Codegen fails with "wrong number of input arguments" while function calls without error

I'm trying to generate a mex file for the following function:

function matInt = testfun(a,b,M) 
matfun = @(x) squeeze(interpn([-1 0 1],[-1,0,1],M,x(1),x(2)));

matInt = integral(@(s) matfun(a*s+(1-s)*b), 0, 1, 'ArrayValued',true);
end

using the following script:

clear; close all;

%% Test arguments
a = [.5;.3];
b  = [-.2;-1];
M = randn(3,3,5,5);

%% Run function
Mint = testfun(a,b,M);   % okay

%% Codegen
% error:
codegen testfun -args {a,b,M}

Running the function (so computing Mint) goes fine without any errors. But generating a mex file using the same input arguments (in the last line) throws the error:

??? Wrong number of input arguments.

Error in ==> testfun Line: 2 Column: 23
Code generation failed: View Error Report

I'm not exactly what goes wrong there, as I do not see how I do not give the correct number of input arguments.

Answer 1

[Mike Hosea]

I can't find where this behavior of INTERPN is documented. To the contrary, we have:

>> help interpn

interpn N-D interpolation (table lookup).

Vq = interpn(X1,X2,X3,...,V,X1q,X2q,X3q,...) interpolates to find Vq,

the values of the underlying N-D function V at the query points in

arrays X1q,X2q,X3q,etc. For an N-D V, interpn should be called with

2*N+1 arguments. [snip]

I mean, the behavior is obviously useful, but the codegen version is only trying to support documented INTERPN behavior. You can rewrite your example to avoid relying on this behavior by using a nested function instead of an anonymous function:

        function matInt = testfun(a,b,M) 
            % matfun = @(x) squeeze(interpn([-1 0 1],[-1,0,1],M,x(1),x(2)));
            function Mq = matfun(x)
                mout = size(M,3);
                nout = size(M,4);
                Mq = zeros(mout,nout,'like',M);
                for j = 1:nout
                    for i = 1:mout
                        Mq(i,j) = interpn([-1 0 1],[-1,0,1],M(:,:,i,j),x(1),x(2));
                    end
                end
            end
            matInt = integral(@(s) matfun(a*s+(1-s)*b), 0, 1, 'ArrayValued',true);
        end

Seems like there should be another, more clever way, but it's not coming to mind at the moment.