This is about matlab.
Let's say I have a matrix like this
A = [1,2,3,4,5;6,7,8,9,10;11,12,13,14,15]
Now I want to know how to get a mean value of a small matrix in A.
Like a mean of the matrix located upper left side [1,2;6,7]
The only way I could think of is cut out the part I want to get a value from like this
X = A(1:2,:);
XY = X(:,1:2);
and mean the values column wise Mcol = mean(XY);.
and finally get a mean of the part by meaning Mcol row-wise.
Mrow = mean(Mcol,2);
I don't think this is a smart way to do this so it would be great if someone helps me make it smarter and faster.
Your procedure is correct. Some small improvements are:
Get XY using indexing in a single step: XY = A(1:2, 1:2)
Replace the two calls to mean by a single one on the linearized submatrix: mean(XY(:)).
Avoid creating XY. In this case you can linearize using reshape as follows: mean(reshape(A(1:2, 1:2), 1, [])).
If you want to do this for all overlapping submatrices, im2col from the Image Processing Toolbox may be handy:
submatrix_size = [2 2];
A_sub = im2col(A, submatrix_size);
gives
A_sub =
1 6 2 7 3 8 4 9
6 11 7 12 8 13 9 14
2 7 3 8 4 9 5 10
7 12 8 13 9 14 10 15
that is, each column is one of the submatrices linearized. So now you only need mean(A_sub, 1) to get the means of all submatrices.
Related
I have an array of mxnx4 dimensions and a function with a 4-element vector input. I want to apply my function to give me an mxnx4 output, such that the function is applied mxn times (i.e. the 3rd dimension of my array becomes in the input of my function). Any ideas on how do do this would be massively appreciated. I have looked at arrayfun, but this seems not to address what I want to do. I also want to maintain the formatting of my function as it requires this formatting elsewhere in my code.
I.e.:
F=#(V)[V(1)+V(2);V(2)+V(3); V(3)+V(4); V(4)+V(1)]
to be applied to array M of dimensions 2x3x4 (m=2; n=3)
M = zeros(2,3,4);
M(:,:,1) = [1 2 3;4 5 6];
M(:,:,2) = [7 8 9;10 11 12];
M(:,:,3) = [13 14 15; 16 17 18];
M(:,:,4) = [19 20 21; 22 23 24];
to generate an array of dimensions 2x3x4
C(1,1,:) = F([1 7 13 19])
C(1,2,:) = F([2 8 14 20])
etc.
I can see a for loop would work, applying the following across the mxn matrix
C(m,n,:)=F(M(m,n,:))
However, I need to run this millions of times so would like a faster approach
This question already has an answer here:
Matlab - Transpose a 3D matrix only in the third dimension
(1 answer)
Closed 5 years ago.
I am trying to figure out how to import large array of data into 3D matrix to a specific order. I have already asked two question but i have not get reliable answer yet and get down voted too. Since then i have done some work and was able to import data to 3D matrix using reshape function. Instead of shooting actual problem, this is a simulation of actual problem.
k=1:27 % create a array of 27 data
r=reshape(k,[3,3,3]) % convert the array into 3 x 3 x 3 matrix,
The results of the first page and second of the matrix is, the data is placed along the columns, but i wanted to place them along rows, The transpose function does not work with ND arrays, I tried to use permute but i did not get the desired result, One solution will be perform transpose to each page, but that will break the 3D matrix in to many 2D matrices.
r(:,:,1) =
1 4 7
2 5 8
3 6 9
r(:,:,2) =
10 13 16
11 14 17
12 15 18
the expected outcome should be,
r(:,:,1) =
1 2 3
4 5 6
7 8 9
Link to the actual problem is,
Thanks
Is this what you want?
result = permute(r, [2 1 3]);
This permutes the first two dimensions. For your example r,
>> k = 1:27;
>> r = reshape(k, [3,3,3]);
>> result = permute(r, [2 1 3]);
>> result
result(:,:,1) =
1 2 3
4 5 6
7 8 9
result(:,:,2) =
10 11 12
13 14 15
16 17 18
result(:,:,3) =
19 20 21
22 23 24
25 26 27
I am trying to plot a figure using three matrices but somehow I couldn't understand. I have three matrices and an array. Suppose,
A =
1 2 3
4 5 4
7 8 9
B =
2 3 13
5 11 10
9 7 6
C =
1 2 3
2 3 13
5 11 10
and an array
Y= [0.001 0.0002 0.0004].
Now I want to plot it in such a way that array values should be on y axis while against 0.001, 0.002 and 0.0004 the matrices value should be arranged.
for examples, the y=0.001, A(1,1)=1, y=0.0002, B(1,1)=2 y=0.0004, C(1,1)=1 for a single line.
and similarly process goes for A(i,j),B(i,j) and c(i,j) points using loop to plot all lines on a single figure.
Thanks
So, the first plot is plot([1,2,1],Y), the next one is plot([2,3,2],Y) and so on?
If so, you could do it like that
X = cat(3,cat(3,A,B),C);
X = reshape(permute(X,[3,1,2]),3,9);
plot(X,Y,'--x');
which gives a plot like this:
Is this what you were looking for? If not, I didn't understand your question well and I'd like to ask you to rephrase it.
You can create a 3D array (tensor) and access it in a loop.
T(:,:,1) = A;
T(:,:,2) = B;
T(:,:,3) = C;
figure;
for idi = 1:size(A,1)
for idj = 1:size(A,2)
plot(squeeze(T(idi,idj,:)).',Y); hold on;
end
end
Accessing the third dimension is not fastest operation (as they are not store sequential in memory) and if the matrices are larger you might consider reshape.
I did not understand you wanted the vector Y to be on the x-axis or y-axis (and neither of those plots make sense to me) but I am sure you can modify the code from here for your needs.
Given A = [3 4 5 6 7 8 9 10 11 1 2 3 4 5 6 8]
Output B = [3 4 5 6 8]
Is there a Matlab function or command to get this result? I am new to Matlab. Just now I am doing it going through for each element and keeping a counter for it. I have very big array so this is taking too much time.
Use a combination of unique and histc:
uA = unique(A); %// find unique values
B = uA(histc(A, uA)>=2); %// select those that appear at least twice
The above code gives the values that appear at least twice. If you want values that appear exactly twice, replace >= by ==.
I want to group my elements using the repeated segments in the array. The breaking is basically depend on where the repeated segments are, in my real data contains ~10000 elements and I want to know if there is a easier way to do that.
Here is a short example to clarify what I want:
Let's say I have an array,
A=[1 5 3 4 4 4 6 9 8 8 9 5 2];
What I want is to break A into [1 5 3],[6 9], and [9 5 2];
What is the easiest to code this using matlab??
Thanks.
For a vectorized solution, you can find out the places where either forward or backward differences to the neighbor are zero, and then use bwlabel (from the Image Processing Toolbox) and accumarray to gather the data.
A=[1 5 3 4 4 4 6 9 8 8 9 5 2];
d = diff(A)==0;
%# combine forward and backward difference
%# and invert to identify non-repeating elments
goodIdx = ~([d,false]|[false,d]);
%# create list of group labels using bwlabel
groupIdx = bwlabel(goodIdx);
%# distribute the data into cell arrays
%# note that the first to inputs should be n-by-1
B = accumarray(groupIdx(goodIdx)',A(goodIdx)',[],#(x){x})
EDIT
Replace the last two lines of code with the following if you want the repeating elements to appear in the cell array as well
groupIdx = cumsum([1,abs(diff(goodIdx))]);
B = accumarray(groupIdx',A',[],#(x){x})
EDIT2
If you want to be able to split consecutive groups of identical numbers as well, you need to calculate groupIdx as follows:
groupIdx = cumsum([1,abs(diff(goodIdx))|~d.*~goodIdx(2:end)])
Here is a solution that works if I understand the question correctly. It can probably be optimised further.
A=[1 5 3 4 4 4 6 9 8 8 9 5 2];
% //First get logical array of non consecutive numbers
x = [1 (diff(A)~=0)];
for nn=1:numel(A)
if ~x(nn)
if x(nn-1)
x(nn-1)=0;
end
end
end
% //Make a cell array using the logical array
y = 1+[0 cumsum(diff(find(x))~=1)];
x(x~=0) = y;
for kk = unique(y)
B{kk} = A(x==kk);
end
B{:}