"Reference to non-existent field" Error

Question

Heba Sakr 2019-9-8

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/479394-reference-to-non-existent-field-error

评论： Guillaume 2019-9-9

I get this error (Reference to non-existent field 'system'.) when I use this line to call "generate_channel_cell" function

H_cell= generate_channel_cell( system ) ;

The fuction code is as following:

function channel_cell = generate_channel_cell(system)
p = inputParser ;
p.FunctionName = 'generate_channel_cell' ;
p.addRequired ('system') ;
Nt = round( p.Results.system.Nt ) ;
Nr = round( p.Results.system.Nr ) ;
Ns = round( p.Results.system.Ns ) ;
K = round( p.Results.system.K ) ;
coeff_size = [K * Nr , K * Nt ] ;   
coeffiecients = (randn(coeff_size ) + 1j .* randn( coeff_size ) ) ;
coeffiecients = (1 / sqrt ( 2 ) ) .* coeffiecients ;
Nr_values = repmat ([ Nr ] , 1 , K ) ;
Nt_values = repmat ([ Nt ] , 1 , K ) ;
channel_cell = mat2cell( coeffiecients , Nr_values , Nt_values ) ;
end % end function

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Answer 1

Guillaume 2019-9-8

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https://ww2.mathworks.cn/matlabcentral/answers/479394-reference-to-non-existent-field-error#answer_390864

编辑：Guillaume 2019-9-8

在 MATLAB Online 中打开

Well, yes, you never use the input parser to parse your input, so the Results never get populated.

p.parse(system);

after you've created the parser but before you use the Results.

edit: on the other hand, your input parser is not very useful, it only checks that one input has been passed but you'll get an error beforehand if it's not the case. You don't check that the input is a structure with the correct fields

You'd be better off with:

function channel_cell = generate_channel_cell(system)
assert(isstruct(system) && all(ismember(fieldnames(system), {'Nt', 'Nr', 'Ns', 'K'}), 'system must be a structure with fields Nt, Nr, Ns and K');
Nt = round( p.Results.system.Nt ) ;
%...

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Heba Sakr 2019-9-8

在 MATLAB Online 中打开

Hi Guillaume, I appreciate your answer.

I understood the part of never using the input parser to parse the inputs, however, your code suggestion did't work for me.

I tried to modify the code as following, the code runs without any errors giving the expected results.

function channel_cell = generate_channel_cell(system)
% p = inputParser ;
% p.FunctionName = 'generate_channel_cell' ;
% p.addRequired ('system') ;
% Nt = round( p.Results.system.Nt ) ;
% Nr = round( p.Results.system.Nr ) ;
% Ns = round( p.Results.system.Ns ) ;
% K = round( p.Results.system.K ) ;
Nt = round(system.Nt ) ;
Nr = round(system.Nr ) ;
Ns = round(system.Ns ) ;
K = round(system.K ) ;

Guillaume 2019-9-9

在 MATLAB Online 中打开

Oops, yes, what I wrote was partly nonsense since I used the input parser that no longer existed. My answer should have read:

function channel_cell = generate_channel_cell(system)
assert(isstruct(system) && all(ismember(fieldnames(system), {'Nt', 'Nr', 'Ns', 'K'}), 'system must be a structure with fields Nt, Nr, Ns and K');
Nt = round(system.Nt);
%...