Multiplying structures together based on field names

Is there any method to multiply structures together according to their field names? For example,

struct1.a=1;
struct1.b=2;
struct2.a=3;
struct2.b=4;

And the result would be:

struct3.a=3
struct3.b=8

[Stephen23]

Note that storing sets of data in one variable makes more sense than lots of sequentially-named variables, and simplifies the code significantly over creating and using dynamically named variables. You might like to read this to learn some of the reasons why:

http://www.mathworks.com/matlabcentral/answers/196952-how-to-create-new-variables-in-batches-with-strcat

This is what David Young proposes in their second solution, and you should seriously consider using a non-scalar structure to store your data rather than multiple individual variables.

Answer 1

[David Young]

A function to multiply two structures field by field. This needs to be copied into a file on your MATLAB path called multiplyStructs.m

function s3 = multiplyStructs(s1, s2)
%MULTIPLYSTRUCTS multiplies structures field by field
%   S3 = MULTIPLYSTRUCTS(S1, S2) returns a structure S3 with those fields
%   that are common to S1 and S2. The value of each such field is the
%   matrix product of the values of the corresponding fields of S1 and S2.

% get common field names
f1 = fieldnames(s1);
fnames = f1(ismember(f1, fieldnames(s2)));

% multiply
for k = 1:length(fnames)
    fname = fnames{k};
    s3.(fname) = s1.(fname) * s2.(fname);   % matrix product
end

end

Test code:

struct1.a = 1;
struct1.b = 2;
struct1.c = 3;  % will be ignored as no match
struct1.d = 4;

struct2.a = 3;
struct2.b = 4;
struct2.d = 10;

struct3 = multiplyStructs(struct1, struct2)

An alternative way, which works only if all the structures have the same fields, but which generalises more readily to multiple structures, is to use this function:

function s = structProd(svec)
%STRUCTPROD returns the field-by-field products of a structure array
%   S = STRUCTPROD(SARR) takes a structure array and returns a scalar
%   structure with the same fields. The value of each field of S is the
%   product of the values of the corresponding fields of SARR, which must
%   all be numerical scalars.
%
%   See also: prod

fnames = fieldnames(svec);

for k = 1:length(fnames)
    fname = fnames{k};
    s.(fname) = prod([svec.(fname)]);
end

end

and then put the structures into an array (which indeed might be the best way to store them to start with. Here's the test code:

struct1.a = 1;
struct1.b = 2;
struct1.d = 4;

struct2.a = 3;
struct2.b = 4;
struct2.d = 10;

struct3.a = -1;
struct3.b = -1;
struct3.d = -1;

struct4.a = 4;
struct4.b = 1;
struct4.d = 2;

struct5 = structProd([struct1 struct2 struct3 struct4])

[David Young]

Note that my second method is equivalent to Stephen Cobeldick's approach - the difference is just that it uses an explicit loop rather than cellfun to iterate over field names.

[Oliz]

Thank you very much for the detailed answer. Your first solution solves my problem :)

It is generic and easily modified for similar purposes.

Answer 2

[Jonathan Campelli]

Hello Oliz,

I've played around with your structures, and I have multiplied struct1.a through struct2.b in the following way:

struct1.a=1
struct1.b=2
struct2.a=3
struct2.b=4

struct3.a=struct2.a*struct1.a    %The multiplication operator provides the right solution.
struct3.b=struct2.b*struct1.b

Once run, the script yields the following results:

struct1 = 

      a: 1

struct1 = 

      a: 1
      b: 2

struct2 = 

      a: 3

struct2 = 

      a: 3
      b: 4

struct3 = 

      a: 3

struct3 = 

      a: 3
      b: 8

Keep having fun with these things.

Best regards,

Jonathan Campelli

[Oliz]

Hi Jonathan,

Thanks for taking the time to answer my question, however the reason I used an example was because my data is a lot more complex. I have around 15 structures each with 12 fieldnames.

As you can imagine, using this method is quite tedious and is the way I am currently implementing it.

I am looking to find an alternative method where I can use something along the lines of struct1*struct2 gives another structure with all the fields or some kind of loop which can recognise the field names and only multiply when they match.

[Jonathan Campelli]

I see. You may find this solution to a related scenario more applicable: http://www.mathworks.com/matlabcentral/answers/66780-multiply-each-element-of-structured-array-assignment-of-structured-array.

[Oliz]

Thanks for this, I saw this earlier when searching through the forum. Unfortunately, I don't think it applies to my case since I don't want to multiply each field in my structure by a constant, but rather dynamically based on values inside another structure sharing the same field name.