I want to splice a list of arguments to pass to a function. For a vector I know that I can use num2cell and call the cell with curly braces (see this question), but in my case the list I want to splice originally has structs and I need to access one of their attributes. For example:
austen = struct('ids', ids, 'matrix', matrix);
% ... more structs defined here
authors = [austen, dickens, melville, twain];
% the function call I want to do is something like
tmp = num2cell(authors);
% myFunction defined using varargin
[a,b] = myFunction(tmp{:}.ids);
The example above does not work because Matlab expected ONE output from the curly braces and it's receiving 4, one for each author. I also tried defining my list of arguments as a cell array in the first place
indexes = {austen.ids, dickens.ids, melville.ids, twain.ids};
[a,b] = myFunction(indexes{:});
but the problem with this is that myFunction is taking the union and intersection of the vectors ids and I get the following error:
Error using vertcat
The following error occurred converting from double to struct:
Conversion to struct from double is not possible.
Error in union>unionR2012a (line 192)
c = unique([a;b],order);
Error in union (line 89)
[varargout{1:nlhs}] = unionR2012a(varargin{:});
What is the correct way for doing this? The problem is that I will have tens of authors and I don't want to pass al of them to myFunction by hand.
As #kedarps rightly pointed out I need to use struct2cell instead of num2cell. The following code does the trick
tmp = struct2cell(authors);
[a, b] = myFunction(tmp{1,:,:}); %ids is the first entry of the structs
I had never heard about struct2cell before! It doesn't even show up in the See also of help num2cell! It would be amazing to have an apropos function like Julia's....
Related
I've been stuck with this for a while and I couldn't find something similar asked previously (or I have failed in doing so)
My situation is fairly simple: I have a cell array of objects. They are all the same object and I have a get function for this kind of object which is: get (obj, attr), where obj is the object in question and attr is a integer from 1-6. Depending on the number the get function returns the corresponding attribute.
I would like to obtain all of my "position" attributes from all my objects which are in the corresponding cell array (this would be attr = 2). I know that cellfun performs a function on all cells, but the question is, how do I use my get function here for all my objects, taking into account that the function is get (obj, attr) ?
Thanks in advance
Firstly, by using get as a custom function you are shadowing the built-in get function - this is bad practise!
With this in mind, and to avoid confusion with the built-in get function which has similar syntax, I'm going to use getattr as a stand-in for your custom function which accpets an object and an integer 1 to 6.
pos = cellfun( #(obj) getattr( obj, 2 ), myCellOfObjects, 'uni', 0 );
By specifying 'uni', 0, the output doesn't have to be scalar and will be put into a cell array. This is useful when, for example, you have a multi-element array for your position.
This is equivalent to the following loop:
pos = cell( numel(myCellOfObjects), 1 );
for ii = 1:numel(pos)
pos{ii} = getattr( myCellOfObjects{ii}, 2 );
end
If ever in doubt about cellfun or arrayfun, just write a loop first - they are essentially the same but more concise.
There is a trick to this some are unaware of: you can pass multiple arguments to cellfun like this:
cellfun(#(obj,attr) get(obj,attr), {obj1,obj2},{attr1,attr2},'uni',0)
if you want to get one attribute of the cellarray (instead of providing an attribute for every object in the cellarray), then you can simply use this
cellfun(#(x) getattr(x,attr),obj,'uni',0)
put into anonymous function for convenience:
get_attr = #(obj,attr) cellfun(#(x) getattr(x,attr),obj,'uni',0)
%use:
get_attr(obj_in_cellarray,'myattribute')
%returns cell array of object attributes
I haven't run any of these functions since you didn't provide any example data / code. Please test and feedback.
I am attempting to loop through the variable 'docs' which is a cell array that holds strings, i need to make a for loop that colllects the terms in a cell array and then uses command 'lower' and unique to create a dictionary.
Here is the code i've tried sp far and i just get errors
docsLength = length(docs);
for C = 1:docsLength
list = tokenize(docs, ' .,-');
Mylist = [list;C];
end
I get these errors
Error using textscan
First input must be of type double or string.
Error in tokenize (line 3)
C = textscan(str,'%s','MultipleDelimsAsOne',1,'delimiter',delimiters);
Error in tk (line 4)
list = tokenize(docs, ' .,-');
Generically, if you get an "must be of type" error, that means you are passing the wrong sort of input to a function. In this case you should look at the point in your code where this is taking place (here, in tokenize when textscan is called), and doublecheck that the input going in is what you expect it to be.
As tokenize is not a MATLAB builtin function, unless you show us that code we can't say what those inputs should be. However, as akfaz mentioned in comments, it is likely that you want to pass docs{C} (a string) to tokenize instead of docs (a cell array). Otherwise, there's no point in having a loop as it just repeatedly passes the same input, docs, into the function.
There are additional problems with the loop:
Mylist = [list; C]; will be overwritten each loop to consist of the latest version of list plus C, which is just a number (the index of the loop). Depending on what the output of tokenize looks like, Mylist = [Mylist; list] may work but you should initialise Mylist first.
Mylist = [];
for C = 1:length(docs)
list = tokenize(docs{C}, ' .,-');
Mylist = [Mylist; list];
end
I'd like to use the substruct function to create a structure for use in subsref. The purpose is to index a string using subsref instead of the usual ( ) notation because I'm subscripting the output of a function. This is a simple example of what I'm trying to do (in my actual code, this is used within a cellfun, so the strings can be of different lengths and the replacement isn't always in the same place):
data = 'quick brown fox';
data2 = strrep(data, 'brown', 'green');
data2(7:end)
Heres where I tried to define this subscripting with substruct:
data = 'quick brown fox';
S = substruct('()', {[7:end]});
subsref(strrep(data, 'brown', 'green'), S)
but this just gives me an error:
Error using substruct (line 30)
SUBSTRUCT takes at least two arguments.
Error in myfile (line 3)
S = substruct('()', {(7:end)});
I've been over and over the documentation for both substruct and subsref, and no where do they mention end. How do I do this?
There is no way to my knowledge to do that with subsref which assumes you know exactly which subs you will need.
Use #function_handle to dynamically determine the end:
f = #(x) x(7:end);
f(strrep(data, 'brown', ''))
ans =
fox
I have a function which accepts a variable number of input variables. The problem is, the number of input arguments I'm going to provide varies. As such, I store all the arguments in a structure:
function grandAvg(datafiles)
% Load up all averaged datafiles
avgs = struct();
for n=1:length(datafiles)
avgs(n).avg = load(datafiles{n});
end
My question is, is there a way to expand this argument for the function? I need a way to convert this:
% DOES NOT WORK
avg = ft_timelockgrandaverage(cfg, avgs);
to this:
% WOULD WORK, BUT DO NOT WANT TO TYPE IT OUT
avg = ft_timelockgrandaverage(cfg, avgs(1).avg, ..., avgs(n).avg);
EDIT TO ADD: So apparently my question wasn't clear. I know how to construct the function using varargin. My question was, if I am trying to use a build-in function which I don't want to or can't modify, how can I provide arguments in a variable manner? I.e., I don't know ahead of time how many argument's I'll be providing, so when I call the function, I'll have to call it with X number of arguments. In effect, I'm looking for a way to turn this:
someVar <1xN struct>
into this:
someVar1 <1x1 struct>
someVar2 <1x1 struct>
...
someVarN <1x1 struct>
in a programmatic manner. Thanks!
An alternative to using a structure array to store your data would be to use a cell array. For example:
nFiles = numel(datafiles); %# Number of files
avgs = cell(1,nFiles); %# Initialize an empty cell array
for iFile = 1:nFiles %# Loop over the files
avgs{iFile} = load(datafiles{iFile}); %# Load the data into each cell
end
avg = ft_timelockgrandaverage(cfg, avgs{:}); %# Pass the contents to a function
The syntax avgs{:} dumps the contents of the cell array into what's called a comma-separated list. It is equivalent to typing avgs{1}, avgs{2}, ... , avgs{end}. The syntax foo(:).bar from the answer you found also creates a comma-separated list, but I find that using cell arrays for such a purpose is generally cleaner than using a structure array.
yes you can use variable length input argument list
varargin
http://www.mathworks.com/help/techdoc/ref/varargin.html
So, after playing around, I've got it. Using the example from above:
Given an 1xN struct named foo, where each foo(n) contains the field bar, I would call the function as:
function(foo(:).bar);
This is the same as typing
function(foo(1).bar, foo(2).bar, ..., foo(N).bar);
In this way, I can dynamically expand or shrink foo and still have no problem calling the function.
You can surely do such a thing, by means of the varargin construct in MATLAB. This will be something like:
avg = ft_timelockgrandaverage(cfg, avgs.avg);
And for the function ft_timelockgrandaverage
function output = ft_timelockgrandaverage(config, varargin)
% your code here
varargin will be a cell array: {avgs(1).avg, avgs(2).avg, ..., avgs(3).avg} which you can process.
I have no idea what's going on here. I'm using R2006b. Any chance someone out there with a newer version could test to see if they get the same behavior, before I file a bug report?
code: (bug1.m)
function bug1
S = struct('nothing',{},'something',{});
add_something(S, 'boing'); % does what I expect
add_something(S.something,'test'); % weird behavior
end
function add_something(X,str)
disp('X=');
disp(X);
disp('str=');
disp(str);
end
output:
>> bug1
X=
str=
boing
X=
test
str=
??? Input argument "str" is undefined.
Error in ==> bug1>add_something at 11
disp(str);
Error in ==> bug1 at 4
add_something(S.something,'test');
It looks like the emptiness/nothingness of S.something allows it to shift the arguments for a function call. This seems like Very Bad Behavior. In the short term I want to find away around it (I'm trying to make a function that adds items to an initially empty cell array that's a member of a structure).
Edit:
Corollary question: so there's no way to construct a struct literal containing any empty cell arrays?
As you already discovered yourself, this isn't a bug but a "feature". In other words, it is the normal behavior of the STRUCT function. If you pass empty cell arrays as field values to STRUCT, it assumes you want an empty structure array with the given field names.
>> s=struct('a',{},'b',{})
s =
0x0 struct array with fields:
a
b
To pass an empty cell array as an actual field value, you would do the following:
>> s = struct('a',{{}},'b',{{}})
s =
a: {}
b: {}
Incidentally, any time you want to set a field value to a cell array using STRUCT requires that you encompass it in another cell array. For example, this creates a single structure element with fields that contain a cell array and a vector:
>> s = struct('strings',{{'hello','yes'}},'lengths',[5 3])
s =
strings: {'hello' 'yes'}
lengths: [5 3]
But this creates an array of two structure elements, distributing the cell array but replicating the vector:
>> s = struct('strings',{'hello','yes'},'lengths',[5 3])
s =
1x2 struct array with fields:
strings
lengths
>> s(1)
ans =
strings: 'hello'
lengths: [5 3]
>> s(2)
ans =
strings: 'yes'
lengths: [5 3]
ARGH... I think I found the answer. struct() has multiple behaviors, including:
Note If any of the values fields is
an empty cell array {}, the MATLAB
software creates an empty structure
array in which all fields are also
empty.
and apparently if you pass a member of a 0x0 structure as an argument, it's like some kind of empty phantom that doesn't really show up in the argument list. (that's still probably a bug)
bug2.m:
function bug2(arg1, arg2)
disp(sprintf('number of arguments = %d\narg1 = ', nargin));
disp(arg1);
test case:
>> nothing = struct('something',{})
nothing =
0x0 struct array with fields:
something
>> bug2(nothing,'there')
number of arguments = 2
arg1 =
>> bug2(nothing.something,'there')
number of arguments = 1
arg1 =
there
This behaviour persists in 2008b, and is in fact not really a bug (although i wouldn't say the designers intended for it):
When you step into add_something(S,'boing') and watch the first argument (say by selecting it and pressing F9), you'd get some output relating to the empty structure S.
Step into add_something(S.something,'test') and watch the first argument, and you'd see it's in fact interpreted as 'test' !
The syntax struct.fieldname is designed to return an object of type 'comma separated list'. Functions in matlab are designed to receive an object of this exact type: the argument names are given to the values in the list, in the order they are passed. In your case, since the first argument is an empty list, the comma-separated-list the function receives starts really at the second value you pass - namely, 'test'.
Output is identical in R2008b:
>> bug1
X=
str=
boing
X=
test
str=
??? Input argument "str" is undefined.
Error in ==> bug1>add_something at 11
disp(str);
Error in ==> bug1 at 4
add_something(S.something,'test'); % weird behavior