I found this octave function, that returns a tuple in octave:
function [ret] = g(x)
ret(1, 1) = cos(x)
ret(1, 2) = sin(x)
end
I don't get the brackets why for [ret], as we are returning one variable, can you explain please? Because ret is a vector, and why vector inside a vector.
The square brackets there don’t form a vector, they collect the output variables. It is part of the function signature.
function [ret] = … is exactly the same as function ret = …. That is, the brackets are optional when there is a single return variable.
The same is true when there are no return variables, function [] = name(…) is the same as function name(…).
Related
I'm working on a MATLAB app that programatically creates anonymous functions to evaluate any native MATLAB function and pass it a list of variables as argument. In the example below, 'formula' contains a string with the function and arguments to be evaluated (e.g., "sum( var1, var2 )" ). The formulas sometimes contain function calls nested within function calls, so the code below would be used recursively until obtaining the final result:
Func2 = str2func( sprintf( '#(%s) %s', strjoin( varNames, ',' ), formula ) );
This evaluates fine for native MATLAB functions. But there's a particular case of a function (named Func1) I made myself that not only needs the list of variables but also an object as argument, like this:
function output = Func1( anObject, varNames )
% do some stuff with the object and the vars
end
For this particular function, I've tried doing this:
Func2 = str2func( sprintf( '#(%s,%s) %s', "objectToPassToFunc1", strjoin( varNames, ',' ), "Func1(objectToPass,""" + strjoin( varNames, '","' ) +""")" ) )
...which doesn't throw an error, but Func1 doesn't receive the objectToPassToFunc1, instead it gets values from one of the variables in varNames. And I don't know why.
So how can I correctly pass the object to Func1????
Matlab doesn't care about the type of arguments you pass to a function. As a matter of fact, the input could be scalar, vector, matrix, and even an object of a class. See the following example.
classdef ClassA
methods
function print(~)
disp('method print() is called.');
end
end
end
This class has only one method. Now, let us define an anonymous function func which accepts one input.
func = #(arg) arg.print;
Notice that we explicitly assume that the input is an object of ClassA. If you pass another type of data to this function, Matlab will throw an error. To test the code,
obj = ClassA;
func = #(arg) arg.print;
func(obj)
To avoid the error, you may need to check the type of the input before using it. For example,
function [] = func(arg)
% check if arg is an object of ClassA
if isa(arg,'ClassA')
arg.print;
end
end
Now you can pass different types for the input without getting an error.
function [TC]=Translate(T0,Base)
end
I know that Translate is a function and T0 and Base his parameter but what is [TC]?
Octave (and matlab) have a rather unique way of returning variables from functions. Instead of defining explicitly what to return from the function using a return keyword, they define from the outset which variables will be returned when the function exits, and octave simply looks for those variables by name at the time the function exits, and returns their values, whatever they may be by that point.
Your function may return nothing:
function returnsNothing();
disp('hello, I return nothing');
end
or it may return one output:
function Out = returnsOne(x)
Out = x+5
disp('This function will return the value of Out');
end
or it may return more than one outputs:
function [Out1, Out2] = returnsTwo(x)
Out1 = x+5;
Out2 = x+10;
end
You would call the last function from the octave terminal (or script) like this:
[a,b] = returnsTwo(5); % this will make a = 10 and b = 15
I have a function called objective in Matlab I evaluate by writing [f, df] = objective(x, {#fun1, #fun2, ..., #funN}) in a script. The functions fun1, fun2, ..., funN have the format [f, df] = funN(x).
Inside objective I want to, for each input in my cell array called fun, evaluate the given functions using the Matlab built-in function feval as:
function [f, df] = objective(x, fun)
f = 0;
df = 0;
for i = 1:length(fun)
fhandle = fun(i);
[fi, dfi] = feval(fhandle, x);
f = f + fi;
df = df + dfi;
end
end
I get the following error evaluating my objective.
Error using feval
Argument must contain a string or function_handle.
I do not get how to come around this error.
You need to reference the elements of fun using curly braces
fhandle = fun{i};
PS
It is better not to use i and j as variable names in Matlab
Alternatively, a solution using cellfun.
A more elegant approach using cellfun
function [f df] = objective( x, fun )
[f, df] = cellfun( #(f) f(x), fun );
f = sum(f);
df = sum(df);
Note the kinky use of cellfun - the cellarray is the fun rather than the data ;-)
If I call a matlab function with:
func(1,2,3,4,5)
it works perfectly.
But if I do:
a=[1,2,3,4,5] %(a[1;2;3;4;5] gives same result)
then:
func(a)
gives me:
??? Error ==> func at 11
Not enough input arguments.
Line 11 in func.m is:
error(nargchk(5, 6, nargin));
I notice that this works perfectly:
func(a(1),a(2),a(3),a(4),a(5))
How can I use the vector 'a' as a parameter to a function? I have another function otherfunc(b) which returns a, and would like to use its output as a paramater like this func(otherfunc(b)).
Comma-seperated lists
(CSL) can be passed to functions as parameter list,
so what you need is a CSL as 1,2,3,4,5 constructed from an array.
It can be generated using cell array like this:
a=[1,2,3,4,5];
c = num2cell(a);
func(c{:});
Maybe you could try with nargin - a variable in a function that has the value of the number of input arguments. Since you have a need for different length input, I believe this can best be handled with varargin, which can be set as the last input variable and will then group together all the extra input arguments..
function result = func(varargin)
if nargin == 5: % this is every element separately
x1 = varargin{1}
x2 = varargin{2}
x3 = varargin{3}
x4 = varargin{4}
x5 = varargin{5}
else if nargin == 1: % and one vectorized input
[x1 x2 x3 x4 x5] = varargin{1}
I've written x1...x5 for your input variables
Another method would be to create a separate inline function. Say you have a function f which takes multiple parameters:
f = f(x1,x2,x3)
You can call this with an array of parameter values by defining a separate function g:
g = #(x) f(x(1),x(2),x(3))
Now, if you have a vector of parameters values v = [1,2,3], you will be able to call f(v(1),v(2),v(3)) using g(v).
Just make the function take a single argument.
function result = func(a)
if ~isvector(a)
error('Input must be a vector')
end
end
Since arguments to functions in Matlab can themselves be vectoes (or even matrices) you cannot replace several arguments with a single vector.
If func expects 5 arguments, you cannot pass a single vector and expect matlab to understand that all five arguments are elements in the vector. How can Matlab tell the difference between this case and a case where the first argument is a 5-vector?
So, I would suggest this solution
s.type = '()';
s.subs = {1:5};
func( subsref( num2cell( otherfunc(b) ), s ) )
I'm not sure if this works (I don't have matlab here), but the rationale is to convert the 5-vector a (the output of otherfunc(b)) into a cell array and then expand it as 5 different arguments to func.
Please not the difference between a{:} and a(:) in this subsref.
You could create a function of the following form:
function [ out ] = funeval( f, x )
string = 'f(';
for I = 1:length(x)
string = strcat( string, 'x(' , num2str(I), '),' );
end
string( end ) = ')';
out = eval( string );
end
In which case, funeval( func, a ) gives the required output.
Use eval:
astr = [];
for i=1:length(a)
astr = [astr,'a(',num2str(i),'),']; % a(1),a(2),...
end
astr = astr(1:end-1);
eval(['func(' astr ');']);
When I pass a function (let's call it f) into my Base function , the Base function doesn't recognize
the f function , without using '' quotes , here's the code :
function y = test(a, b, n ,f)
if ( rem(n,2) ~= 0 )
error ( 'n is not even' )
end
% create a vector of n+1 linearly spaced numbers from a to b
x = linspace ( a, b, n+1 );
for i = 1:n+1
% store each result at index "i" in X vector
X(i) = feval ( f, x(i) );
end
y=sum(X);
end
And this is f.m :
function [y] = f (x)
y = 6-6*x^5;
When I run from command line with quotes :
>> [y] = test(0,1,10,'f')
y =
52.7505
but when I remove them :
>> [y] = test(0,1,10,f)
Error using f (line 2)
Not enough input arguments.
Where is my mistake ? why can't I execute [y] = test(0,1,10,f) ?
Thanks
The function feval expects either the function name (i.e., a string) or a function handle as input. In your code, f is neither a name, nor a handle. Use either the string 'f' or the handle #f when calling your base function test.
If, as posted in the comments, function handles are not allowed per assignment in the call to the base function, you can still use the function handle to create a string with the name of the function. This functionality is provided by the function func2str:
functionName = func2str(#f);
test(0,1,10,functionname);
Try passing #f as the argument instead of 'f', and also change the line to
X(i) = f(x(i));
The thing is that just f is not a function handle. Also there's no need to use feval in this case.