How to increase the size of a matrix. I have a matrix of size 36 rows and 3000 columns. I want to convert it to an array of size 200 rows and 3600 columns without affecting the information contained in the original matrix.
In any programming language you will need to create the new array and do copy of the previous elements in the new array.
If you are using java i would prefer to use Arraylist.
You can always create 2D array using arrayList in java.Something like this
ArrayList<ArrayList<Type>>
But in C/C++ you will need to do entire copy operation.
There is a way to do this using zero padding.
Assuming matrix A has p rows and q columns and you want to convert it to a matrix B having m rows and n columns (m>p and n>q). You can use
B = padarray(A,[m-p n-q],0,'post');
B will contain the matrix A and the rest of the matrix will be all zeros.
Example:
a=randi(10,[2 2]);
a =
10 10
2 9
b = padarray(a,[4-2 3-2],0,'post');
b =
10 10 0
2 9 0
0 0 0
0 0 0
Related
What is the fastest way of calculating the maximum value, with it's corresponding index, of each 'slice' of a 3D array?
Say you have A with n slices (here I just made each slice 10 by 10, but this can be changed to any size):
A = rand(10,10,n);
You can reshape it to n-columns matrix, then take the maximum of each column:
[val,ind] = max(reshape(A,[],n),[],1);
The first output val will be an n-element vector with all the maximum values, and the second output ind will be their row index in the reshaped A.
Then you get the size of the slices:
sz = size(A);
and use it to find the row (r) and column (c) of each maximum element in each slice:
[r,c] = ind2sub(sz(1:2),ind)
So in this example (using rand and 10x10x6 array for A) you would get something like this at the end (but with different values):
val =
0.99861 0.98895 0.98681 0.99991 0.96057 0.99176
r =
9 7 3 8 2 9
c =
1 1 8 10 10 5
If you have a matrix A with n layers, you can apply max function in two steps to get a 1 x 1 x n matrix with max of each layer
A = rand(10,10,n);
layer_max = max(max(A,[],1),[],2); % 1 x 1 x n matrix, use squeeze to remove extra dims
layer_max = squeeze(layer_max);
I have a vector from 1 to 40 and want to shuffle it in such a way that each block of four integers (ten blocks in total) are shuffled only with themselves.
For example: 3 4 2 1 | 7 6 5 8 | 9 11 10 12 | ...
My original idea was to append ten permutation vectors to eachother and then add a 1 to 40 vector to the big permutation vector, but it didn't work at all as expected and was logically wrong.
Has anyone an idea how to solve this?
data = 10:10:120; % input: values to be permuted
group_size = 4; % input: group size
D = reshape(data, group_size, []); % step 1
[~, ind] = sort(rand(size(D)), 1); % step 2
result = D(bsxfun(#plus, ind, (0:size(D,2)-1)*group_size)); % step 3
result = result(:).'; % step 4
Example result:
result =
20 10 30 40 60 50 70 80 110 100 120 90
How it works
Reshape the data vector into a matrix D, such that each group is a column. This is done with reshape.
Generate a matrix, ind, where each column contains the indices of a permutation of the corresponding column of D. This is done generating independent, uniform random values (rand), sorting each column, and getting the indices of the sorting (second output of sort).
Apply ind as column indices into D. This requires converting to linear indices, which can be done with bsxfun (or with sub2ind, but that's usually slower).
Reshape back into a vector.
You can use A = A(randperm(length(A))) to shuffle an array.
Example in Octave:
for i = 1:4:40
v(i:i+3) = v(i:i+3)(randperm(4));
end
I have a vector like
A=[1,2,3,4,5,6,7,8,9,10];
I would like to insert 2 zero every 5 number.
The result would be A=[1,2,3,4,5,0,0,6,7,8,9,10,0,0].
I know I could preallocate the space and then use a for cycle to assign the variable, but I was wandering if there was some more elegant way.
This works even if A doesn't contain an integer number of blocks:
A = [1,2,3,4,5,6,7,8,9,10,11,12]; % input vector
m = 5; % block size
n = 2; % number of zeros to be added after each block
B = zeros(1, numel(A)+floor(numel(A)/m)*n); % preallocate to appropriate size
B(mod(0:end-1, m+n)<m) = A; % logical index. Fill values of A at desired positions of B
The result in this example is
B =
1 2 3 4 5 0 0 6 7 8 9 10 0 0 11 12
With A having number of elements a multiple of 5, you could use some reshaping and concatenation with zeros, like so -
reshape([reshape(A,5,[]) ; zeros(2,numel(A)/5)],1,[])
I want to merge multiple arrays of unique occurrences to a single array. To get the arrays in the first place I use this code, where image series is a slice from a tiff image imported using imread:
a = unique(img_series);
occu = [a,histc(img_series(:),a)];
I do that multiple times, because the tiff image I'm using has multiple hundred images stacked, which my RAM will not support to import at once. So each 'occu' looks something like this (first number is the unique value, second number is the number of occurrences):
occu1 occu2 .....
0 1 1 2
12 1 10 1
14 1 12 1
15 1 14 2
.. .. .. .. .....
Now I want to merge them all together, or better merge them in each iteration, when I'm reading another stacked image.
The merged results should be a 2D matrix similar to the one above. The number of occurrences of the same values should be added to one another, as this is the whole point of counting them. So the result of the above example should be this:
occu_total
0 1
1 2
10 1
12 2
14 3
15 1
.. ..
I found the join command, but that one does not seem to work here. I guess I could do it the long way of searching the matching number and add the occurrences together and so on, but there must be a quicker way of doing it.
A = [0 1;12 1; 14 1;15 1];B = [1 2;10 1;12 1;14 2];
tmp = [A;B]; %// merge arrays into a single one
tmp(:,1) = tmp(:,1)+1;%// remove zero occurrences by adding 1 to everything
C = accumarray(tmp(:,1),tmp(:,2)); %// add occurrences all up
D = [1:numel(C)].'; %// create numbered array
E = [D C];
E((C==0),:)=[]; %// get output
E(:,1) = E(:,1)-1;%// subtract the 1 again
E =
0 1
1 2
10 1
12 2
14 3
15 1
Job for accumarray. This takes the first argument as your dictionary key, and adds the values of the each key together. The addition and subtraction of 1 is done because 0 cannot be an index in MATLAB. To circumvent this (assuming you have no negative numbers), you can simply add 1 and remove that afterwards, shifting all your indices to positive integers. If you hit negative numbers, subtract tmp(:,1) = min(tmp(:,1)+1 and add E(:,1) = min(tmp(:,1)-1
In a matrix, how can we sum part by part of the elements? Consider the primary matrix in a way that can be divided into smaller m by n matrix. then i want to sum the whole elements of each m by n matrix together and put the number instead of the m by n matrix
for example consider the following matrix, i want to sum every four elements and create another matrix:
A = [1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16];
And after summing i want to have:
B = [14 22
46 54];
I this example i summed 4 elements as a matrix of 2 by 2 then for example the result of summing 1,2,5 and 6 seats in the first element of the new matrix.
Let
m = 2; %// number of rows per block
n = 2; %// number of columns per block
You can do the sum with blockproc (from the Image Processing Toolbox), which is very suited for this task:
B = blockproc(A, [m n], #(x) sum(x.data(:)));
Or, if you build the appropriate indices, you can use accumarray:
[ii jj] = ndgrid(1:size(A,1), 1:size(A,2));
B = accumarray([ceil(ii(:)/n) ceil(jj(:)/m)], A(:))
One approach -
B = squeeze(sum(reshape(sum(reshape(A,m,[])),size(A,1)/m,n,[]),2))
Another approach if you would like to avoid squeeze, which is sometimes slower -
B = reshape(sum(reshape(reshape(sum(reshape(A,m,[])),size(A,1)/m,[])',n,[])),[],size(A,1)/m)'