How can I index a MATLAB array returned by a function without first assigning it to a local variable?

Question

For example, if I want to read the middle value from magic(5), I can do so like this:

M = magic(5);
value = M(3,3);

to get value == 13. I'd like to be able to do something like one of these:

value = magic(5)(3,3);
value = (magic(5))(3,3);

to dispense with the intermediate variable. However, MATLAB complains about Unbalanced or unexpected parenthesis or bracket on the first parenthesis before the 3.

Is it possible to read values from an array/matrix without first assigning it to a variable?

Solution 1

It actually is possible to do what you want, but you have to use the functional form of the indexing operator. When you perform an indexing operation using (), you are actually making a call to the subsref function. So, even though you can't do this:

value = magic(5)(3, 3);

You can do this:

value = subsref(magic(5), struct('type', '()', 'subs', {{3, 3}}));

Ugly, but possible. ;)

In general, you just have to change the indexing step to a function call so you don't have two sets of parentheses immediately following one another. Another way to do this would be to define your own anonymous function to do the subscripted indexing. For example:

subindex = @(A, r, c) A(r, c);     % An anonymous function for 2-D indexing
value = subindex(magic(5), 3, 3);  % Use the function to index the matrix

However, when all is said and done the temporary local variable solution is much more readable, and definitely what I would suggest.

Solution 2

There was just good blog post on Loren on the Art of Matlab a couple days ago with a couple gems that might help. In particular, using helper functions like:

paren = @(x, varargin) x(varargin{:});
curly = @(x, varargin) x{varargin{:}};

where paren() can be used like

paren(magic(5), 3, 3);

would return

ans = 16

I would also surmise that this will be faster than gnovice's answer, but I haven't checked (Use the profiler!!!). That being said, you also have to include these function definitions somewhere. I personally have made them independent functions in my path, because they are super useful.

These functions and others are now available in the Functional Programming Constructs add-on which is available through the MATLAB Add-On Explorer or on the File Exchange.

Solution 3

How do you feel about using undocumented features:

>> builtin('_paren', magic(5), 3, 3)               %# M(3,3)
ans =
    13

or for cell arrays:

>> builtin('_brace', num2cell(magic(5)), 3, 3)     %# C{3,3}
ans =
    13

Just like magic :)

---

UPDATE:

Bad news, the above hack doesn't work anymore in R2015b! That's fine, it was undocumented functionality and we cannot rely on it as a supported feature :)

For those wondering where to find this type of thing, look in the folder fullfile(matlabroot,'bin','registry'). There's a bunch of XML files there that list all kinds of goodies. Be warned that calling some of these functions directly can easily crash your MATLAB session.

Solution 4

At least in MATLAB 2013a you can use getfield like:

a=rand(5);
getfield(a,{1,2}) % etc

to get the element at (1,2)

Solution 5

unfortunately syntax like magic(5)(3,3) is not supported by matlab. you need to use temporary intermediate variables. you can free up the memory after use, e.g.

tmp = magic(3);
myVar = tmp(3,3);
clear tmp

View more solutions

Comments

[second]

well what do you know! though i agree it's pretty ugly, and probably less readable than a temp-var solution. +1 for impressive obscure matlab knowledge!

[Joe Kearney]

That's disgusting, but a very clear answer. Good work! Should've guessed there'd be a back way into it. I'll think I'll carry on with the temp variable.

[Sam Roberts]

Bear in mind that the intermediate variable is still fully created though. So if the purpose is to save memory by not having to create a temporary local variable, no luck.

[Mechanical snail]

@SamRoberts: You can't really get around that in a strict-evaluation language like Matlab. The main reason people want this is conciseness/readability, not memory savings.

[Joe Kearney]

I agree that this is more concise, and clearing is a good idea in a loop, as you say, but the question was specifically whether the intermediate assignment can be avoided.

[Joe Kearney]

This is a slightly more general version of the second half of gnovice's answer; also good.

[gerrit]

What about myfunc().attr?

[T. Furfaro]

@gerrit, how does at help? and the x.attr() field isn't available unless you have the database toolbox.

[gerrit]

@T.Furfaro Huh? If myfunc() returns a structure that includes an attribute attr, then to access attr currently I need to do S = myfunc(); S.attr. The question is if we can have a helper function like getattr(myfunc(), 'attr') in analogy to the paren and curly helpers. I don't understand what this has to do with the database toolbox.

[T. Furfaro]

@gerrit Sorry, total confusion ( I wasn't aware that your "attr" was arbitrary -- in the db tb there's such a field explicity defined ). I believe what you're looking for is getfield()

[Shai]

but this is exactly what subref does... but in a more general way.

[Admin]

yes, more general way, but not friendly... to much ugly in my opinion.

[Rody Oldenhuis]

@SamRoberts: true, but it does save you from the burden of calling clear on the temporary (which no-one ever does) -- the temporary tends to stick around longer

[Amro]

@RodyOldenhuis: I dont recall now, I guess I must have read it in some buried code ;)

[vim]

Wonderful! I stumbled around the internet for hours to find this! I'm going to stick with python.

[mmumboss]

This is actually a nice method. Any drawbacks?

[rayryeng]

Hot damn. I didn't know this was possible. In the end, I'm still going to stick with the temp variable!

[Dominik]

The colon (:) operator must be used with apostrophes ':' to avoid the error Undefined function or variable "builtin".

[Amro]

@Dominik: right, say you want to slice the 2nd column, that would be: builtin('_paren', magic(5), ':', 2) (in certain places it does work without the quotations directly as : as opposed to ':', like when running in the command prompt directly not from inside a function. I guess that's a bug in the parser!)

[knedlsepp]

Any way to use this with end, as in vectorReturningFunction()(1:end-1)?

[knedlsepp]

I don't suppose there is some way to use end with this?

[Amro]

@knedlsepp: No, unfortunately the whole end-trickery doesn't work in this syntax, you'll have to be explicit in your indexing.. (Same limitation applies for most other listed answers)

[Daniel]

@mmumboss: That's undocumented behaviour, this functionality may disappear without notice in future versions. Besides this no disadvantages.

[gnovice]

@knedlsepp: Unfortunately, I haven't been able to figure out a way to get end to work with the subsref syntax I used above.

[Dev-iL]

@knedlsepp - According to docs of end: "In that context, end = (size(x,k)) when used as part of the kth index.". So I would say that the possibility of using end with subsref has a slight causality problem: because you're trying to find size(x) before x is known. However, "for the sake of freedom of shooting oneself in the foot", I suppose what you want is possible if you recursively do subsref (within the indexing term i.e. subsref(...,{1:size(subsref(...))})) and obtain the result so that you could know its size... :)

[knedlsepp]

@Dev-iL: Would have been cool to have a feasible solution for this, but I think in the end I can still live without it. :-)

[Dev-iL]

@knedlsepp - I managed to do this using a crazily complicated command that relies on matlab's interface with python, and python's negative-indexing ability. Here goes: vectorReturningFunction = @()1:10; endminus = 4; double(py.array.array('d',py.ast.literal_eval(wrap_system(['‌​python -c "import sys; print eval(sys.argv[1])[0:-int(sys.argv[2])]" ' regexprep(char(py.array.array('d',reshape(vectorReturningFun‌​ction(),1,[])).tolis‌​t()), ' ', '') ' ' num2str(endminus)])))). where wrap_system() is: function out = wrap_system(cmd); [~,out] = system(cmd);. If you post a question I'll explain more..

[BIOStheZerg]

(I know this is a year-old discussion, but...) Well, if you use the other possibility, creating a custom function (anonymous or not), you can convert any (-1, 0, inf,...) index to end inside the function...

[nekomatic]

I've edited in a link to the add-on containing a range of these functions, which is available through the MATLAB add-on explorer or the file exchange.

[njspeer]

As of MATLAB2017b, this functionality is documented.

[Edric]

This solution can be refined a tiny bit by using substruct like so: subsref(magic(5), substruct('()', {3, 3})).

[Cris Luengo]

It is actually not strange: MATLAB keeps a list of defined functions, there is not that much searching to do. feval does the “normal” thing and therefore can make full use of this list. builtin must search elsewhere so it finds only built in functions. Likely this case is not optimized nearly as much as the “normal” case, because why would you put money into optimizing something not used very often?

[Rody Oldenhuis]

Just out of curiosity - if you issue [ subsref(A,substruct('{}',{':'})) ], it will not behave the same as [ A{:} ]...any ideas for a workaround there?

[gnovice]

@RodyOldenhuis: Using the functional form for indexing appears to also require you to specify the number of output arguments you want from the function, the default being one. The output from the operation is a comma-separated list, but only the first one is captured. Any attempt to capture multiple output arguments would not give you a simple way to concatenate them into a vector. It's all a moot point, however, since you could probably just use cell2mat: cell2mat(A)

[Minh Nghĩa]

@SamRoberts Will it be automatically cleared right after that?

[Sam Roberts]

@MinhNghĩa The intermediate variable will be cleared at the same time as any other variable - i.e. when it goes out of scope. That would not usually be immediately after the statement, but rather at the end of the function (although it could be different if the user does something like press Ctrl-C).

[ZR Han]

How do I get a column or a row of the output? Such as a(1, :). I've tried getfield(rand(5), {1, 1:5}) and getfield(rand(5), {1:5, 1}) which work fine, but are not elegant.

[Cris Luengo]

The clear statement will significantly slow down your code, it's better to leave it out unless M is terribly big and you're running out of memory somewhere.

[Andreas GS]

@JoeKearney understood. Perhaps it's my novice level of Matlab, but intermediate values are computed in every answer given, if only implicitly in some. Is that correct? In any case, thanks for the feedback!

[Jacob Lee]

Nice; I'd just run across these two functions, and now I have a solid use case for them.

[John]

@ZRHan: You can use getfield(rand(5), {1, ':'})