I wanna ask a question about cell array. Suppose I have a cell array
C={[2 1], [3 5], [15 6]};
I'd like to get all first value of each cell, which are [2, 3 , 15]
however, when I type
C{:}(1);
or
C(:)(1);
There would be some mistakes.
So what is the simplist way to get those numbers.
If every cell is a vector and has the same amount of elements, one way is to create a matrix by stacking all of these cells together and extract out the first column. Use vertcat to help you do that:
CMat = vertcat(C{:});
out = CMat(:,1);
If every cell does not have the same amount of elements, one way is to use cellfun. Use an anonymous function to extract out the first element over each cell array:
out = cellfun(#(x) x(1), C);
The benefit of the above approach is that it doesn't matter if each cell is a vector or matrix. It'll extract the first element for a vector or the top-left corner for a matrix.
Related
I have a 1x24 cell array called chaining, whose each cell contains a 119x119 matrix:
I want to find the sum of each corresponding elements of all the cells, and store them in a matrix called result. That is, the (j,k)th element of result should contains the sum of the (j,k)th elements of all the matrices in the cell array chaining.
The code I wrote to do this is:
for j=1:size(chaining,2)
for k=1:size(chaining,2)
result(j,k) = sum(chaining{1,:}(j,k));
end
end
But this gives error because apparently MATLAB can't aggregate cell arrays for some reason (i.e., the chaining{1,:} part).
Could anyone please show me how to go about doing this?
how about
result = sum( cat(3, chaining{:}), 3 );
What just happened here?
First, we convert the cell array into a 3D array by "stacking" the 2D cell elements on the third dimension:
cat(3, chaining{:})
Once we have a 3D array of size 119-by-119-by-24 we can sum along the third dimension and get result of size 119-by-119:
sum( ..., 3 );
For more info see cat and sum help pages.
BTW,
If you insist on chaining{1,:}(jj,kk) type of solution (not recommended), you might find subsref command useful.
I'm writing a MATLAB function to read out data into an n-dimensional array (variable dimension size). I need to be able to access a specific point in the Matrix (to write to it or read it, for example), but I don't know ahead of time how many indexes to specify.
Currently I have a current_point vector which I iterate through to specify each index, and a max_points vector which specifies the size of the array. So, if for example I wanted a 3-dimensional array of size 1000-by-15-by-3, max_points = [1000 15 3], and current_point iterates from [1, 1, 1] to [1000, 15, 3] ([1, 1, 1] -> [1000, 1, 1] -> [1, 2, 1] -> [1000, 2, 1] ->...). What I'd like to be able to do is feed current_point as an index to the matrix like so:
output_matrix(current_point) = val
But apparently something like output_matrix([1 2 3]) = val will just set outputmatrix(1:3) = 30. I can't just use dummy variables because sometimes the matrix will need 3 indexes, other times 4, other times 2, etc, so a vector of variable length is really what I need here. Is there a simple way to use a vector as the points in an index?
Using the function sub2ind to create a linear index is the typical solution to this problem, as shown in this closely-related question. You could also compute a linear index yourself instead of calling sub2ind.
However, your case may be simpler than those in the other questions I linked to. If you're only ever indexing a single point with your current_point vector (i.e. it's just an n-element vector of subscripts into your n-dimensional matrix), then you can use a simple solution where you convert current_point to a cell array of subscripts using the function num2cell and use it to create a comma-separated list of indices. For example:
current_point = [1 2 3 ...]; % A 1-by-n array of subscripts
subCell = num2cell(current_point); % A 1-by-n cell array of subscripts
output_matrix(subCell{:}) = val; % Update the matrix point
The operation subCell{:} creates the equivalent of typing subCell{1}, subCell{2}, ..., which is the equivalent of typing current_point(1), current_point(2), ....
I know it is too late but for anybody who will find this topic. the easiest way that work for me is to use: diag(A (x(:),y(:)) );
unfortunately this works only if you need to get values from the matrix, not for changing values
You can use the sub2ind function to get the linear index from the subscript.
Example:
A=magic(4)
A =
16 2 3 13
5 11 10 8
9 7 6 12
4 14 15 1
selectElement={2,3}; %# get the element at position 2,3 in A.
indx=sub2ind(size(A),selectElement{:});
A(indx)
ans =
10
In the above example, I've stored the subscripts (can be any number of dimensions) as a cell. If you have it stored as a vector, simply use num2cell() to convert it to a cell.
You can now easily assign a value to this as A(indx)=value;. I've used different variables than yours to keep the answer general, but the idea is the same and you just need to replace the variable names.
You also mentioned in your post that you're looping from (1,1,1) till some value, (1000,15,3) and assigning a value to each of these points. If you're looping along the columns, you can replace this entire operation with a vectorized solution.
Let finalElement={1000,15,3} be the final step of the loop. As before, find the linear index as
index=sub2ind(size(A),finalElement{:});
Now if you have the values you assign in the loop stored as a single vector, values, you can simply assign it in a single step as
A(1:index)=values;
I have a vector, let say as=[1 3 4] and I have 30 by 30 cell array. I want to check if the elements of vector as intersect with the elements of each cell or not? If so, I want to find the indices of the cell.
Assuming cellarr to be input cell array, see if this approach works for you -
%// Find intersecting elements for each cell
int_idx = cellfun(#(x) intersect(x,as),cellarr,'UniformOutput', false)
%// Find non empty cells that denote intersecting cells.
%// Then, find their row and column indices
[row_ind,col_ind] = find(~cellfun('isempty',int_idx))
Another approach with ismember to find matches among each cell and if there is any match within a cell, find the indices of it -
[row_ind,col_ind] =find(cell2mat(cellfun(#(x) any(ismember(x,as)),cellarr,'un', 0)))
And another -
%// Vertically concatenate all numeric array from cells
vertcat_cells = vertcat(cellarr{:})
%// Get all good matches
matches = any(any(bsxfun(#eq,vertcat_cells,permute(as,[1 3 2])),2),3)
%// Reshape matches into the size of cellarr and get indices of matches
[row_ind,col_ind] = find(reshape(matches,size(cellarr)))
Hi I have a cell array which is called vector, with dimensions 69083x2 , now i want to reshape this cell array to 3212762x2, but reshape(vector,3212762,2) does not work. I get this error:
To RESHAPE the number of
elements must not change.
Can anyone tell me how I can do this ?
Do you mean you wish to make the cell array larger? reshape is to store the same elements in a different 'shape', for eg., a 3x2 cell array as a 6x1 cell array - note that the total number of elements remains 6 in both cases.
If you wish to enlarge the cell array, just assign something to the last element of the enlarged cell array like so:
vector(3212762, 2) = {[]}
Now vector would be of size 3212762x2.
Just like sundar mentioned
vector(3212762, 2) = 0
will give you 3212762x2 matrix with the new rows assigned to 0.
I'm writing a MATLAB function to read out data into an n-dimensional array (variable dimension size). I need to be able to access a specific point in the Matrix (to write to it or read it, for example), but I don't know ahead of time how many indexes to specify.
Currently I have a current_point vector which I iterate through to specify each index, and a max_points vector which specifies the size of the array. So, if for example I wanted a 3-dimensional array of size 1000-by-15-by-3, max_points = [1000 15 3], and current_point iterates from [1, 1, 1] to [1000, 15, 3] ([1, 1, 1] -> [1000, 1, 1] -> [1, 2, 1] -> [1000, 2, 1] ->...). What I'd like to be able to do is feed current_point as an index to the matrix like so:
output_matrix(current_point) = val
But apparently something like output_matrix([1 2 3]) = val will just set outputmatrix(1:3) = 30. I can't just use dummy variables because sometimes the matrix will need 3 indexes, other times 4, other times 2, etc, so a vector of variable length is really what I need here. Is there a simple way to use a vector as the points in an index?
Using the function sub2ind to create a linear index is the typical solution to this problem, as shown in this closely-related question. You could also compute a linear index yourself instead of calling sub2ind.
However, your case may be simpler than those in the other questions I linked to. If you're only ever indexing a single point with your current_point vector (i.e. it's just an n-element vector of subscripts into your n-dimensional matrix), then you can use a simple solution where you convert current_point to a cell array of subscripts using the function num2cell and use it to create a comma-separated list of indices. For example:
current_point = [1 2 3 ...]; % A 1-by-n array of subscripts
subCell = num2cell(current_point); % A 1-by-n cell array of subscripts
output_matrix(subCell{:}) = val; % Update the matrix point
The operation subCell{:} creates the equivalent of typing subCell{1}, subCell{2}, ..., which is the equivalent of typing current_point(1), current_point(2), ....
I know it is too late but for anybody who will find this topic. the easiest way that work for me is to use: diag(A (x(:),y(:)) );
unfortunately this works only if you need to get values from the matrix, not for changing values
You can use the sub2ind function to get the linear index from the subscript.
Example:
A=magic(4)
A =
16 2 3 13
5 11 10 8
9 7 6 12
4 14 15 1
selectElement={2,3}; %# get the element at position 2,3 in A.
indx=sub2ind(size(A),selectElement{:});
A(indx)
ans =
10
In the above example, I've stored the subscripts (can be any number of dimensions) as a cell. If you have it stored as a vector, simply use num2cell() to convert it to a cell.
You can now easily assign a value to this as A(indx)=value;. I've used different variables than yours to keep the answer general, but the idea is the same and you just need to replace the variable names.
You also mentioned in your post that you're looping from (1,1,1) till some value, (1000,15,3) and assigning a value to each of these points. If you're looping along the columns, you can replace this entire operation with a vectorized solution.
Let finalElement={1000,15,3} be the final step of the loop. As before, find the linear index as
index=sub2ind(size(A),finalElement{:});
Now if you have the values you assign in the loop stored as a single vector, values, you can simply assign it in a single step as
A(1:index)=values;