MATLAB: Loop in a loop question

MATLABstructure array instantaneous speed

I have a 1 x 521 structure array called 'data' with 20 fields. One of the field is 'GS', which stands for Ground speed. I am trying to find the instantaneous Ground Speed for all arrays, and make a new field called 'instantaneous_GS'. The following is the code I am using.
for n = 1 : length(data)
   for i = 1 : length(data(n).GS)
  data(n).instantenous_GS = (data(n).GS(i) + data(n).GS(i + 1))/2;
   end
end
I am a new MATLAB user, and have no clue what is the problem with my code. Can anyone help me? Thanks.

Best Answer

To get all the GS values in to one array, use the [] operator to concatenate them all then conv() to get the array of means (the average between pairs like you were doing)
for k = 1 : length(data)
    data(k).instantaneous_GS = conv([data(k).GS], [.5, .5], 'valid')
end
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