MATLAB Incorrect number of right hand side elements - matlab

Question

Matlab's syntax is infuriating, specifically with structs. In the Bioinformatics toolkit, there is a method called jcampread(File) which is described here.
In the description, the method jcampread() takes a Filepath and outputs into a struct called jcampStruct. From my understanding, in Matlab, you don't declare return variable types like you do in C: you just give the return variable a name and it somehow knows that the return of jcampread() method will be a jcampStruct. How it does, I have no idea, but it does.
I put in the code exactly how their example shows in step 4 of the Example section, and I get the following error message back from Matlab:
Incorrect number of right hand side elements in
dot name assignment. Missing [] around left hand
side is a likely cause.
Error in jcampread>ntupleRead (line 510)
dataBlock.ZName = name{Zidx};
Error in jcampread (line 192)
dataBlocks = ntupleRead(fid);
This site says the problem occurs "when f has more than one matrix element." Code is below:
»f.a = [1 0]
f =
a: [1 0]
»f.b = [1 2]
f =
a: [1 0]
b: [1 2]
»f = setfield(f,'a',[2 2])
f =
a: [2 2]
b: [1 2]
»f(2).a=1
f =
1x2 struct array with fields:
a
b
»f = setfield(f,'a',[2 2])
??? Error using ==> setfield
Incorrect number of right hand side elements in dot name assignment.
Missing [] around left hand side is a likely cause.
I assume this means the matrix f looks like this:
f = [ [a1; b1]; [a2; b2]; ]
f = [ [[2 2]; [1 2]]; [[1]; []]; ]
When they tried to update f.a which was set to
f.a = [[2 2]; [1]]
...to a single element [2 2], it doesn't like that because f.a is currently a matrix with 2 vector elements. Basically if you are going to reassign f.a (all elements of the attribute a of matrix f), you have to reassign f.a to have the same number of elements as it currently has.
I think that is why this error is occuring in the setfield example.
My question: how does this apply to jcampread()? jcampStruct is literally a structure with the same attributes, and those attributes are assigned only once. I do not understand:
a. How matlab knows the return value of jcampread() is a jcampStruct, and
b. Why (given that it knows (a)), the 'Incorrect number of right hand..' error message is firing here.
Can anyone clear this up for me?

Answer 1

You are creating a non scalar structure and there is no way to assign at once, i.e. without a loop, a different value to the same field of each sub-structure.What does it mean?
Scalar structure
s.a = 1;
size(s)
ans =
1 1
Now, adding fields doesn't change the size of the structure:
s.b = 2;
size(s)
ans =
1 1
Non-scalar structure
However, assigning a value to the same field, but into a position > 1 of the structure, will grow it a non-scalar one:
s(2).a = 3
size(s)
ans =
1 2
Also, notice how the sub-structure in position 2 replicates/pre-allocates the fields of the initial structure even though you assigned to a alone:
s(2)
ans =
a: 3
b: []
Pointers
Additionally, the field s(2).b is just an empty pointer:
whos s
Name Size Bytes Class
s 1x2 496 struct
and by adding a scalar double (8 bytes), we get
s(2).b = 4;
whos s
Name Size Bytes Class
s 1x2 608 struct
Pro of non-scalar structure
What you can do with a non-scalar structure, is retrieve one field across all sub-structure (considering you don't run into concatenation issues):
for ii = 1:100
s(ii).a = rand(1,2);
end
cat(1,s.a)
the last command will concatenate all values of a single field from all sub-structure into a 100 by 2 array.
Cons
To assign different values across the sub-structures, even if the same field, you need to loop (as above in the for loop).
At most you could deal() the same values into one field across all sub-structures:
clear s
[s(1:100)] = deal([1, 2]);