How must I declare a function handle that refers to a function you've written? What syntax is necessary?

I keep getting the error, "??? Output argument [...] not assigned during call to" my function psi_en whenever I try any of the following commands:

Case 1:

quad_fun = @(E) mass_ab_E(E).*exp(-depth*mass_E(E)*rho_water) ...
                                                      .*psi_en(E,spectrum);
max_energy = spectrum(length(spectrum),1);
y = quad(quad_fun,0,max_energy);

Case 2:

quad_fun = @(E) mass_ab_E(E).*exp(-depth*mass_E(E)*rho_water);
max_energy = spectrum(length(spectrum),1);
y = quad(@(E) quad_fun.*psi_en(E,spectrum),0,max_energy);

Case 3:

psi_E = @(E) psi_en(E,spectrum); % error points to both lines as problematic
quad_fun = @(E) mass_ab_E(E).*exp(-depth*mass_E(E)*rho_water).*psi_E(E);

I think it is angry that I'm trying to define a function handle that refers to a function I've written! I can execute the function directly, e.g. "psi_en(0.02,spectrum)" returns "ans = 0.0041", but it takes issue with this function handle declaration. What must I do, and why is it unhappy?!

psi_en.m contains the following; it is intended to assign values to psi_en based on where the quad integration variable E falls within the spectrum histogram (first column is the energy bin, second column is the energy's weighting factor psi_en):

function y = psi_en(en,spect)
 if en < 0
     error('energy must be positive!')
 elseif en == 0
     y = 0;
 else
     if (en > 0) & (en <= spect(1,1))
         y = spect(1,2);
     end
     for i = 1:length(spect)-1
         if (en > spect(i,1)) & (en <= spect(i+1,1))
             y = spect(i+1,2);
         end
     end
     if en > spect(length(spect))
         error('Energy surpasses data!')
     end
 end

[Walter Roberson]

Please show the code for psi_en

[Daniel]

I have edited the post to include this content. Thank you!

Answer 1

[Walter Roberson]

You have flows of control where y is not set, or at least not obviously so.

When you have a function that returns a value, you should make sure that the output value is always defined (except possibly in cases you are going to error() out of.) You could initialize y to inf for example.

[Daniel]

My problem seems more than that: I initialized y = 0 before entering the if statements, and now it's giving only 0. It appears not to be using the if statements; that is, it seems I am not properly calling the function. "psi_E = @(E) psi_en(E,spectrum);", "quad_fun = @(E) mass_ab_E(E).*exp(-depth*mass_E(E)*rho_water).*psi_E(E);" and "y = quad(quad_fun,0,max_energy);" -- it is not working as I think it should. Why is it so difficult to use this quad function?!

[Walter Roberson]

quad passes a vector argument -- your E will be a vector, and that will reach your function psi_en through the argument en. You then do tests suitable for _scalars_ on the _vector_ . When "if" is applied to vector (or array) of values, the test is considered true if and only if _every_ value in the vector (or array) is non-zero -- if *all* of the values meet the condition.

If you are only getting the 0 output, the implication is that none of your "if" statements was true for all elements in the vector.

You need to either switch over to using a loop over the "en" values, or else switch over to using logical indexing (a better approach.)

Answer 2

[Kevin Holst]

Is 'spectrum' actually getting passed into your function psi_en? It looks like, if it isn't and 'en' in passed in correctly, 'y' would never be set.

[Daniel]

Well, I'm certainly trying to pass it (cf. the cases of code excerpts)! The function works when I execute it directly, but I think the problem is trying to assign a function handle to to an expression that includes the function -- because I'm not actually executing the function in that case, but only including it in an expression, but MATLAB gets confused thinking I'm trying to call the function. Must I work with strings or something?

[Kevin Holst]

I don't think the problem lies in assigning a function handle to to an expression that includes the function, there's something else at play. Which line of code in your main function is producing the error?

[Daniel]

It's the lines that involve the function handle. Assigning a value at the beginning to later be overwritten within the if statements solves the problem, as Roberson suggested, but it appears still not to be "working" as I like -- that is, psi_en is then always that value, apparently...