I'm wondering how to write a function in Matlab with two different ways of passing the same input.
One example of this is the built-in function lsqcurvefit. You can either pass the input like this:
x = lsqcurvefit(fun,x0,xdata,ydata,lb,ub,options)
where the first 4 arguments are required and the last 3 are optional, OR you can call it like this:
x = lsqcurvefit(problem)
where problem is a struct containing the same inputs. In this case there is only one required input.
My question is: how do I write a function that can take the same input in two different ways?
I looked at documentation and can't seem to find it because I just don't know what words or terminology to search for... Can someone point me to this? I'm sure it's very simple. Maybe it's trivial and I'm just missing it?
Ben
EDIT: It seems what I may have been looking for is 'function overloading' and 'function signature'.
I am not sure on how to write code on matlab but I believe that the concept is the same for all programming languages so " just do method overriding where by one method will passes 4 parameters and other pass 2.
e.g
//assume that the following method are defined within the class
public void FirstFunction( int x, int y,int c,int f){
System.out.print(x + y + c + f);
}
public void SecondFunction( int m,int s){
System.out.print(m - s);
}
//within the main method on java you call these method
//so it will depend on the number of parameters you have passed the it will call.
Related
I'd like to be able to view the structure of objects in Matlab/GNU Octave the same way as I do in R (using the str() function). Is there a function that does this? An example task would be returning nr rows and cols in matrix, but also all the arguments for a given function.
I'm aware that I could use both size() and help() (but not for function files) separately to get this information.
There are several useful functions for displaying some information about Matlab objects (I can't say anything about Octave compatibility), but I'm not sure they'll provide the same detail as R's str(). You can display all of the methods of a class with the methods function, e.g.:
methods('MException')
which returns
Methods for class MException:
addCause getReport ne throw
eq isequal rethrow throwAsCaller
Static methods:
last
The what function will return similar results. Or methods can be used on an object of a given class:
ME = MException('Test:test','Testing');
methods(ME)
Similarly, you can view the properties with properties and the events with events.
Suppose I want to declare some variables then declare a function:
x = 2;
function y = function(x)
y = (x^2)+1;
end
y = function(x);
disp(y)
Matlab returns the error "Function keyword use is invalid here..."
Why can't I declare variables or write any text before declaring a function? Is there good reason or is it a quirk?
EDIT:
To clarify, I do know how to get around this problem (but thanks for the suggestions nonetheless) but I suppose I'm asking why the Matlab team made this decision. By making a function declaration the first line of a file, does it have implications for memory management, or something?
The REPL prompt of Scala can have a function defined after a variable. So this is a choice (a quirk if you want) from Matlab's inner internals.
If a function is defined in a file, there are two possibilities:
The main function of that file. Then the file must begin with the function declaration: in your example, function y = fun(x). I'm using fun as the function's name. I don't think function can be used as a function's name.
See here for more details.
A nested function. In this case, the function declaration and definition can be within another function of the preceding case.
See here for more details.
As you can see, in either case the file begins with a function declaration (namely that of the main function).
The function can also be defined as an anonymous function. Then no declaration is needed, and the function can be defined anywhere. But there's a restriction: the function can contain only a single statement (so it cannot define internal variables other than the output). Therefore this method can only be used for simple functions.
In your example, the function could be defined anonymously as fun = #(x) x^2+1.
See here for more details.
Others have given good info about nested functions and such.
But the reason for the error you get is that "function" is a reserved word in Matlab. You cannot have a function with this name.
function y = my_function(x)
y = (x^2)+1;
end
And stick it in another file called my_function.m
Is there an idiomatic way in Matlab to bind the value of an expression to the nth return value of another expression?
For example, say I want an array of indices corresponding to the maximum value of a number of vectors stored in a cell array. I can do that by
function I = max_index(varargin)
[~,I]=max(varargin{:});
cellfun(#max_index, my_data);
But this requires one to define a function (max_index) specific for each case one wants to select a particular return value in an expression. I can of course define a generic function that does what I want:
function y = nth_return(n,fun,varargin)
[vals{1:n}] = fun(varargin{:});
y = vals{n};
And call it like:
cellfun(#(x) nth_return(2,#max,x), my_data)
Adding such functions, however, makes code snippets less portable and harder to understand. Is there an idiomatic to achieve the same result without having to rely on the custom nth_return function?
This is as far as I know not possible in another way as with the solutions you mention. So just use the syntax:
[~,I]=max(var);
Or indeed create an extra function. But I would also suggest against this. Just write the extra line of code, in case you want to use the output in another function. I found two earlier questions on stackoverflow, which adress the same topic, and seem to confirm that this is not possible.
Skipping outputs with anonymous function in MATLAB
How to elegantly ignore some return values of a MATLAB function?
The reason why the ~ operator was added to MATLAB some versions ago was to prevent you from saving variables you do not need. If there would be a syntax like the one you are searching for, this would not have been necessary.
I have a function that returns two values, like so:
[a b] = myfunc(x)
Is there a way to get the second return value without using a temporary variable, and without altering the function?
What I'm looking for is something like this:
abs(secondreturnvalue(myfunc(x)))
not that i know of. subsref doesn't seem to work in this case, possibly because the second variable isn't even returned from the function.
since matlab 2009b it is possible to use the notation
[~, b] = function(x)
if you don't need the first argument, but this still uses a temporary variable for b.
Unless there is some pressing need to do this, I would probably advise against it. The clarity of your code will suffer. Storing the outputs in temporary variables and then passing these variables to another function will make your code cleaner, and the different ways you could do this are outlined here: How to elegantly ignore some return values of a MATLAB function?.
However, if you really want or need to do this, the only feasible way I can think of would be to create your own function secondreturnvalue. Here's a more general example called nth_output:
function value = nth_output(N,fcn,varargin)
[value{1:N}] = fcn(varargin{:});
value = value{N};
end
And you would call it by passing as inputs 1) the output argument number you want, 2) a function handle to myfunc, and 3) whatever input arguments you need to pass to myfunc:
abs(nth_output(2,#myfunc,x))
example
add(int a, int b) Here we pass two argument int a, int b,
can we pass n argument with different data types like int,float,long,
is it possible?
I means i need to write only one method which handles n Arguments.
Yes, it is possible to pass arguments with different data types to Objective-C methods. I assume you're speaking about Objective-C, since the question is tagged iphone.
For example, you could have a method like:
- (void)foo:(int)foo bar:(float)bar baz:(long)baz {
// ...
}
If you are talking about C, you could have a function:
void myfunc(int foo, float bar, long baz) {
// ...
}
If you want to write functions that have variable sized argument lists (so it can take 1 parameter, or 2 parameters, or 3, ...), I suggest you take a look at this blog post which discusses variable argument lists in Objective-C/Cocoa or Apple's technical Q&A on variable argument lists.
- (int)add:(int)number1 to:(int)number2 {
return number1 + number2;
}
call it using [obj add:2 to:4]; where obj is the object receiving the add message.
You may use any type of arguments in the above to achieve what you have in mind. For example:
- (void)add:(int)number1 to:(float)number2 {
//print result here
}
Standard type conversions etc would apply like in any programming language.
If you are talking about totally arbitrary number of arguments, take a look at Variadic functions in Objective C (Google it). Plenty of good tutorials in the first search results page.
Anything is possible, but I am pretty certain it would require some serious hacking to get it to work for variable data types. If you can change the requirements to be a variable number of specific data type, then you should look at an excellent tutorial at "Cocoa with Love" : variable-argument-lists-in-cocoa