Codegen failed to infer output size of built-in function, ??? Dimension 1 is fixed on the left-hand side but varies on the right ([1 x :?] ~= [:? x :?])

I tried to codegen a large file containing downsample(), below is a simplified one. For some reason, I have to specify the output size. However, coder failed to infer the size of output of dowmsample().

function a = zzz_tmp0329_33(n)%#codegen
coder.varsize('a', [1 Inf], [0 1] );
b = [1,2,3,4];
a = downsample(b, n);
end

Below is the script to run codegen

% ZZZ_TMP0329_33_SCRIPT   Generate static library zzz_tmp0329_33 from
%  zzz_tmp0329_33.
% 
% Script generated from project 'zzz_tmp0329_33.prj' on 14-Sep-2022.
% 
% See also CODER, CODER.CONFIG, CODER.TYPEOF, CODEGEN.

%% Create configuration object of class 'coder.CodeConfig'.
cfg = coder.config('lib','ecoder',false);
cfg.GenerateReport = true;
cfg.ReportPotentialDifferences = false;
cfg.GenCodeOnly = true;

%% Define argument types for entry-point 'zzz_tmp0329_33'.
ARGS = cell(1,1);
ARGS{1} = cell(1,1);
ARGS{1}{1} = coder.typeof(0);

%% Invoke MATLAB Coder.
codegen -config cfg zzz_tmp0329_33 -args ARGS{1}

The error says ??? Dimension 1 is fixed on the left-hand side but varies on the right ([1 x :?] ~= [:? x :?]). Why would it say downsample a vector leads to size [:? x :?] ?

Do you know why coder cannot determine the size of output from downsample()?

Answer 1

[David Fink]

Thank you for reporting this!

There have been some changes to downsample - try a recent version of MATLAB - R2021b or newer, where this is fixed.

[Xingwang Yong]

It works on R2022a on my PC. Unfortunately, I have to use this on a workstation, which only has R2020a installed because the OS is CentOS7 and the newest version it supports is R2020a. Do you know how I can solve this with R2020a?

[David Fink]

In R2020a, you could reshape the output of downsample to the expected 1x?:

a = reshape(downsample(b, n), 1, []);

which (similar to upgrading to R2021b or newer) would allow you to remove the coder.varsize declaration.