Loren on the Art of MATLABDecember 13th, 2005Use Dynamic Field ReferencesUse dynamic field references, the notation .(), where possible, instead of setfield, getfield, and eval. HistoryIn MATLAB 6.5 (Release 13), we introduced dynamic field references into MATLAB. What are they and why should you use them? In MATLAB, there are 3 basic types of arrays, traditional ones where you index using parentheses (), cell arrays where you can address the contents using curly braces {}, and structures (known in MATLAB as the type struct) where you access elements using the dot . notation. Before R13, if you had collected information into a variable, call it ind, about which parts of an array you wanted to either index into or to change, you could do so using the variable in a straight-forward manner - if your array was a regular or cell array. By that, I mean that you could do things like:
y = x(ind)
x(ind) = 17
% With cells, you typically access the contents
% of one cell at a time.
c{ind(1)} = x(ind(1))
However, there was no symmetry with how to accomplish the same task if your array was a struct and you had the fieldnames in a variable; instead you had to use setfield and getfield. There was extra overhead for accessing structure elements compared to other array types. And that's why dynamic field indexing was introduced. ExamplePreferred:fldnm = 'fred'; s.(fldnm) = 18; y = s.(fldnm) Not Recommended:s = setfield(s,'fldnm',18); y = getfield(s,'fldnm'); eval(['s.' fldnm ' = 18']); eval(['y = s.',fldnm]) Benefits
Contraindications
13 Responses to “Use Dynamic Field References”Leave a Reply |
|
|||
|
These postings are the author's and don't necessarily represent the opinions of The MathWorks. |
I just stumbled across Loren’s blog quite by accident, and I really enjoy reading through the older postings. I think there is a solution to the second point in the Contraindications section of the article about dynamic field references: “If you don’t know the overall structure of your struct, … it’s hard to program generically with .()”
Use ‘fieldnames’ to retrieve the names of all fields at one level of the structure. I’ve done something similar to the following a number of times:
fn = fieldnames(mystruct);
for lp=1:length(fn)
doSomeOperation( mystruct.( fn{lp} ) );
end
I’ve become more comfortable just using the field names as the loop arguments outright.
fn = fieldnames(mystruct);
for lp=fn
doSomeOperation( mystruct( lp{:} ) );
end
This works very well for homogenous structures or when your ‘doSomeOperation’ function knows how to handle different input types.
For example, you might define doSomeOperation like this:
function x = doSomeOperation(y)
if ~isnumeric(y)
x = [];
else
x = doSomethingForReal(y);
end
Anyway, it’s a thought.
I am writing Matlab code using this notation in order to manage fields in Matlab. I think it is a very good method. I am wondering if codes written using this notation could lead some problems when Matlab Compiler is applied, as it happens for the “eval” function.
Eventually I ask you whether the use of “feval” can solve this problem or not.
Thank you very much.
Pietro Garofalo
Pietro-
There are no issues with using dynamic field referencing and the compiler. I don’t see how using feval helps in any way. This method should be robust.
–Loren
I happen to pass by your blog and saw your entries on MATLAB.
I would like to ask you regarding structure used in MATLAB.
But I am a beginner of MATLAB but I have to work on a project on B-splines.
>>>> My problem is that I currently can only plot one spline on the axes. My data will always be over-written the moment I plot another axes. However, when I read from the MATLAB book. It seems that the example are assigning data to structure in a less interactive way with the user. Is there any ways that I can work on the data clicked by the user. Then retrieving the relevant data upon selecting on the spline drawn.
Another problem of mine is that, after I have shifted the original spline to another location. After I apply ‘cla’ function, all the splines have been deleted. Is there any other functions that I can used to delete the original spline after shifting the spline to a new location without deleting other splines on the axes.
Thanks a million,
Magdalene
I’m using some super dumb techniques to process multiple variables with a differing prefix..
var_str = {’gamrf’,'pt’,’s’};
for var = 1:length(var_str)
string = strcat(var_str{var},’mean_max = squeeze(max(shiftdim(’,var_str{var},’mean,2)));’); cmd = char(string); eval(cmd);
string = strcat(var_str{var},’mean_min = squeeze(min(shiftdim(’,var_str{var},’mean,2)));’); cmd = char(string); eval(cmd);
end
This is really hitting things with a hammer.. Can someone direct me to an enlightening way of digging myself out of hammerland?
Paul-
Follow the ideas in this blog article instead! Do NOT name your variables specially. Instead use some base name for a structure and have the fields be the various names you want to refer to. That is one of the main points of the article.
–Loren
Hi Loren,
I stumbled upon this page while searching a solution to following problem. I searched usergroups to no avail.
I need to retrieve value of a field from the structure and name of the field is based on user input. Following is the sample code:
test.fourarray=zeros(3,3,3,3);
fname=’fourarray’;
eval([’test.’ ‘fourarray’]);
ERROR: ??? Index exceeds matrix dimensions.
I just can’t figure out what’s happening!
Why do I get above error (which occurs btw irrespective of the size and type of the field)?
Shalin-
Something else must be going on (or there’s a bug in the version you are running). I just tried your code in R2008b and it works fine.
However, you don’t need the eval statement. Try this instead:
and add whatever trailing indexing you want.
–Loren
Yes something else was wrong (may be spurious variables in workspace). It worked alright after clearing unused workspace variables.
Thanks…
Hi Loren,
regarding number 6. I have a similar problem.
I have perform an simulation with Simulink and have stored some variables with Simulink ToWorkspace blocks at my workspace. Due to this I have now variables with a special name at my workspace e.g.
a = 3;
b = 4;
c = 5;
Now I want to save the simulation results at a data structure (e.g. D). At a cell array (e.g. simResults), I have stored all names of the variables which will be stored by the ToWorkspace blocks at the workspace:
simResults = {’a';’b';’c'};
When I try to save the simulation results at my data structure D, I can’t solve it without eval(). Can you please help me, to do it without?
D = struct();
for i = 1:length(simResults)
% Desired, something without eval()
% D.(simResults{i}) = .(simResults{i});
% Possible, at the moment only with eval()
D.(simResults{i}) = eval(simResults{i});
end
-Frank
Frank-
You can’t do that without eval. There is not way to reference a variable without naming it unless you somehow create an appropriate string to evaluate.
You are better off creating the structure and using that to start with, if that’s an option. Or use a cell array, one cell per variable, similarly.
–Loren
Hi Loren,
Are dynamic fields the same thing people call in Perl or Python hatches (previously known as associative arrays)? I am in the bioinformatic area and most scientists use Python or Perl and usually pick on me ’cause Matlab doesn’t have hatches. I found dynamic fields some time ago but still have doubts whether they are the same thing.
Thanks,
A
Alex-
Sort of but not really (and I think you mean hash perhaps, not hatch?)… structs are more like structures in C. the containers.Map class is more like associative arrays in perl or python. The notation .() simply allows variable “indexing” into a struct with a variable fieldname rather than just a fixed literal string.
–Loren