Search within a structure using a string

Hello Im trying to check if a string exist within a strucure:

The string is the structure path, which I created using sprintf I can not detect if the string coincide with the path of an existent structure in the workspace (Clicks).

Eval dosent work for structures neither fieldnames ... there is any way to check if exist or not in the workspace???

thanks in advance

prof_l=unique(prof_list);
shore_l=unique(shore_list);

% read clicks and extract shoreline

met=('fix');  
%
for i1=1:numel(prof_l)
    p=prof_l(i1,1);

for i2=1:1:numel(shore_l)
    s=shore_l(i2,1);

      file=(sprintf('Clicks.prof_%u.shore_%u.%s',p,s,met));  

if exist(eval(file),var) ==1;
    disp('hola')

end
end
end

Answer 1

[Walter Roberson]

Here is the test you asked for. I would point out that if the answer is True, that the structure exists, chances are high that you are planning to use eval() to get at the value stored, and that you should be strongly avoiding doing that.

http://matlab.wikia.com/wiki/FAQ#How_can_I_create_variables_A1.2C_A2.2C....2CA10_in_a_loop.3F

function tf = struct_exists(candidate)

    %Written 20150905 - Walter Roberson

    %{
      Here is a structure exist. In this version it *only* handles flat structures, no
      indexing, no dynamic fieldnames.

      The empty string and [] are accepted as valid arguments (and return false). Other
      non-string arguments will generate a warning (and false)

      The last field given is _not_ tested to see if it is a structure (only whether it
      exists) so that this routine can be used to test whether a field exists. As a design
      decision this extends to the case where the passed name has no subfields given and
      so is the last field name given: in this case true is returned if the name exists
      whether or not the name is a structure.

      The use of warnings instead of error was a design decision, one that is most
      debatable for the case of invalid input. I used warnings because this is
      supposed to be an existence test, and if the answer is "No that is not a valid
      arrangement of structure references" then Okay, that's why you *tested* rather
      than just went ahead and used the string.
    %}

    tf = false;

    if ~( ischar(candidate) || (isdouble(candidate) && isempty(candidate)) )
      warning('struct_exist: only character strings or [] permitted as argument')
      return
    end

    if isempty(candidate); return; end

    if regexp(candidate, '[^a-zA-z0-9_.]')
      warning('struct_exist: only fieldnames and ''.'' permitted');
      return;
    end

    parts = regexp(candidate, '\.', 'split');

    if  ~ evalin('caller', sprintf('exist(''%s'', ''var'')', parts{1}));
      warning( sprintf('struct_exist, "%s" is not a variable', parts{1}) );
      return;
    end

    for K = 1 : length(parts) - 1
      subname = strjoin(parts(1:K), '.');
      try
        fn = evalin('caller', sprintf('fieldnames(%s)', subname));
      catch ME
        %some leading fieldname was not a structure
        fn = {};
      end
      if ~ismember(parts{K+1}, fn)
        %some leading prefix does not have the next part as a field
        % no warning because this is a normal use case
        return
      end
    end

    tf = true;
  end

[Jules Ray]

thanks Walter, interesting is the slipt between substructures

Answer 2

[per isakson]

Try

>> S.a.b.c = 'abc';
>> S.('a').('b').c
ans =
abc

and

str = Clicks.( sprintf('prof_%u',p) ).( sprintf('shore_%u',s) ).met;
if exist( str, 'var' ) == 1
    disp('hola')
end

[Jules Ray]

Your methods give an error with exist that is not designed for structures :(

i solve it in a very dirty way, by using try...

doesVarExist = true
    try
    Clicks.(sprintf('prof_%u',p)).(sprintf('shore_%u',s)).method;
    catch
    doesVarExist = false
    end

      if doesVarExist == 1

wierd that matlab not includes this type of routines for work with structures...

[per isakson]

"Your methods give an error with exist that is not designed for structures" &nbsp I think my method is sound! My problem is that I have to backward engineer your code.

I assume that the value of Clicks.prof1.shore2.met is a string. (Note that I just picked 1 and 2.) That's why I assigned the value to a variable called str. Next, I assumed that you want to test whether there exists a variable, the name of which is the value of str.

Your "try-catch-end" construct tests whether the fields exist. That is best done with the function isfield.

Please don't write "give an error" without further detail. It's just not helpful.

[Jules Ray]

Hey thanks a lot for your help

I think this happens with any structure array in matlab, not only with my specific case, I think "exist" fails because matlab can not detect an existing array inside a structure

check this, based in your example:

a.b.c.d='ABC'

exist(a.b.c.d) %Ok, works with the entire structure

ans =

       0

exist(a.b.c) %but fails when I'm trying to detect a substructure
Undefined function 'exist' for input
arguments of type 'struct'.

[per isakson]

"I think "exist" fails" &nbsp See exist, Check existence of variable, function, folder, or class. That's right, exist cannot test for existence of fields in a structure. There is a special function for that, isfield

>> S.a.b.c = 'abc';
>> isfield( S.a.b, 'c' )
ans =
     1
>> isfield( S.a.b, 'd' )
ans =
     0
>> isfield( S, 'a' )
ans =
     1
>> isfield( S, 'z' )
ans =
     0

Note that the values of S.a.b and S are structures.

[Jules Ray]

>> S.a.b.c = 'abc'; %isfield "FAILS" in the attemp to check a substructure inside the structure. obtaining 0 all the time
%
>> isfield( S.a.b, 'c' ) %ok it works with a single string
ans =
   1
>> isfield( S.a.b, 'S.a' )%lets test a complex string 
ans =
   0
>> isfield( S, 'S.a' )
ans =
   0

PD:

 isfield( S, 'S'.'a' )
 isfield( S, 'S'.'a' )
                 |
 Error: Unexpected MATLAB expression.

[per isakson]

"in the attemp to check a substructure inside the structure" &nbsp I don't understand what exactly you want to check. In plain English, what is your intention with

isfield( S.a.b, 'S.a' )
isfield( S, 'S.a' )

Under what circumstances should they return true and false, respectively? &nbsp 'S.a' isn't a legal field name.

&nbsp

To check whether a value is a structure use isstruct

>> isstruct( S )
ans =
     1
>> isstruct( S.a )
ans =
     1
>> isstruct( S.a.b )
ans =
     1
>> isstruct( S.a.b.c )
ans =
     0
>> ischar( S.a.b.c )
ans =
     1
>>