Error when trying to use if exist command. variable exists but it is not recognising it

Hi

I'm trying to run a section of code that find's variables I've imported from a c3d file. Unfortunately from file to file the location of the variable changes.

Therefore the code I'm running is designed to search if the variable I want exists under on sturcture. If variable doesn't exist then search for it under an alternative location.

With some files I'm getting the error "Reference to non-existent field"

Can anyone Identify where my error in my code might be? In the instance that I'm getting this error the marker.LkneeMoments does exist but it's skipping past that straight to the moment.LKneeMoments which doesn't exist (hence the error message). Changing to use elseif instead of if doesn't work either.

if  exist('markers.LKneeMoments','var');
        WAPhaseMoments.KneeMoment = markers.LKneeMoments(WAStartFrame:WAEndFrame,:);
        fprintf('\t\t\t Imported WA Knee Moments\n')

      else exist('moments.LKneeMoments','var');
          WAPhaseMoments.KneeMoment = moments.LKneeMoments(WAStartFrame:WAEndFrame,:);
          fprintf('\t\t\t Imported WA Knee Moments\n')
end

Any suggestions are more than welcome

Answer 1

[Guillaume]

Notwithstanding the syntax error in your else branch, the problem you have is that you're not passing a variable name to exist. The variables are either markers or moments. The .LKneeMoments is not part of the variable names. It's a field of the variables and you test its existence with isfield.

So, if you're sure that both variables markers and moments exist (you should be!) and you're simply unsure whether or not they have the required field your code should be:

if  isfield(markers, 'LKneeMoments')    %semicolon should not be there
        WAPhaseMoments.KneeMoment = markers.LKneeMoments(WAStartFrame:WAEndFrame,:);
        fprintf('\t\t\t Imported WA Knee Moments\n')

elseif isfield(moments, 'LKneeMoments')
        WAPhaseMoments.KneeMoment = moments.LKneeMoments(WAStartFrame:WAEndFrame,:);
        fprintf('\t\t\t Imported WA Knee Moments\n')
end

If you're not sure that either markers or moments exist then, a) how come? and b) combine exist and isfield:

if  exist('markers', 'var') && isfield(markers, 'LKneeMoments')    %semicolon should not be there
        WAPhaseMoments.KneeMoment = markers.LKneeMoments(WAStartFrame:WAEndFrame,:);
        fprintf('\t\t\t Imported WA Knee Moments\n')

elseif exist('moments', 'var') && isfield(moments, 'LKneeMoments')
        WAPhaseMoments.KneeMoment = moments.LKneeMoments(WAStartFrame:WAEndFrame,:);
        fprintf('\t\t\t Imported WA Knee Moments\n')
end

Note that because of the shortcircuit behaviour of &&, isfield won't try to check the field of an inexistant variable if exist is false.

[Sean Byrne]

Thanks,

I tried modifying the exist to use the is field instead however I still get the same result.

It's not reading that the RKneeMoments field exists at all in the markers variable but just moving on to the elseif.

Additionally if I use elseif instead of else it does not run any code but just skips down to the next line as if nothing went wrong and just continues running the rest of the code.

If I use else it does try to find that field and when it can't it stops and displays the error.

Any thoughts?

[Guillaume]

"if I use elseif instead of else it does not run any code but just skips down to the next line"

which means that LKneeMoments is not a field of either variable. If you want a warning when that happens:

if isfield ...
   ...
elseif isfield ....
   ...
else
   warning('LKneeMoments not found in either moments or markers');
end

Using else instead of elseif makes absolutely no sense. The else branch would be equivalent to

else
    isfield(moments, 'LKneeMoments');  
    WAPhaseMoments.KneeMoment = moments.LKneeMoments(WAStartFrame:WAEndFrame,:);

which basically asks whether LKneeMoments is a field of moments and immediately discards the answer. Then gets the value of the field regardless of the answer. Of course, if the answer was that the field does not exist, then asking for its value will throw an error. Exactly what you saw.