you were probably doing was typing this:
testinit;
This would display the values (because you didn't end each line in the function with a semicolon to suppress displaying them), but it would not store them in variables in the Command Window for you to use later.
This is a result of how variables are stored in MATLAB, as described by the documentation on variable scope:
MATLAB stores variables in a part of memory called a workspace. The base workspace holds variables created during your interactive MATLAB session and also any variables created by running scripts. Variables created at the MATLAB command prompt can also be used by scripts without having to declare them as global.
Functions do not use the base workspace. Every function has its own function workspace. Each function workspace is kept separate from the base workspace and all other workspaces to protect the integrity of the data used by that function. Even subfunctions that are defined in the same file have a separate function workspace.
So, if you want to share variables between functions, the simplest way is to pass them back and forth via their input and output argument lists.
It should also be noted that the names you give variables in the output argument list of the function don't have to match the names of the variables you place those output values in. For example, given this function:
function [a, b, c] = testinit
a = 4;
b = 3;
c = 2;
You can make this call in the Command Window:
[x, y, m] = testinit;
And you will get x = 4, y = 3, and m = 2.
If you run [x, y, m] = testinit in the console you should get the variables. The output variables can have any allowed name, not necessary x, y and m.
In addition you should put ; after each variable assignment in the function to avoid their output to the console. You can control the console output while calling the function.
If you want simply initialize new variables just by typing testinit, use assignin as in #BenVoigt's answer.
assignin('base','x',4)
However, this is dangerous, since some variable may already exist in the calling environment and will be overwritten. You can avoid it adding EXIST tests (inside EVALIN):
function testinit
if ~evalin('base','exist(''x'',''var'')')
assignin('base','x',4)
end
if ~evalin('base','exist(''y'',''var'')')
assignin('base','y',3)
end
if ~evalin('base','exist(''m'',''var'')')
assignin('base','m',2)
end
You can also use 'caller' instead of 'base' if you plan to call the function from another function.
The variables are local to the function so you cannot access them from the command line. As #BenVoigt said, you can use asignin but it's very dirty, and I don't think that it's what you really want to do.
I advise you do go in debug mode
Add a break point or a keyboard to your function like that:
function [x, y, m] = testinit
x=4
y=3
m=2
keyboard
After execute your function, and the command line will remain in the environment of the function.
The usual >> kill be replaced by a K>>. At that point you can access all your local variables.
To quit the debug mode type dbquit, or press shift+F5
I'm writing a program in MATLAB to solve integrals, and I have my function in a .M-file. Now I wonder how I can write a program in the .MAT-file that lets the user set a value that exists in the both files. The .M-file looks like this:
function fh = f(y)
fh = 62.5.*(b-y).*(40-20.*exp(-(0.01.*y).*(0.01.*y)));
and as you can see, the function depends on two variables, y and b. I want the user to set b. I tried putting b = input('Type in the value of b: ') in the .M-file but for some reason the user would then have to put in the same value four times.
Can I ask for the value of b in the .MAT-file?
Firstly, m-files store code (i.e. functions), while MAT-files store data (i.e. variables). You can save workspace variables to a MAT-file using the function SAVE and load them into a workspace from a file using the function LOAD. If you have a user choose a value for b, then save it to a MAT-file ('b_value.mat', for example), you can simply load the value from the MAT-file inside your m-file function like so:
function fh = f(y)
load('b_value.mat','b');
fh = 62.5.*(b-y).*(40-20.*exp(-(0.01.*y).*(0.01.*y)));
However, this is not a very good way to handle the larger problem I think you are having. It requires that you hardcode the name of the MAT-file in your function f, plus it will give you an error if the file doesn't exist or if b isn't present in the file.
Let's address what I think the larger underlying problem is, and how to better approach a solution...
You mention that you are solving integrals, and that probably means you are performing numerical integration using one or more of the various built-in integration functions, such as QUAD. As you've noticed, using these functions requires you to supply a function for the integrand which accepts a single vector argument and returns a single vector argument.
In your case, you have other additional parameters you want to pass to the function, which is complicated by the fact that the integration functions only accept integrand functions with a single input argument. There is actually a link in the documentation for QUAD (and the other integration functions) that shows you a couple of ways you can parameterize the integrand function without adding extra input arguments by using either nested functions or anonymous functions.
As an example, I'll show you how you can do this by writing f as an anonymous function instead of an m-file function. First you would have the user choose the parameter b, then you would construct your anonymous function as follows:
b = input('Type in the value of b: ');
f = #(y) 62.5.*(b-y).*(40-20.*exp(-(0.01.*y).^2));
Note that the value of b used by the anonymous function will be fixed at what it was at the time that the function was created. If b is later changed, you would need to re-make your anonymous function so that it uses the new value.
And here's an example of how to use f in a call to QUAD:
q = quad(f,lowerLimit,upperLimit);
In your m file declare b as a global
function fh = f(y)
global b
fh = 62.5.(b-y).(40-20.*exp(-(0.01.y).(0.01.*y)));
This allows the variable to be accessed from another file without having to create another function to set the value of b. You could also add b to the arguments of your fh function.