Convert cell array of structures to numeric vector

Suppose I have

ale{1}.a = 0;
ale{2}.a = 1;
ale{3}.a = 1.5

ale = 1x3 cell array
    {1x1 struct}    {1x1 struct}    {1x1 struct}

ale_vec = NaN(length(ale),1);
for ii=1:length(ale)
    ale_vec(ii) = ale{ii}.a;
end
disp(ale_vec)

         0
    1.0000
    1.5000

I would like to generate

%ale_vec = [0,1,1.5]

Is there a quick way to do this without using a loop?

Thanks!

Answer 1

[Yukthi S]

Hi @Alessandro,

The function arrayfun can be used to extract the values of the field a from each structure within the cell array ale and store them in a vector called ale_vec without explicitly using a for loop.

ale{1}.a = 0;
ale{2}.a = 1;
ale{3}.a = 1.5;

% Use arrayfun to extract the 'a' field from each element in the cell array
ale_vec = arrayfun(@(x) x.a, [ale{:}]);

% Display the result
disp(ale_vec);

         0    1.0000    1.5000

You can find more about arrayfun in the below MathWorks documentation:

https://www.mathworks.com/help/matlab/ref/arrayfun.html#:~:text=Description-,B%20%3D%20arrayfun(func%2CA),-applies%20the%20function

[Voss]

@Alessandro: I thought you wanted to know a quick way without a loop, but the answer you've accepted uses arrayfun, which uses a loop and is slower than the "direct" method I showed, which uses only indexing and concatenation:

N = 7;
ty = zeros(N,1);
tv = zeros(N,1);
M = 10.^(0:N-1).';
for ii = 1:N
    ale = num2cell(struct('a',num2cell(rand(1,M(ii)))));
    ty(ii) = timeit(@()run_arrayfun(ale));
    tv(ii) = timeit(@()run_direct(ale));
end
format long g
T = table(M,ty,tv,'VariableNames',{'# of cells','arrayfun time','direct time'})

T = 7x3 table
    # of cells    arrayfun time        direct time     
    __________    _____________    ____________________

           1       1.16775e-05               4.7871e-06
          10       3.92943e-05     1.48168650793651e-05
         100      0.0002868775     7.41060714285714e-05
        1000      0.0027108175             0.0007648575
       10000      0.0277998175             0.0077908175
      100000      0.2808028175             0.0920128175
     1000000      2.9622488175             0.9987388175

function V = run_arrayfun(C)
V = arrayfun(@(x) x.a, [C{:}]);
end

function V = run_direct(C)
S = [C{:}];
V = [S.a];
end

Answer 2

[Voss]

ale{1}.a = 0;
ale{2}.a = 1;
ale{3}.a = 1.5

ale = 1x3 cell array
    {1x1 struct}    {1x1 struct}    {1x1 struct}

This will work for the given example:

S = [ale{:}];
ale_vec = [S.a]

ale_vec = 1×3

0 1.0000 1.5000

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[Voss]

If the structs inside the cells of the cell array have different sets of fields, then they cannot be put together into a single struct array (which is what [ale{:}] does). In that case, you can use cellfun which (like arrayfun) implements a loop:

% cell array containing structs with different field sets
ale{1}.a = 0;
ale{2}.a = 1;
ale{3}.a = 1.5;
ale{3}.b = -2;
ale{:}

ans = struct with fields:
    a: 0

ans = struct with fields:
    a: 1

ans = struct with fields:
    a: 1.5000
    b: -2

ale_vec = cellfun(@(s)s.a,ale)

ale_vec = 1×3

0 1.0000 1.5000

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