I've got a nested struct array like
A(1).B(1).var1 = 1;
A(1).B(2).var1 = 2;
Now I want to change the values of var1 to using the elements of the vector x = [3; 4] for each of the respective values.
The result should be
A(1).B(1).var1 = 3;
A(1).B(2).var1 = 4;
I have tried
% Error : Scalar structure required for this assignment.
A(1).B.var1 = x;
% Error : Insufficient number of outputs from right hand side of equal sign to satisfy assignment.
[A(1).B.var1] = x(:);
Curiously, if x is a cell array, the second syntax works
x = {3, 4};
[A(1).B.var1] = x{:};
Luckily, it's not too complicated to convert my numeric vector to a cell array using mat2cell, but is that the only way to do this assignment without a for loop?
What's the correct syntax for multiple assignment to a nested struct array? Can I use numeric vectors or do I have to use cell arrays?
The statement
[A(1).B.var1] = x{:};
is shorthand for
[A(1).B.var1] = deal(x{:});
(see the documentation for deal).
Thus you can also write
[A(1).B.var1] = deal(3,4);
I'm not aware of any other way to assign different values to a field in a struct array in a single command.
If your values are in a numeric array, you can easily convert it to a cell array using num2cell (which is simpler than the mat2cell you found).
data = [3,4];
tmp = num2cell(data);
[A(1).B.var1] = tmp{:};
In general, struct arrays are rather awkward to use for cases like this. If you can, I would recommend that you store your data in normal numeric arrays, which make it easier to manipulate many elements at the same time. If you insist on using a struct array (which is convenient for certain situations), simply use a for loop:
data = [3,4];
for ii = 1:length(A(1).B)
A(1).B(ii).var1 = data(ii);
end
The other alternative is to use table.
Related
I have a cell array (let's say size 10) where each cell is a structure with the same fields. Let's say they all have a field name x.
Is there a way to retreive in a vector the value of the field x for all the structure in the cell array? I would expect the function to return a vector of size 10 with in position 1, the value of the field x of the structure in cell 1 etc etc...
EDIT 1:
The structure in the cell array have 1 field which is the same for all but some others which are different.
First convert your cell array of structures, c, (with identical field names in the same order) to a structure array:
c = cell2mat(c)
Then, depending on the data types and sizes of the elements of the field, you may be able to use
[c.x]
to extract your vector of field x values in the "standard" way.
It is also possible that you can skip the conversion step and use cellfun(#(e)e.x, c) to do the extraction in one go.
The below code creates a cell array of structures, and extracts field 'x' of each structure to a vector v.
%create a cell array of structures
s1.a = 'hello';
s1.x = 1;
s2.a = 'world';
s2.x = 2;
c{1} = s1;
c{2} = s2;
v = zeros(1,2);
%extract to vector
for idx=1:size(c,2)
v(1,idx) = c{idx}.x;
end
Let's say you have
c = {s1, s2, s3, ...., sn};
where common field is 'field_1', then you have two options
Use cell2mat.
cc = cell2mat(c); % which converts your cell array of structs into an array of structs
value = [cc.field_1]; % if values are number
or
value = {cc.field_1}; % if values are characters, for example
Another option is to use cellfun.
If the field values are characters, you should set "UniformOutput" to "false"
value = cellfun(#(x) x.field_1, c, 'UniformOutput', false)
The first option is better. Also, try to avoid using cell/cellfun/arrayfun whenever you can, vectors are way faster and even a plain for loop is more effecient
I have a fairly simple question in Matlab. I want to copy n items of structure array (sumRT.P) to a matrix (m). In C, I would just use a for loop, like this:
for i = 1:n
m(i) = sumRT(i).P;
end
But I bet there's a simpler way to copy an array in Matlab (that's the whole point of language right?). I tried this:
m = sumRT(1:n).P;
But this just copies the first item in sumRT.P to m, resulting in a 1 X 1 matrix. Note, if I type, sumRT(2).P for example, I can see the second item. Same for any number up to n. Why is this wrong and how do I fix it?
It depends on the data types in your structure array. If they are types of variables, or if they are variables of the same size in arrays of different dimensions, then you can't put them into an array, but you can make them into a cell:
m={sumRT(1:n).P}
and cells are pretty simple to deal with, so this oughtn't be a big problem.
If they are all scalar numerical values, you can create a matrix:
m=cell2mat({sumRT(1:n).P})
Try the following:
m = squeeze(cell2mat(struct2cell(sumRT(1:n))));
This converts the struct array to a cell array, and then to a (numeric) array, and then squeezes it by remoiving singleton dimensions.
Example:
>> sumRT(1).P = 10; sumRT(2).P = 20; sumRT(3).P = 30;
>> n = 2; %// copy first two elements only
>> m = squeeze(cell2mat(struct2cell(sumRT(1:n))))
m =
10
20
How to create an empty array in matlab that accepts elements from a matrix when you do not know the no.of elements it is going to contain ?
Use the [] operator. Example:
x = [];
If you wanna be specific in the type of the empty matrix, use the empty property. Examples:
emptyDoubleMatrix = double.empty; % Same as emptyDoubleMatrix = [];
emptySingleMatrix = single.empty;
emptyUnsignedInt8Matrix = uint8.empty;
This works for empty matrices of classes as well. Example:
emptyFunctionHandleMatrix = function_handle.empty;
You can use the empty matrix/vector notation, [], and Matlab will set up a placeholder for it.
x = []
Now if you want to append a scalar, say num, to it, you cannot index it because it is empty.
However, you can either:
Use array concatenation to concatenate itself with another scalar:
x = [x num]
Use the end+1 notation, to address the first available location:
x(end+1) = num
Both of the above two notations also work when you want to append a row or a column vector to an existing row vector or column vectors. But when you are concatenating vectors/matrices, remember to be consistent with the dimensions.
I have a set of strings vals, for example:
vals = {'AD', 'BC'}
I also have a struct info, inside of which are structs nested in fields corresponding to the elements in the array vals (that would be 'AD' and 'BC' in this example), each in turn storing a number in a field named lastcontract.
I can use a for loop to extract lastcontract for each of the vals like this:
for index = 1:length(vals)
info.(vals{index}).lastcontract
end
I'd like to find a way of doing this without a loop if at all possible, but I'm not having luck. I tried:
info.(vals{1:2}).lastcontract
without success. I think arrayfun may be the appropriate way, but I can't figure out the right syntax.
It is actually possible here to manage without an explicit loop (nor arrayfun/cellfun):
C = struct2cell(info); %// Convert to cell array
idx = ismember(fieldnames(info), vals); %// Find fields
C = [C{idx}]; %// Flatten to structure array
result = [C.lastcontract]; %// Extract values
P.S
cellfun would be more appropriate here than arrayfun, because you iterate vals (a cell array). For the sake of practice, here's a solution with cellfun:
result = cellfun(#(x)info.(x).lastcontract, vals);
There has a loop in my program, and during each iteration an ID will be generated. I want to store these IDs into a two dimensional array, i.e., A. The first column of A stores the iteration number, i.e., A(1,1) = 1 and A(2,1) = 2. The second column of A stores the ID generated during each iteration, i.e., A(1,2) stores the ID generated during the first iteration. The tricky part is that these IDs can be either a numerical value or a string. For instance, A(1,2) = 12345; A(2,2) = abcde
Which kind of data structure should I use to store this mixed-value matrix?
You have two good options, a cell array or an array of structures.
To use a cell array you need to use braces:
A{1,1} = 1;
A{2,1} = 2;
A{1,2} = 12345;
A{2,2} = 'abcd';
You cannot use most vectorized code with cell arrays, although you can convert numeric subsets to numeric arrays, for example:
col1 = cell2mat(A(:,1));
To use an array of structures, you need to define fields. This has the advantage that you can name your columns of data.
A(1).iteration = 1;
A(2).iteration = 2;
A(1).result = 12345;
A(2).result = 'abcd';
To access a single row of data, use A(1), like this
>> A(1)
ans =
iteration: 1
result: 12345
To access a column of data, use brackets or braces
>> [A.iteration] %This results a numeric array, or an error if not possible
ans =
1 2
>> {A.result} %This returns a cell array, as discussed above.
ans =
[12345] 'abcd'
Which option you use depends on the nature of your task and what method is more suitable to your style. I usually start with a cell array, and eventually convert to an array of structs to take advantage of the named fields.