How to check whether a structure exists?

Question

Ben Wang 2011-5-7

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评论： Walter Roberson 2017-10-17

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

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Answer 1

Walter Roberson 2011-5-7

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exist('structurename','var')

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

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John McGraw 2017-10-17

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This does not work for me:

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

Walter Roberson 2017-10-17

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

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Answer 2

Céldor 2012-3-31

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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

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Jan 2012-3-31

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 2012-3-31

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.

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Answer 3

Matt Fig 2011-5-7

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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

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Ben Wang 2011-5-7

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