'Reference to non-existent field' when using nested for loops

Hi,

I'm trying to calculate some simpe things for data that is in structures and put it in a new 'outcome' structure which can later be exported to excel.

I have the following code:

def = {'Acc_X','Acc_Y','Acc_Z','Gyr_X','Gyr_Y','Gyr_Z'};
RMSdef = {'RMS_Acc_X','RMS_Acc_Y','RMS_Acc_Z','RMS_Gyr_X','RMS_Gyr_Y','RMS_Gyr_Z'};
for k = 1:size(sensors,2)
    for j = 1: length(def)
            for u = 1:nEVENTS/2
                % Max values & timing of maximal values
                [OUTCOME(k).event(u).(MAXdef{j}),OUTCOME(k).event(u).(idxMAXdef{j})]  = max(abs(Cycle(k).event(u).(def{j})));
                % RMS
                OUTCOME(k).event(u).(RMSdef{j}) = rms(Cycle(k).event(u).(def{j}));
                % Sample entropy
                OUTCOME(k).event(u).(E{j}) = SampEn(2,0.2*std(Cycle(k).event(u).(def{j})),(Cycle(k).event(u).(def{j})));
            end
    end
end

This works perfectly.

Now I want to do something similar:

jerkdef = {'Jerk_X','Jerk_Y','Jerk_Z'};
accdef = {'Acc_X','Acc_Y','Acc_Z'};
for k = 1:size(sensors,2)    
    for q = 1: length(accdef)
        for u = 1:nEVENTS/2
            % Calculate jerk
            JERK(k).event(u).(jerkdef{q}) = diff(Cycle(k).event(u).(accdef{q}));
            % Norm of jerk
            JERK(k).event(u).NormJerk = ...
                sqrt(((JERK(k).event(u).Jerk_X).^2 + (JERK(k).event(u).Jerk_Y).^2 + (JERK(k).event(u).Jerk_Z).^2));
        end
    end
end

For some reason, matlab gives me "Reference to non-existent field 'Jerk_Y' ", but when I run this line seperately:

JERK(1).event(1).(jerkdef{2}) = diff(Cycle(1).event(1).(accdef{2}));

It does work.

I might be missing something very very very obvious but currently, I'm lost..

Thanks in advance

Answer 1

[Geoff Hayes]

Inti - the cell array jerkdef has the different Jerk* fields for your struct, but you only add these fields on each iteration of the second for loop

for k = 1:size(sensors,2)    
    for q = 1: length(accdef)  % <---- SHOULD THIS BE jerkdef INSTEAD??
        for u = 1:nEVENTS/2
            % Calculate jerk
            JERK(k).event(u).(jerkdef{q}) = diff(Cycle(k).event(u).(accdef{q}));
            % Norm of jerk
            JERK(k).event(u).NormJerk = ...
                sqrt(((JERK(k).event(u).Jerk_X).^2 + (JERK(k).event(u).Jerk_Y).^2 + (JERK(k).event(u).Jerk_Z).^2));
        end
    end
end

Here is the line that adds the field

JERK(k).event(u).(jerkdef{q}) = diff(Cycle(k).event(u).(accdef{q}));

but this will just add the qth field. When q is 1, then all we have added is the Jerk_X field and so an attempt to calculate something using Jerk_Y and Jerk_Z will fail. You need to add these fields first before you try to access them.

[Inti Vanmechelen]

Hi Geoff,

Thank you for your reply.

Since 'jerkdef'and 'accdef' have the same length, I don't think it matters for the command

for q = 1: length(accdef)

if I use 'jerkdeg' or 'accdef'? (Correct me if I'm wrong please)

I'm aware that I'm probably doing something wrong with order of the for loops, I am just very struck that everything works in the first code I gave above (where I do the exact same thing - adding a new field in every iteration of the for-loop). I have a hard time understanding why one works and the other one does not. If you would see an explanation there, I'd be very happy to hear it :)

I'll try to add the fields first and then calculate the jerk, thank you for the suggestion.

[Geoff Hayes]

Inti - I may be mistaken but the problem isn't with the line

JERK(k).event(u).(jerkdef{q}) = diff(Cycle(k).event(u).(accdef{q}));

which is similar to your

JERK(1).event(1).(jerkdef{2}) = diff(Cycle(1).event(1).(accdef{2}));

The error is coming from the lines

% Norm of jerk
JERK(k).event(u).NormJerk = ...
    sqrt(((JERK(k).event(u).Jerk_X).^2 + (JERK(k).event(u).Jerk_Y).^2 + (JERK(k).event(u).Jerk_Z).^2));

where you are trying to access Jerk_Y and Jerk_Z before you have added them as a fields/members in the struct. Or am I misunderstanding something?

[Inti Vanmechelen]

I finally see what you mean.

No, you were absolutely correct, I was just staring through it...

I've put the two command seperately in loops and now it does wrok...

Should've known that one, thank you!