# How to convert a structure array into vector

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SS on 9 Aug 2019
Edited: Jos (10584) on 9 Aug 2019
Hello.
I have a structure array (1 x 50000) with 10 fields. The elements in the fields are arrays (1 x 50). I want to convert this structure array into a scalar structure with same fields such that, its size is (50000*50 x 1).
SS on 9 Aug 2019
Hi.
The input is something like,
Main(1).fel1=[1,2,3,4,5];
Main(2).fel1=[10,20];
Main(1).fel2=[2,4,6,8,10];
Main(2).fel2=[150,200];
The output should be,
Main(1).fel1=1, Main(2).fel1=2, Main(3).fel1=3.........Main(7).fel1=20
Main(2).fel2=2, Main(2).fel2=4, ..........................Main(7).fel2=200

Stephen on 9 Aug 2019
Edited: Stephen on 9 Aug 2019
S(1).F1 = [1,2,3,4,5];
S(2).F1 = [10,20];
S(1).F2 = [2,4,6,8,10];
S(2).F2 = [150,200];
F = fieldnames(S);
C = num2cell(struct2cell(S),3);
C = cellfun(@(c)[c{:}],C,'uni',0);
C = num2cell(vertcat(C{:}));
T = cell2struct(C,F,1)
Giving:
T =
7x1 struct array with fields:
F1
F2
Checking the first field's values:
>> S.F1 % input
ans =
1 2 3 4 5
ans =
10 20
>> T.F1 % output
ans = 1
ans = 2
ans = 3
ans = 4
ans = 5
ans = 10
ans = 20
Stephen on 9 Aug 2019
"How can this be implemented if, the size of the arrays is not same throughtout."
Something like this should work:
1. decide what default value/array should go into those fields that are unassigned (e.g. empty numeric, NaN).
2. measure the sizes of all vectors
3. preallocate a cell array of the correct size, using your default value.
4. loop over the fieldnames
5. concatenate the data (comma-separated lists will likely be useful here), convert to cell.
6. assign to the cell array using the length of the concatenated data.
7. convert to structure using struct2cell.

Jos (10584) on 9 Aug 2019
Why on earth store scalar values like that? Why not have a simple, highly efficient M-by-N matrix, rather than a cumbersome M-by-1 structure array, with N fields, each storing a single scalar value.
% a structure array with N = 2 fields. Each field holds overall and in total 7 elements
S(1).F1 = [1,2,3,4,5];
S(2).F1 = [10,20];
S(1).F2 = [2,4,6,8,10];
S(2).F2 = [150,200];
A = arrayfun(@(f) [S.(f{:})].', fieldnames(S), 'un', 0)
A = [A{:}] % a 7-by-2 matrix
Jos (10584) on 9 Aug 2019
One option is to keep the fields blank, or fille the empty spots in my output with NaNs.
One can use my function PADCAT to pad shorter vectors (being concatenated fields) with NaNs:
S(1).F1 = [1,2,3,4,5];
S(2).F1 = [10,20];
S(1).F2 = 2 ;
S(2).F2 = [150,200];
A = arrayfun(@(f) [S.(f{:})].', fieldnames(S), 'un', 0)
A = padcat(A{:}) % a 7-by-2 matrix
PADCAT can be found on the File Exchange: