There are 3 matrices A,B,C:
A=[0 1;2 3]
B=[4 5;6 7]
C=[8 9;10 11]
How to create a new matrix D(2,2) so as its elements are arrays of a type
D = [{A(1,1), B(1,1), C(1,1)} {{A(1,2), B(1,2), C(1,12};
{A(2,1), B(2,1), C(2,1)} {A(2,2), B(2,2), C(2,2)}]
For example: Using an operator D(1,1) gives the result
0, 4, 8
The bracket {} are only illustrative and do not represent a matlab syntax...
You could stack the matrices along the third dimension:
D = cat(3,A,B,C);
Then you could access as:
>> D(1,1,:)
ans(:,:,1) =
0
ans(:,:,2) =
4
ans(:,:,3) =
8
if you want to get a 1D-vector:
>> squeeze(D(1,1,:)) %# or: permute(D(1,1,:),[1 3 2])
ans =
0
4
8
If you prefer to use cell arrays, here is an easier way to build it:
D = cellfun(#squeeze, num2cell(cat(3,A,B,C),3), 'UniformOutput',false);
which can be accessed as:
>> D{1,1}
ans =
0
4
8
You are almost there:
D = [{[A(1,1), B(1,1), C(1,1)]} {[A(1,2), B(1,2), C(1,2)]};
{[A(2,1), B(2,1), C(2,1)]} {[A(2,2), B(2,2), C(2,2)]}]
(you see the additional branches?)
D is now a cell array, with each cell containing a 1x3 matrix.
To access the cell array use this syntax:
D{1,1}
Related
I have vector c:
c = [2 5 3];
I want to generate vectors with their lengths equal to each value in c in a consecutive order. So, I should obtain 3 vectors:
c1 = [1 2];
c2 = [3 4 5 6 7];
c3 = [8 9 10];
Next, I want to align these vectors in a 1x3 cell array:
out = {c1 c2 c3};
This may seem straightforward, but I can't figure how to do it automatically. Any ideas?
You could use mat2cell to accomplish this. We first create an array from 1 to sum(c) and then use mat2cell to group the array into pieces where each piece is the size of each element of c.
out = mat2cell(1:sum(c), 1, c);
This reduces the need for intermediate variables and gives you your cell array directly.
out{1} =
1 2
out{2} =
3 4 5 6 7
out{3} =
8 9 10
I've always been told that almost all for loops can be omitted in MATLAB and that they in general slow down the process. So is there a way to do so here?:
I have a cell-array (tsCell). tsCell stores time-arrays with varying length. I want to find an intersecting time-array for all time-arrays (InterSection):
InterSection = tsCell{1}.time
for i = 2:length{tsCell};
InterSection = intersect(InterSection,tsCell{i}.time);
end
Here's another way. This also assumes there are no duplicates within each original vector.
tsCell_time = {[1 6 4 5] [4 7 1] [1 4 3] [4 3 1 7]}; %// example data (from Divakar)
t = [tsCell_time{:}]; %// concat into a single vector
u = unique(t); %// get unique elements
ind = sum(bsxfun(#eq, t(:), u), 1)==numel(tsCell_time); %// indices of unique elements
%// that appear maximum number of times
result = u(ind); %// output those elements
Here's a vectorized approach using unique and accumarray, assuming there are no duplicates within each cell of the input cell array -
[~,~,idx] = unique([tsCell_time{:}],'stable')
out = tsCell_time{1}(accumarray(idx,1) == length(tsCell_time))
Sample run -
>> tsCell_time = {[1 6 4 5],[4 7 1],[1 4 3],[4 3 1 7]};
>> InterSection = tsCell_time{1};
for i = 2:length(tsCell_time)
InterSection = intersect(InterSection,tsCell_time{i});
end
>> InterSection
InterSection =
1 4
>> [~,~,idx] = unique([tsCell_time{:}],'stable');
out = tsCell_time{1}(accumarray(idx,1) == length(tsCell_time));
>> out
out =
1 4
I want to assemble a matrix from 5 vectors using a for loop in Matlab.
How can I do that? Or is there a simple alternative to do the same?
Here's an example: I have 5 vectors:
A = [1 2 3]
B = [1 2 3]
C = [1 2 3]
D = [1 2 3]
E = [1 2 3]
I want to create a matrix Z of size 3 x 5 from these vectors.
for loops are generally a bad idea in Matlab. Use vectorized syntax:
Z = [A;B;C;D;E]'; %'// vertical concatenation (5x3), then transpose to get 3x5
>> size(Z)
ans =
3 5
You can also transpose the vectors first and then concatenate them:
Z = [A' B' C' D' E']; %'// this comment is a syntax highlighting hotfix
>> size(Z)
ans =
3 5
If you really insist on using a for loop, here's how you would do that:
Z(3,5) = 0; %// preallocate a 3x5 numerical matrix
for i=1:3
Z(i,1) = A(i);
Z(i,2) = B(i);
Z(i,3) = C(i);
Z(i,4) = D(i);
Z(i,5) = E(i);
end
But it really makes no sense at all to do it this way...
I have a 2-dimensional array of structs 'cell' which each contain a number of vectors:
cell(1,1).U = [1 2 3];
cell(1,2).U = [4 5 6];
cell(2,1).U = [7 8 9];
cell(2,2).U = [0 1 2];
I would like to extract the data into a 3-D array A(i,j,k).
Using cell-style extraction returns a 1x3 cell, the contents of which are
>> {cell.U}
ans = {[cell(1,1).U] [cell(1,2).U] [cell(2,1).U] [cell(2,2).U]}
And thus, converting this to a matrix using cell2mat(), as in:
cell2mat(ans)
Returns a 3x4 array.
Ideally, I would like a 2x2x3 array so that the indices i and j are preserved. Is there any way (short of looping) to accomplish this?
Use CAT and RESHAPE. Also, don't call your variable cell, since that's a built-in function.
>> c(1,1).U = [1 2 3];
c(1,2).U = [4 5 6];
c(2,1).U = [7 8 9];
c(2,2).U = [0 1 2];
>> out = cat(1,c.U)
>> out = reshape(out,2,2,3)
out(:,:,1) =
1 4
7 0
out(:,:,2) =
2 5
8 1
out(:,:,3) =
3 6
9 2
Let's say I have a one-dimensional array:
a = [1, 2, 3];
Is there a built-in Matlab function that takes an array and an integer n and replicates each
element of the array n times?
For example calling replicate(a, 3) should return [1,1,1,2,2,2,3,3,3].
Note that this is not at all the same as repmat. I can certainly implement replicate by doing repmat on each element and concatenating the result, but I am wondering if there is a built in function that is more efficient.
I'm a fan of the KRON function:
>> a = 1:3;
>> N = 3;
>> b = kron(a,ones(1,N))
b =
1 1 1 2 2 2 3 3 3
You can also look at this related question (which dealt with replicating elements of 2-D matrices) to see some of the other solutions involving matrix indexing. Here's one such solution (inspired by Edric's answer):
>> b = a(ceil((1:N*numel(a))/N))
b =
1 1 1 2 2 2 3 3 3
a = [1 2 3];
N = 3;
b = reshape(repmat(a,N,1), 1, [])
As of R2015a, there is a built-in and documented function to do this, repelem:
repelem Replicate elements of an array.
W = repelem(V,N), with vector V and scalar N, creates a vector W where each element of V is repeated N times.
The second argument can also be a vector of the same length as V to specify the number of replications for each element. For 2D replication:
B = repelem(A,N1,N2)
No need for kron or other tricks anymore!
UPDATE: For a performance comparison with other speedy methods, please see the Q&A Repeat copies of array elements: Run-length decoding in MATLAB.
>> n=3;
>> a(floor((0:size(a,2)*n-1)/n)+1)
ans =
1 1 1 2 2 2 3 3 3
Some exotic alternatives. Admittedly more funny than useful:
Assign the (first) result of meshgrid to a vector:
b = NaN(1,numel(a)*n); %// pre-shape result
b(:) = meshgrid(a,1:n);
Build a matrix that multiplied by a gives the result:
b = a * fliplr(sortrows(repmat(eye(numel(a)),n,1))).';
Use ind2sub to generate the indices:
[~, ind] = ind2sub([n 1],1:numel(a)*n);
b = a(ind);
If you have the image processing toolbox, there is another alternative:
N = 3;
imresize(a, [1 N*numel(a)],'nearest')
% To get b = [1 1 1 2 2 2 3 3 3]
N = 3;
a = [1 2 3];
temp_a = a(ones(N,1),:);
b = reshape(temp_a,1,numel(temp_a));
% To get b = [1 2 3 1 2 3 1 2 3]
N = 3;
a = [1 2 3];
temp_a = a(ones(N,1),:);
b = reshape(temp_a',1,numel(temp_a));