Mean of structure's line - matlab

Question

I have a structure like
struct =
Fields Subject1 Subject2 Subject3 Subject4
1 30000x1 double 30000x1 double 30000x1 double 30000x1 double
2 30000x1 double 30000x1 double 30000x1 double 30000x1 double
3 30000x1 double 30000x1 double 30000x1 double 30000x1 double
4 30000x1 double 30000x1 double 30000x1 double 30000x1 double
where 1,2,3 and 4 are conditions
I would like to calculate the mean for each condition, so for each LINE of the struture.
I tried with :
for i = 1:length(struct)
mean_condition(i) = mean([strut(i)]);
end
but I obtain this error
Error using sum
Invalid data type. First argument must be numeric or logical.
Error in mean (line 117)
y = sum(x, dim, flag)/size(x,dim);
How can I fix it ?

Answer 1

While structfun allows you to perform an operation over the fields of a structure, it only works with scalar arrays. Because you have a structure array, you'll need to use an explicit loop or an implicit arrayfun loop.
As an example of the latter:
condition(1).subject1 = 1:10;
condition(1).subject2 = 1:20;
condition(2).subject1 = 1:30;
condition(2).subject2 = 1:40;
results = arrayfun(#(x)mean(structfun(#mean, x)), condition).';
Which gives us:
results =
8
18
Which we can verify with:
>> [mean([mean(condition(1).subject1), mean(condition(1).subject2)]); mean([mean(condition(2).subject1), mean(condition(2).subject2)])]
ans =
8
18
Depending on MATLAB version, the *fun functions may be slower than the explicit loop due to additional function call overhead. This is certainly the case with older versions of MATLAB, but engine improvements have started to bring parity to their performance.
For completeness sake, an explicit loop version:
results = zeros(numel(condition, 1));
for ii = 1:numel(condition)
tmpnames = fieldnames(condition(ii));
tmpmeans = zeros(numel(tmpnames, 1));
for jj = 1:numel(tmpnames)
tmpmeans(jj) = mean(condition(ii).(tmpnames{jj}));
end
results(ii) = mean(tmpmeans);
end

Answer 2

Since the fields of all the structs in the array have the same size, you can perform this computation very easily as follows:
s = struct();
s_len = 4;
for i = 1:s_len
s(i).Subject1 = repmat(i,30,1);
s(i).Subject2 = repmat(i,30,1);
s(i).Subject3 = repmat(i,30,1);
s(i).Subject4 = repmat(i,30,1);
end
m = reshape(mean(cell2mat(struct2cell(s))),s_len,1);
The variable m is then a row vector of double values in which each row contains the mean of the respective condition:
m =
1 % mean of condition 1
2 % mean of condition 2
3 % mean of condition 3
4 % mean of condition 4