Matlab is really driving me crazy on this point. I just want to access the inner array of a 2d array.
E.g.:
A = [1,1; 2,2; 3,3]
B = [4,4; 5,5; 6,6]
C = [7,7; 8,8; 9,9]
D = [0,0; 1,2; 3,4]
E = [A,B,C,D]
how do I get e.g. B out of E again?
By this I mean in the exact same writing styl like X = [4,4; 5,5; 6,6]
The syntax you use concatenates the array to a new one, it is not an array of arrays.
If you want an array of arrays, you could use a cell, E = {A,B,C,D}. Then you can get B back using E{2}.
Cells are general containers, each element can contain whatever you want, it does not have to be the same data type. See What is a cell?
You concatenated A, B, C, D horizontally into a new array E. That is not array of arrays, as the other answer pointed out. Suppose the new array is what you want. If you like to extract original B from E, you will need to know A and B's size, in this case both are 3x2. So You can do following:
X = E(:, 3:4); % 3 is size(A,2)+1, numel(3:4) is size(B,2)
Also I think you did not really mean "writing style", since that is just a way to write assignment.
Related
I'm pretty new to matlab, so I'm guessing there is some shortcut way to do this but I cant seem to find it
results = eqs\soltns;
A = results(1);
B = results(2);
C = results(3);
D = results(4);
E = results(5);
F = results(6);
soltns is a 6x1 vector and eqs is a 6x6 matrix, and I want the results of the operation in their own separate variables. It didn't let me save it like
[A, B, C, D, E, F] = eqs\soltns;
Which I feel like would make sense, but it doesn't work.
Up to now, I have never come across a MATLAB function doing this directly (but maybe I'm missing something?). So, my solution would be to write a function distribute on my own.
E.g. as follows:
result = [ 1 2 3 4 5 6 ];
[A,B,C,D,E,F] = distribute( result );
function varargout = distribute( vals )
assert( nargout <= numel( vals ), 'To many output arguments' )
varargout = arrayfun( #(X) {X}, vals(:) );
end
Explanation:
nargout is special variable in MATLAB function calls. Its value is equal to the number of output parameters that distribute is called with. So, the check nargout <= numel( vals ) evaluates if enough elements are given in vals to distribute them to the output variables and raises an assertion otherwise.
arrayfun( #(X) {X}, vals(:) ) converts vals to a cell array. The conversion is necessary as varargout is also a special variable in MATLAB's function calls, which must be a cell array.
The special thing about varargout is that MATLAB assigns the individual cells of varargout to the individual output parameters, i.e. in the above call to [A,B,C,D,E,F] as desired.
Note:
In general, I think such expanding of variables is seldom useful. MATLAB is optimized for processing of arrays, separating them to individual variables often only complicates things.
Note 2:
If result is a cell array, i.e. result = {1,2,3,4,5,6}, MATLAB actually allows to split its cells by [A,B,C,D,E,F] = result{:};
One way as long as you know the size of results in advance:
results = num2cell(eqs\soltns);
[A,B,C,D,E,F] = results{:};
This has to be done in two steps because MATLAB does not allow for indexing directly the results of a function call.
But note that this method is hard to generalize for arbitrary sizes. If the size of results is unknown in advance, it would probably be best to leave results as a vector in your downstream code.
Is there a way to create an array of sets in Matlab.
Eg: I have:
a = ones(10,1);
b = zeros(10,1);
I need c such that c = [(1,0); (1,0); ...], i.e. each set in c has first element from a and 2nd element from b with the corresponding index.
Also is there some way I can check if an unknown set (x,y) is in c.
Can you all please help me out? I am a Matlab noob. Thanks!
There are not sets in your understanding in MATLAB (I assume that you are thinking of tuples on Python...) But there are cells in MATLAB. That is a data type that can store pretty much anything (you may think of pointers if you are familiar with the concept). It is indicated by using { }.
Knowing this, you could come up with a cell of arrays and check them using cellfun
% create a cell of numeric arrays
C = {[1,0],[0,2],[99,-1]}
% check which input is equal to the array [1,0]
lg = cellfun(#(x)isequal(x,[1,0]),C)
Note that you access the address of a cell with () and the content of a cell with {}. [] always indicate arrays of something. We come to this in a moment.
OK, this was the part that you asked for; now there is a bonus:
That you use the term set makes me feel that they always have the same size. So why not create an array of arrays (or better an array of vectors, which is a matrix) and check this matrix column-wise?
% array of vectors (there is a way with less brackets but this way is clearer):
M = [[1;0],[0;2],[99;-1]]
% check at which column (direction ",1") all rows are equal to the proposed vector:
lg = all(M == [0;2],1)
This way is a bit clearer, better in terms of memory and faster.
Note that both variables lg are arrays of logicals. You can use them directly to index the original variable, i.e. M(:,lg) and C{lg} returns the set that you are looking for.
If you would like to get logical value regarding if p is in C, maybe you can try the code below
any(sum((C-p).^2,2)==0)
or
any(all(C-p==0,2))
Example
C = [1,2;
3,-1;
1,1;
-2,5];
p1 = [1,2];
p2 = [1,-2];
>> any(sum((C-p1).^2,2)==0) # indicating p1 is in C
ans = 1
>> any(sum((C-p2).^2,2)==0) # indicating p2 is not in C
ans = 0
>> any(all(C-p1==0,2))
ans = 1
>> any(all(C-p2==0,2))
ans = 0
I have a big cell array A=cell(a,b,c,d) and a row vector B with dimensions 1-by-b.
I want to build a loop in MATLAB that does the following:
for i=1:n
B = Calculate_row(input1,input2) %this is a function that creates my B row
A{a,:,c,i} = B(:)
end
anyway if I try to do A{a,:,c} = B(:) I receive the following error:
Expected one output from a curly brace or dot indexing expression, but there were b results.
And if I try to do A(a,:,c) = B(:) I receive the following error:
Conversion to cell from double is not possible.
Is there a way to do this? (I know a less elegant way that probably works would be to assign each value to the cell separately, but I would prefer not to do it).
One way to do this is to make B a cell array and then take advantage of comma-separated-lists:
B_cell = num2cell(B);
[A{a,:,c}] = B_cell{:} %// or [A{a,:,c,i}] = B_cell{:} if tim's comment is correct
Have a look at Loren Shure's article Deal or No Deal and also this answer for more.
The problem with your syntax, A{a,:,c} = B(:), is that the RHS (i.e. B(:)) is just one single matrix whereas the LHS is a comma-separated-list of b results. So you are basically requesting that 1 output be assigned to b variables and MATLAB doesn't like that, also hence the error message.
The problem with A(a,:,c) = B(:) is that indexing a cell array with () returns a cell array and you can't just assign a matrix (i.e. B(:)) to a cell array hence you second error.
I'm starting to develop a project which uses multi-dimensional arrays very often.
my arrays are mostly 2 , 3 dimensional or so.
As a 2D array sample consider 'A', I may have 2 or more 1D arrays in a cell.
sth like
A=[1, [78,9] [10,65], 9;
2 , 3 , 6;
7 , [9,1] , [91,41,96][10,-1]]
As you saw in 'A(1,2)' there are two 1D arrays.
I don't know which structure I should use to achieve such thing.
moreover I want to be able to have access to all those 1D arrays.
please share your knowledge with me.
try using cell or struct i would recommend cell.
E.g. preinitialize A1:
A1=cell(3,3)
(that would be a 3x3 cell array/matrix). Then you can adress elements with curly brackets ({}). E.g.:
A1{1,1}= 1;
A1(1,1)={1};
both work. You can also define many cells in one line. E.g:
A1(2,:) = {2,3,6};
For the cases with multiarray entries use another cell structure:
B= {[78,9], [10,65]};
A1(1,2) = {B};
and so on. Pay attention to use curly brackets around B (or the coordinate in A1)! Otherwise he would try to merge the cells from B in A1 but that won't do any good because B is a 1x2 cell and you want to give it as argument to one cell in A1.
If you want to return the value inside a cell you have to use curly brackets again:
A1{1,1}
would return 1.
It depends on what you want to do with such a structure. You could use cell arrays for each 1d array entry, and create a matrix of such cell arrays:
a = {1, 2};
b = {-1, 4, 6};
M = [a b];
Alternatively, you can define a sparse 3d array.
I have a cell array with strings and a numeric array in Matlab. I want every entry of the cell array to be concatenated with the number from the corresponding position of the array. I think that it can be easily solved using cellfun, but I failed to get it to work.
To clarify, here is an example:
c = {'121' '324' '456' '453' '321'};
array = 1:5
I would like to get:
c = {'1121' '2324' '3456' '4453' '5321'}
A special version of sprintf that outputs directly to a cell array is called sprintfc:
>> C = sprintfc('%d%d',[array(:) str2double(c(:))]).'
C =
'1121' '2324' '3456' '4453' '5321'
It is also a bit different in the way it handles array inputs, by preserving the shape.
You're correct, you can use cellfun – you just need to convert the array to a cell array as well using num2cell. Assuming array is a vector of integers:
c = {'121' '324' '456' '453' '321'};
array = 1:5;
c2 = cellfun(#(c,x)[int2str(x) c],c,num2cell(array),'UniformOutput',false)
which returns
c2 =
'1121' '2324' '3456' '4453' '5321'
In your case, you can also accomplish the same thing just using cell2mat and mat2cell:
c2 = mat2cell([int2str(array(:)) cell2mat(c.')],ones(1,length(array))).'
Another one-liner, this one without undocumented functions (with thanks to #chappjc for showing me the "format" input to num2str):
strcat(num2str(array(:),'%-d'),c(:)).'