average of field in a structure with an if condition

So I have a structure S with two fields field1 and field2 What i want to do is If field1 is equal to a value x , I sum (then get the average of) the values in field2 I can do it with a for loop and if inside of it but is there a more faster way to do it.

Example

S.a   field1  field2
     2         4
     0         6
     1         5
     2         3

If x=2, the result i want to get is 3.5 (the average of values in field2) where field1 =2

P.S. It is a structure with other fields (not shown here) not a matrix

What I have been doing is

       Tot=0;
       for i=1:S.numElements
            if S.a(i).field1 == 1 
              Tot=Tot+S.a(i).field2;
            end
       end
       Avg= Tot/S.numElementsBis (I have this value stored in another field)

Answer 1

[Guillaume]

It's not clear if your S is a structure array with scalar fields:

S = struct('a', {2; 0; 1; 2}, 'b', {4; 6; 5; 3})
meanb = mean([S([S.a] == 2).b])

Or a scalar structure with fields of arrays:

S = struct('a', [2; 0; 1; 2], 'b', [4; 6; 5; 3])
meanb = mean(S.b(S.a == 2))

Either way as illustrated above getting the mean is easy.

[etudiant_is]

This is not exactly working for my case. I tried to edit my question to explain better. See above

[Guillaume]

Right, it's actually a substructure of S that is a structure array. If you replace S by S.a, a by field1 and b by field2, the same answer apply:

S.a = struct('field1', {2; 0; 1; 2}, 'field2', {4; 6; 5; 3})
meanb = mean([S.a([S.a.field1] == 2).field2])

By the way, your S.numElements appears to be redundant as it's the same as numel(S.a). It's actually dangerous to store the size of the array explicitly in case you change the size of the array without updating S.numElements.

[etudiant_is]

That works, thanks. But if I want to check two conditions for the mean at the same time, it gives an error, why?

meanb = mean([S.a([S.a.field1] == 2 || [S.a.field1] == 5).field2])

You are right about the redundancy. I am updating it everytime correctly but I will probably remove it later.

[Guillaume]

|| is a short-circuit logical operator (it does not evaluate the second argument if the first is true), it only works with scalar.

With vectors, you have to use | (single bar).

[etudiant_is]

Thanks, with this it works