MATLAB: Passing structure field name in function

structures

I have a function that accepts a structure and a field_identifier (character format) as inputs. The structure loaded has substructures in it, for which the field_identifier loaded in the function matches. The format is the following: structure(i).substructure.field_identifier

I have two checkpoints, ~isstruct and ~isfield for each of the inputs and it works fine. After that I initialize a for loop, defined as:

i=1;
for i=1:size(structure(i).substructure.field_identifier,1)

With this, I get the error that field_identifier is not a valid field. I understand the problem lies in the way I am passing the field identifier, but I have tried several things and cannot figure a way of passing field_identifiers as arguments for the for loop within this function. Can someone tell me what am I doing wrong?

Thanks in advance for the advice!

Best Answer

 structure(i).substructure.(field_identifier)

This is clearly explained in the documentation: http://www.mathworks.com/help/matlab/matlab_prog/generate-field-names-from-variables.html

Note that it is recommended to avoid using i and j for loop variable names, because these are names of the inbuilt imaginary unit.

Related Solutions

MATLAB: How to address the fields of a struct with function input arguments

You shouldn't be using dynamic variable names in the first place as you lose syntax checking, optimisation and ease of debugging. But anyway, to access dynamic variables you have to use eval:

s = eval(sprintf('Struct_Nr%d.Field_Nr%d', x, y))

Note that dynamic fields can be accessed using brackets:

s = struct('Field_1', 0, 'Field_2', 1)
fidx = 1;
fldval = s.(sprintf('Field_%d', fidx))

Finally, don't use ans as variable name. It's used by matlab.

MATLAB: Is there a MATLAB function that can check if a field exists in a MATLAB structure

There is no MATLAB function that examines every level of a structure of structures, or nested structure, to determine if a field exists. The 'isfield' function examines only the top level of a nested structure. To determine if a field exists at any other level, you can use either of the following methods.

1. To determine if a field exists in a particular substructure, use 'isfield' on that substructure instead of the top level. In the example, the value of a.b is itself a structure, and you can call 'isfield' on it.

a.b.c = 1;
isfield(a.b,'c')
ans =
logical
1

Note: If the first input argument is not a structure array, then 'isfield' returns 0 (false). While there are other MATLAB data types that have properties you can access with dot notation, those other data types are not structure arrays.

2. To determine if a field exists at any level in a nested structure, create a new function that examines all levels of the structure. Open the MATLAB Editor and paste the following function into it. Save the function as a MATLAB file, named 'myIsField.m'.

function isFieldResult = myIsField (inStruct, fieldName)
% inStruct is the name of the structure or an array of structures to search
% fieldName is the name of the field for which the function searches
isFieldResult = 0;
f = fieldnames(inStruct(1));
for i=1:length(f)
if(strcmp(f{i},strtrim(fieldName)))
isFieldResult = 1;
return;
elseif isstruct(inStruct(1).(f{i}))
isFieldResult = myIsField(inStruct(1).(f{i}), fieldName);
if isFieldResult
return;
end
end
end

Call 'myIsField' on a nested structure.

a.b.c = 1;
myIsField(a,'c')
ans = 
logical 
1

Best Answer

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