Using Set Field for multiple depths

Hi,

I'd like to set a particular element of a structure array with a dynamic index to a field. So, for example, if fieldNames={'a' 'b' 'c'} and fieldValue=3, I want to set myStruct.a.b.c=3. I can hack together something that uses the eval function, say for example

str=['myStruct'];
for ii=1:length(fieldnames)
str=[str '.('''];
str=[str fieldnames{ii}];
str=[str ''')'];
end
eval([str '=' num2str(fieldValue)])

But is there a way to do this without using eval? I tried looking at setfield, but I can't get it to work. Running

x.a.b.c=3;
fieldnames={'a' 'b' 'c'};
getfield(x,fieldnames{:})

successfully returns 3, but running

setfield(x,fieldnames{:},5)

doesn't seem to do anything

Thanks

Brendan

Answer 1

[Walter Roberson]

You will not be able to do this using setfield() or getfield(), not in any useful way.

You should refer to subsref() and subsassgn(). They are a bit clumsy to use, but they can handle the task.

[Brendan]

Thanks for the subsref comment. I don't really understand your comment about getfield(), it seems like I can use getfield in the way I describe, just not setfield.

Answer 2

[Daniel Shub]

If it is always 3 deep then you can just do

myStruct.(fieldnames{1}).(fieldnames{2}).(fieldnames{3}) = fieldValue;

A little bit more general is

switch length(fieldnames)
  case 1
    myStruct.(fieldnames{1}) = fieldValue;
  case 2
    myStruct.(fieldnames{1}).(fieldnames{2}) = fieldValue;
  case 3
    myStruct.(fieldnames{1}).(fieldnames{2}).(fieldnames{3}) = fieldValue;
end

You can obviously extend this to any N you want.

A truly robust solution would be to do this recursively.

function S = rsetfield(S, field, V)
  if length(field) > 1
    S.(field{1}) = rsetfield(S.(field{1}), field(2:end), V);
  else
    S.(field{1}) = V;
  end
end

You would want to do some input checking to make sure everything is happy. I have no idea how inefficient this is.