Function handle in varargin not recognized

When i am trying to input a function manually the scripts doesent recognize x ( Unrecognized function or variable 'x'. ) see pictures

How am i using the function handle wrong?

Thanks

function drawfunction(varargin)
switch nargin
    case 0
        disp('0 inparam. chosen')
        f = @(x) exp(atan(x)+sin(x))+log(2+x.^4)-(cos(3*x)/(1+x.^2));
        x = linspace(-3,8);
        color = 'r:';
        
    case 1
        disp('1 inparam. chosen')
        f = @(x) varargin(1);
        x = linspace(-3,8);
        color = 'r:';
        
    case 2
        disp('2 inparam. chosen')
        f = @(x) varargin(1);
        x = varargin(2);
        color = 'r:';
        
    case 3
        disp('3 inparam. chosen')
        f = @(x)varargin(1);
        x = varargin(2);
        color = varargin(3);
        
    otherwise
        error('Unexpexted inputs')
end

plot(f(x),color)

No input:

One input: (same function)

Answer 1

[Voss]

You're calling drawfunction with an expression that MATLAB tries to evaluate before sending the result of that evaluation in to drawfunction.

What you mean to do is to send a function handle to drawfunction, so call it like this:

drawfunction( @(x)exp(atan(x)+sin(x)) ) % ( etc., long expression )

Then, inside drawfunction, varargin{1} (note the curly braces, by the way) is that function handle you passed in, so there's no need to do this:

f = @(x)varargin(1) % neither one of these is correct
f = @(x)varargin{1}

It's just this:

f = varargin{1}; % varargin{1}, i.e., the first input argument, is already a function handle

[Voss]

Another way to specify a function as an input argument is as a character vector, e.g.:

func = 'exp(atan(x)+sin(x))';

If you want drawfunction to handle arguments like that, you can convert them to function handles inside drawfunction using str2func:

f = str2func(['@(x)' varargin{1}]); % assuming the function is a function of x

You can handle both char-vector and function-handle inputs by checking the class of varargin{1} and converting it if necessary:

if ischar(varargin{1}) % convert character vector to function_handle
    f = str2func(['@(x)' varargin{1}]); % assuming the function is a function of x
else % already a function_handle
    f = varargin{1};
end

And, by the way, now that that's a few lines of code, so you don't have to copy and paste it several times for the case when nargin is 2, 3, 4, etc., you can modify your argument handling structure a little bit:

function drawfunction(varargin)

if nargin >= 1 % at least one argument given
    if ischar(varargin{1})
        f = str2func(['@(x)' varargin{1}]);
    else
        f = varargin{1};
    end
else
    f = @(x)exp(atan(x)+sin(x)); % default function
end

if nargin >= 2 % at least two arguments given
    x = varargin{2};
else
    x = linspace(-3,8); % default x
end

if nargin >= 3 % at least three arguments given
    color = varargin{3};
else
    color = 'r:'; % default color/line-spec
end

if nargin >= 4
%     error('Unexpexted inputs');
    warning('drawfunction: Extra arguments ignored.');
else
    disp(sprintf('%d inparam. chosen',nargin));
end

plot(x,f(x),color);

end

[Mikael Freyhult]

Thank you very much!

I understand I will need to "create the function handle" when calling the function, or as you described use srting2func. I was a bit confused as why it didn't just pass the argument to varagin{1} but then i understand that it evaluates it "before" sending it into the function.

Sorry if my terminology is under par :P

If i may ask a subsequent question, is there any way to, without changing the code to enter a blank argument as varagin{1} and varagin{2} then inputing varagin{3}?

I didn't manage it by leaving blanks as -> drawfunction( , ,'b:')

to only change the plot, see image

i tried increasing number of cases to include only for example varagin{3} but it did not work

%%% Code below

function drawfunction(varargin)

switch nargin

case 0

disp('0 inparam. chosen')

f = @(x) exp(atan(x)+sin(x))+log(2+x.^4)-(cos(3*x)/(1+x.^2));

x = linspace(-3,8);

color = 'r:';

case 1

disp('first inparam. chosen')

f = varargin{1};

x = linspace(-3,8);

color = 'r:';

case 2

disp('1,2 inparam. chosen')

f = varargin{1};

x = varargin{2};

color = 'r:';

case 3

disp('1,2,3 inparam. chosen')

f = varargin{1};

x = varargin{2};

color = varargin{3};

case 4

disp('Third inparam. chosen')

f = @(x) exp(atan(x)+sin(x))+log(2+x.^4)-(cos(3*x)/(1+x.^2));

x = linspace(-3,8);

color = varargin{3};

case 5

disp('2,3 inparam. chosen')

f = @(x) exp(atan(x)+sin(x))+log(2+x.^4)-(cos(3*x)/(1+x.^2));

x = varargin{2};

color = varargin{3};

case 6

disp('1,3 inparam. chosen')

f = varargin{1};

x = linspace(-3,8);

color = varargin{3};

otherwise

error('Unexpexted inputs')

end

plot(f(x),color)

[Voss]

As you've discovered, you cannot send missing arguments like that

drawfunction( , , 'b:')

If you want to use an input argument, then all previous input arguments must also be specified.

However, you can write your function such that it handles certain values specially. For instance, you might use the empty array [] as a special value that indicates to the function that the default value for that argument should be used (this is only useful if [] is not a valid value for that argument).

If you do that, then you could call drawfunction like this:

drawfunction([], [], 'b:')

3 inparam. chosen

to get the default function and x values. Of course, you also have to set up drawfunction to handle [] appropriately, which is easy if you use the argument handling structure I showed in my last comment:

function drawfunction(varargin)

if nargin >= 1 && ~isempty(varargin{1}) % at least one argument given and it's not empty
    if ischar(varargin{1})
        f = str2func(['@(x)' varargin{1}]);
    else
        f = varargin{1};
    end
else
    f = @(x)exp(atan(x)+sin(x)); % default function
end

if nargin >= 2 && ~isempty(varargin{2}) % at least two arguments given and argument 2 is not empty
    x = varargin{2};
else
    x = linspace(-3,8); % default x
end

if nargin >= 3 && ~isempty(varargin{3}) % at least three arguments given and argument 3 is not empty
    color = varargin{3};
else
    color = 'r:'; % default color/line-spec
end

if nargin >= 4
%     error('Unexpexted inputs');
    warning('drawfunction: Extra arguments ignored.');
else
    disp(sprintf('%d inparam. chosen',nargin));
end

plot(x,f(x),color);

end

(Note that I changed

plot(f(x),color);

to

plot(x,f(x),color);

because I think that makes more sense.)