I am trying to create an array where each element is an empty array.
I have tried this:
var result = Array.fill[Array[Int]](Array.empty[Int])
After looking here How to create and use a multi-dimensional array in Scala?, I also tried this:
var result = Array.ofDim[Array[Int]](Array.empty[Int])
However, none of these work.
How can I create an array of empty arrays?
You are misunderstanding Array.ofDim here. It creates a multidimensional array given the dimensions and the type of value to hold.
To create an array of 100 arrays, each of which is empty (0 elements) and would hold Ints, you need only to specify those dimensions as parameters to the ofDim function.
val result = Array.ofDim[Int](100, 0)
Array.fill takes two params: The first is the length, the second the value to fill the array with, more precisely the second parameter is an element computation that will be invoked multiple times to obtain the array elements (Thanks to #alexey-romanov for pointing this out). However, in your case it results always in the same value, the empty array.
Array.fill[Array[Int]](length)(Array.empty)
Consider also Array.tabulate as follows,
val result = Array.tabulate(100)(_ => Array[Int]())
where the lambda function is applied 100 times and for each it delivers an empty array.
Related
If I define my array like this:
array[_, 1..2] of J: A;
where J is a enum, what function can I use to get the length of the first dimension of A? The length function returns 14 (and states that it doesn't work for multiple dimensions). I had it hard-coded like so in my testing, but it won't always be 13:
constraint forall(row in 1..13)(S[A[row, 2]] >= C[A[row, 1]]);
The length function is really only meant for getting the number of elements. When you want the retrieve indexes of an array, you should instead use the index_set function. This returns the actual index set, which you can iterate over: i in index_set(A)
For multidimensional arrays there are specific functions for each index set of the array. For example, to get the first index set from a two-dimensional array (as in your code fragment) you can use the function index_set_1of2
I have a struct array named Lst. Every struct has the following form:
Point (x,y)
Type (1-6)
I want get the separate array of points for each type. How can I get it?
Lst(Lst.Type==1);
won't work since Type is not a field of Lst but of Lst(i).
In addition, is there a way to save the indexes of each item or an alternative way to then combine them again to the original order?
L1 = Lst([Lst.Type]==1); will give you the subset L1 of Lst where Type == 1.
Likewise, you can use idx1 = find([Lst.Type]==1) to memorize your indexes.
EDIT: the above uses the [] operator to aggregate the field elements Type of Lst into an array. To your comment/question, you could use the exact same operator also to obtain an array of specific field elements X of a subset of the structured array, as in
X1 = [Lst([Lst.Type]==1).X];
I want to replace the value of the fields in a structure array. For example, I want to replace all 1's with 3's in the following construction.
a(1).b = 1;
a(2).b = 2;
a(3).b = 1;
a([a.b] == 1).b = 3; % This doesn't work and spits out:
% "Insufficient outputs from right hand side to satisfy comma separated
% list expansion on left hand side. Missing [] are the most likely cause."
Is there an easy syntax for this? I want to avoid ugly for loops for such simple operation.
Credits go to #Slayton, but you actually can do the same thing for assigning values too, using deal:
[a([a.b]==1).b]=deal(3)
So breakdown:
[a.b]
retrieves all b fields of the array a and puts this comma-separated-list in an array.
a([a.b]==1)
uses logical indexing to index only the elements of a that satisfy the constraint. Subsequently the full command above assigns the value 3 to all elements of the resulting comma-separated-list according to this.
You can retrieve that the value of a field for each struct in an array using cell notation.
bVals = {a.b};
bVals = cell2mat( bVals );
AFAIK, you can't do the same thing for inserting values into an array of structs. You'll have to use a loop.
As input to a function, I am getting an array of target elements, T, and an array of structs S where each one has a .elems field, which is a list of integers (elements).
I'm sure there's a simple way to do this in Matlab. How do I get the indices i of all structs where a specific element t of T is in S(i).elems contains t?
So I think you'll need to do this with an arrayfun. I did:
S = ... (1-by-N array of structs);
T = ... (1-by-K array of numbers);
indices = find(arrayfun(#(i)any(ismember(T, S(i).elems)), 1:numel(S)));
any(ismember(T, S(i)elems)) tests is any of the things in T are in S(i).elems. The arrayfun repeats this for each struct in S. find extracts indices from the logical array that is returned by the arrayfun.
I am writing a function to remove some values from a cell array, like so:
function left = remove(cells, item);
left = cells{cellfun(#(i) ~isequal(item, i), cells)};
But when I run this, left has only the first value, as the call to cells{} with a logical array returns all of the matching cells as separate values. How do I group these separate return values into a single cell array?
Also, perhaps there is already a way to remove a given item from a cell array? I could not find it in the documentation.
You have to use () instead of {} to index the cells:
function left = remove(cells, item)
left = cells(cellfun(#(i) ~isequal(item, i), cells));
Using () for indexing will give you a subset of cells, while using {} will return the contents of a subset of cells as a comma-separated list, and only the first entry of that list will get placed in left in your example.
You can check out this MATLAB documentation for more information on using cell arrays.
EDIT: Response to comment...
If you have an operation that ends up giving you a comma-separated list, you can place the individual elements of the list into cells of a cell array by surrounding the operation with curly braces. For your example, you could do:
left = {cells{cellfun(#(i) ~isequal(item, i), cells)}};
The inner set of curly braces creates a comma-separated list of the contents of cells that are not equal to item, and the outer set then collects this list into a cell array. This will, of course, give the same result as just using parentheses for the indexing, which is the more sensible approach in this case.
If you have a function that returns multiple output arguments, and you want to collect these multiple values into a cell array, then it's a bit more complicated. You first have to decide how many output arguments you will get, or you can use the function NARGOUT to get all possible outputs:
nOut = 3; %# Get the first three output arguments
%# Or...
nOut = nargout(#some_fcn); %# Get all the output arguments from some_fcn
Then you can collect the outputs into a 1-by-nOut cell array outArgs by doing the following:
[outArgs{1:nOut}] = some_fcn(...);
It should be noted that NARGOUT will return a negative value if the function has a variable number of output arguments, so you will have to choose the value for nOut yourself in such a case.