How to check whether a structure exists?

Dear all:

I have a simple question and I can't find an answer on the web...

I have a function that takes arguments in. One argument is a structure array, and inside the function I want to test whether this argument is specified and feed into the function.

I tried isexist(), but this is not for a structure. isstruct() can test whether it is a structure, but the name must exist first. isfield() can check if a structural field exist, but not the structure itself.

So my question is, how to test whether a structure exist?

Any help is appreciated.

Cheers

Ben

Answer 1

[Walter Roberson]

exist('structurename','var')

[EDITED, Jan Simon 25-Feb-2012 22:00 UTC]: exists -> exist

[Ben Wang]

Hi Walter:

It doesn't work for me, I have the following message:

>> exists('param_fix','var')

??? Undefined function or method 'exists' for input arguments of type 'char'.

then I tried:

>> exists(param_fix,'var')

??? Undefined function or method 'exists' for input arguments of type 'struct'.

Maybe I miss something obvious here...

Cheers

Ben

[Teja Muppirala]

I think Walter meant EXIST not EXISTS

if exist('param_fix','var') && isstruct(param_fix)

.

.

.

[Walter Roberson]

Yeh, Mathworks used the wrong command name :)

[Roger Parkyn]

If you want to also check whether specific fields exist go to this file on the File-Exchange: https://au.mathworks.com/matlabcentral/fileexchange/28142-recursively-check-fields-of-a-structure-exist

[John McGraw]

This does not work for me:

exist(nonExistantFileStruct, 'var')
??? Undefined function or method 'exist' for input arguments of type 'struct'.

[Walter Roberson]

Pass in the name of the structure, not the structure itself.

Answer 2

[Céldor]

This thread is old but if someone still needs an answer then

exist('var_name') works and returns 0 if a structure does not exist in memory

Regards

[Jan]

This is no sufficient solution in opposite to the advice Walter gave 329 days ago: "exist('var_name')" replies a non-zero if there is a corresponding .m, .p, .mex file or folder in the path, or if there is a Java class with this name. Because all these chances are checked, "exist()" is slow when used without 2nd argument. See "help exist".

[Walter Roberson]

But it can return non-zero if there happens to be something else by the same name, such as a function or directory. It is better to use the optional second argument to be specific that you are inquiring about a variable.

Answer 3

[Matt Fig]

I am not entirely sure what you want to do, but here is an example input parsing... You don't have to make your function error as I do, but the checks are there.

function [] = takestwoargs(A,B)
% Takes two arguments, the first is a structure, the second
% is a double.  Both are required.

if nargin~=2
    error('Two input arguments required.')
elseif ~isstruct(A)
    error('The first input must be a structure.')
elseif ~isa(B,'double')
    error('The second argument must be a double.')
end

%

%

%

%

EDIT In response to your comment below. To make defaults, you could do something like this:

function [] = takestwoargs(A,B)
% Takes two arguments, the first is a structure, the second
% is a double.  Both are required.

N = nargin;  % Check how many inputs were passed.
if N==0
    A = struct('string','hello','yes','lengths',4);
    B = 5;% Defaults.
elseif N==1
    B = 5;% Default.
else
   % Or make defaults and issue a warning that 
   % argumenst beyond the second are ignored....
   error('Two many input arguments.')
end

if ~isstruct(A)
    % Or make a default and issue a warning...
    error('The first input must be a structure')
elseif ~isa(B,'double')
    % Same as above...
    error('The second argument must be a double')
end

[Ben Wang]

Hi Matt:

Thanks for helping out here. The function looks something like the following:

function [] = likelihood(A,B)

and B is a structure.

I guess I could use

if nargin~=2

error('Two input arguments required.')

to test whether B exists, but I don't want to put a restriction since if it is not parse to the function, I could use a default value specified inside the function.

Sorry I didn't make myself clear before. Is there a function I can test B exists in the workspace?

Cheers

Ben

[Matt Fig]

See my edit above.

Answer 4

[Jason Garcia]

Or alternatively, you could try:

x =

    x: 1
    y: 2
    z: 3

>> any(strcmp(fieldnames(x),'d'))

ans =

     0

>> any(strcmp(fieldnames(x),'y'))

ans =

     1

[Jan]

More efficient: isfield(x, 'y')

[Jason Garcia]

Ha, ya. :)