How to get an array from a Rosbag

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Paolino De Risi on 16 Sep 2021

Commented: Cam Salzberger on 17 Sep 2021

Hi, I'm trying to plot a data from a rosbag, here what i did until now:

                          bag=rosbag('/home/paolino/bagfiles/subset.bag');
                         
                          wh_gaz=select(bag,'Topic','/joint_states');
                         
                          wh_gazStruct=readMessages(wh_gaz,'DataFormat','struct');

This struct is a 19153x1 cell and each element is a 1x1 struct with a velocity record, a 2x1 array.

Now, what i need is to create 1 column array with each wh_gazStruct.Velocity first element, but I can't find a solution. Until now I tryed:

wh_gaz_mat=cellfun(@(m) double(m.Velocity),wh_gazStruct);

but since the .Velocity record has two elements it doesn't work. How may I solve my problem?

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Cam Salzberger on 16 Sep 2021

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//www.tianjin-qmedu.com/matlabcentral/answers/1454909-how-to-get-an-array-from-a-rosbag#answer_789164

我和示例代码提供了一些指导 similar question here . That should help get you moving in the right direction!

-Cam

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Cam Salzberger on 17 Sep 2021

Well, I don't know exactly what your messages look like. But if I set up my messages like this (to simulate using readMessages with DataFormat='struct'):

                                   msg = rosmessage('sensor_msgs/JointState','DataFormat','struct');
                                  
                                   n = 4;
                                  
                                   msgs = cell(n, 1);
                                  
                                   fork = 1:n
                                  
                                   msgs{k} = msg;
                                  
                                   msgs{k}.Velocity = [k ; -k];
                                  
                                   end

Then I can do this:

[wh1, wh2] = cellfun(@getwheelsvelocities, msgs);

with your code as-provided, and it works just fine.

If you have any messages that do not have two entries in the Velocity field, then this code will fail.

-Cam

s manbetx 845

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R2021a

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