MATLAB: Determine total length/size of a structure array with fields as structure arrays

Question

I have a structure array containing fields as structure arrays of varying length. For example: 's' is a structure 'data' is a field in 's', and also a structure array itself and ``` length(s(n).data) ~= length(s(m).data) ``` I want to preallocate an array that takes a time stamp from every field s.data.timestamp. Is there a way to do this without using a for loop twice? This is what I have so far: ``` % find the total length count=0; for x=1:length(s) count=count+length(s(x).data); end % preallocate timestamp array timestamp=zeros(1,count); % populate timestamp array index=1; for x=1:length(s) for y=1:length(s(x).data) timestamp(index)=s(x).data(y).timestamp; index=index+1; end end ``` I thought about just overestimating the length that I would need based on the length of 's' and an average length of 'data', but the actual length of each 'data' field/substructure varies widely. Would I be better off just overestimating the heck out of it and trimming the resulting array afterward? Zer…

Accepted Answer

This will work if every structure array `data` has the same fields and are row vectors (i.e. 1-by-N):

```
allData = [s.data];               %# Concatenate all data arrays into one
timestamp = [allData.timestamp];  %# Collect all the time stamps
```

If the `data` structure arrays are column vectors (i.e. N-by-1), you need to use `vertcat` instead:

```
allData = vertcat(s.data);        %# Concatenate all data arrays into one
timestamp = [allData.timestamp];  %# Collect all the time stamps
```

The above solutions work due to the fact that accessing a single field of a structure array returns a comma-separated list.

MATLAB: Determine total length/size of a structure array with fields as structure arrays

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