matlab coder error Directly accessing field or property of nonscalar struct or object not supported for code generation.

This works in Matlab but not in Coder, Why?

s1=string({OPS_FLT(:).ACFT_ID})

s2=OPS_FLT(2).ACFT_ID

uuindex=find(strcmpi(s1,s2))

My array is defineded as:

OPS_FLT(1).ACFT_ID="apple"

OPS_FLT(2).ACFT_ID="orange"

I need to find orange from the array but the code has to be compatible in coder. It says: Directly accessing field or property of nonscalar struct or object not supported for code generation in ({OPS_FLT(:).ACFT_ID})

[cui]

It is hoped that future versions will support this feature, for example, the current R2023a supports "end+1" C/C++ code generation, which would greatly reduce the concerns of developers.

Answer 1

[Andy]

Hi Upol. Presently, referencing a component from an array of non-scalar values (accessing a field in a struct array) is not allowed in code generation.

As a workaround, for the time being you can rewrite the code to avoid the non-scalar access:

function uuindex = foo
    OPS_FLT = struct('ACFT_ID', {"apple", "orange"});   
    
    uuindex=[];
    for k = numel(OPS_FLT)
      if OPS_FLT(k).ACFT_ID == OPS_FLT(2).ACFT_ID
        uuindex = [uuindex, k];
      end
    end
end

*EDIT: I've modified code from version I original posted since it used another feature that also would not compile.

[Xie Yiru]

hello I need your help about this question. I used houghpeaks() and houghlines() in matlab. But I can't transform them to C because I can not controll the size of the funciton. I can post my code.

clear 
clc 
close all; 
oldpath = path;
path(oldpath,'E:\ZKIT_ws2017_18\Conti\Aufgabe_1')
image=imread('Kreuz_1.png'); 
A=neutest_mex(image);

function A = neutest_mex(n)
%coder.extrinsic('rgb2gray')
I2 = rgb2gray(n); 
%coder.extrinsic('imbinarize')
I=imbinarize(I2);
theta = zeros(1,180,'double');
rho = zeros(1,381,'double');
H=zeros(381,180,'double');
%coder.extrinsic('hough')
[H,theta,rho] = hough(I); 
%{
imshow(H,[],'XData',theta,'YData',rho,...
            'InitialMagnification','fit');
xlabel('\theta'), ylabel('\rho');
axis on, axis normal, hold on;
%}
%coder.extrinsic('houghpeaks')

P = houghpeaks(H,1);
lines = struct('point1',[0,0],'point2',[0,0],'theta',0,'rho',0);
lines = repmat(line,1,0);
%coder.extrinsic('houghlines')
FillGap=double(0);
MinLength=double(0);
FillGap =double(0);
MinLength= double(0);
lines = houghlines(I,theta,rho,P,'FillGap',5,'MinLength',7); 
A=lines.theta; 
end

i can not use the output from the houghlines. do you know why?

[Andy]

Hi Xie,

When running this through codegen "lines" ends up typed as a 1x:? struct array. As such you getting the error message about directly accessing a field of a nonscalar struct.

Could you try converting the line "A = lines.theta" into a for loop that copies each individual theta out to A? In other words change: A=lines.theta into:

A=zeros(1, size(lines,2));
for i=1:size(lines,2)
    A(i)=lines(i).theta; 
end

Also, I think you may have a problem with the following:

lines = struct('point1',[0,0],'point2',[0,0],'theta',0,'rho',0);
lines = repmat(line,1,0);

You pass "line" to repmat rather than "lines", "line" is a matlab function that returns an object.

This will end up causing a runtime error when you run the generated mex. You can resolve it either by commenting out this code or changing what you pass to repmat from "line" to "lines" (which is what I think you meant to do).

[TripleSSSS]

I'd suggest using

A=coder.nullcopy(zeros(1, size(lines,2)));

instead of

A=zeros(1, size(lines,2));