How to enlarge matrix by repeating matrix rows? [duplicate] - matlab

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Matlab: repeat every column sequentially n times [duplicate]
(3 answers)
Closed 4 years ago.
Initial matrix is A = [ [1 2 3; 4 5 6; 7 8 9]. Every row is to be replicated 3 times such that the output matrix is
B = [1 2 3;1 2 3;1 2 3;4 5 6; 4 5 6; 4 5 6; 7 8 9; 7 8 9; 7 8 9]
B = replicate(permute(A,[3 2 1]),3,1)


you mean like that?
kron(A,ones(3,1))
ans =
1 2 3
1 2 3
1 2 3
4 5 6
4 5 6
4 5 6
7 8 9
7 8 9
7 8 9


Since R2015a, there is a dedicated function for this: repelem.
A = [1 2 3; 4 5 6; 7 8 9]
B = repelem(A,3,1)
B =
1 2 3
1 2 3
1 2 3
4 5 6
4 5 6
4 5 6
7 8 9
7 8 9
7 8 9


Or just indexing:
A = [1 2 3; 4 5 6; 7 8 9]; % original matrix
m = 3; % row repetition factor
n = 1; % column repetition factor
B = A(ceil(1/m:1/m:size(A,1)), ceil(1/n:1/n:size(A,2)));

Related

Determining how many elements in a matrix are equal or bigger compared to another matrix element-wise

Let's say:
a = 1 2 3
4 5 6
7 8 9
b = 3 2 1
6 5 4
9 8 7
So in MATLAB: a = [1 2 3; 4 5 6; 7 8 9]; b = [3 2 1; 6 5 4; 9 8 7];. I want to know how many elements in a are equal or bigger than the element in the same place in b. So in this example, the result will be 6.

Let
a = [1 2 3
4 5 6
7 8 9];
b = [3 2 1
6 5 4
9 8 7];
then the expression
c = a>=b;
gives you the positions of the elements where a is larger than b.
sum(c(:));
Gives you the number of such elements.

Compare two matrices row by row and store indices of non-equal rows in Matlab

What is Matlab-way two compare two matrices row by row and store indices of non-equal rows (without for loops)?

I would do it in this way:
A = [1 2 3 4 5 6; 4 5 6 7 8 9; 1 3 5 7 9 1; 4 5 6 7 8 9; 1 3 5 7 9 1];
B = [0 2 3 4 5 6; 4 5 6 7 8 9; 1 5 5 7 9 1; 4 5 6 7 8 9; 1 3 5 7 9 1];
ind = find(sum(A - B, 2) ~= 0); %returns [1; 3]

How do I add mirrored padding around a matrix?

I have a matrix and want to add padding around it but the padded values have to be mirrored.
I have tried using A = padarray(B,[1 1],'symmetric','both');
but it mirrors the edge values of matrix B.
Meaning if
B = [1 2 3;
4 5 6;
7 8 9];
the result will be
A = [1 1 2 3 3;
1 1 2 3 3;
4 4 5 6 6;
7 7 8 9 9;
7 7 8 9 9]
But I need A to look like this:
A = [5 4 5 6 5;
2 1 2 3 2;
5 4 5 6 5;
8 7 8 9 8;
5 4 5 6 5]
Is there some function like padarray I can use for that or do I have to do it manually?

You could use symmetric with [2 2] and remove the extra parts,
B = [1 2 3; 4 5 6; 7 8 9];
c = padarray(B,[2 2],'both','symmetric');
c(end-1,:) = [];
c(:,end-1) = [];
c(:,2) = [];
c(2,:) = [];
gives,
c =
5 4 5 6 5
2 1 2 3 2
5 4 5 6 5
8 7 8 9 8
5 4 5 6 5

repmat, the size of matrix or number of use

I have a matrix for which I extract each column and do repmat function for each of them to build another matrix. Since i I have to do this for a large number of vectors(each column of my first matrix) it takes so long(relative to which I expect). If I do this for the whole matrix and then do something to build them, does it takes less time?
Consider this as an example:
A=[1 4 7;2 5 8;3 6 9]
I want to produce these
A1=[1 2 3 1 2 3 1 2 3
1 2 3 1 2 3 1 2 3
1 2 3 1 2 3 1 2 3]
A2=[4 5 6 4 5 6 4 5 6
4 5 6 4 5 6 4 5 6
4 5 6 4 5 6 4 5 6]
A3=[7 8 9 7 8 9 7 8 9
7 8 9 7 8 9 7 8 9
7 8 9 7 8 9 7 8 9]

As an alternative to #thewaywewalk's answer and using kron and repmat:
clear
A=[1 4 7;2 5 8;3 6 9];
B = repmat(kron(A',ones(3,1)),1,3);
A1 = B(1:3,:)
A2 = B(4:6,:)
A3 = B(7:end,:)
Which results in the following:
A1 =
1 2 3 1 2 3 1 2 3
1 2 3 1 2 3 1 2 3
1 2 3 1 2 3 1 2 3
A2 =
4 5 6 4 5 6 4 5 6
4 5 6 4 5 6 4 5 6
4 5 6 4 5 6 4 5 6
A3 =
7 8 9 7 8 9 7 8 9
7 8 9 7 8 9 7 8 9
7 8 9 7 8 9 7 8 9
Or as #Divakar pointed out, it would be advisable to create a single 3D array and store all your data in it (general solution):
n = 3; %// # of times you want to repeat the arrays.
A=[1 4 7;2 5 8;3 6 9];
B = repmat(kron(A',ones(n,1)),1,n);
C = zeros(n,n*size(A,2),3);
C(:,:,1) = B(1:n,:);
C(:,:,2) = B(n+1:2*n,:);
C(:,:,3) = B(2*n+1:end,:);

Try if this fits your needs:
A = [1 4 7;2 5 8;3 6 9];
n = 3; %// size(A,1)
cellArrayOutput = arrayfun(#(x) repmat( A(:,x).',n,n ), 1:size(A,2), 'uni',0)
instead of different variable names, everything is stored in a cell array.
if you insist on different names, I'd recommend to use structs:
A = [1 4 7;2 5 8;3 6 9];
n = 3;
structOutput = struct;
for ii = 1:size(A,2)
structOutput.(['A' num2str(ii)]) = repmat( A(:,ii).', n, n );
end
which gives you:
>> structOutput.A1
ans =
1 2 3 1 2 3 1 2 3
1 2 3 1 2 3 1 2 3
1 2 3 1 2 3 1 2 3
and so on.
I don't expect to much performance plus, you should share your full code for further help.

How do I generate the following matrix and vector from the given input data in MATLAB?

Suppose I have the inputs data = [1 2 3 4 5 6 7 8 9 10]
and num = 4. I want to use these to generate the following:
i = [1 2 3 4 5 6; 2 3 4 5 6 7; 3 4 5 6 7 8; 4 5 6 7 8 9]
o = [5 6 7 8 9 10]
which is based on the following logic:
length of data = 10
num = 4
10 - 4 = 6
i = [first 6; second 6;... num times]
o = [last 6]
What is the best way to automate this in MATLAB?

Here's one option using the function HANKEL:
>> data = 1:10;
>> num = 4;
>> i = hankel(data(1:num),data(num:end-1))
i =
1 2 3 4 5 6
2 3 4 5 6 7
3 4 5 6 7 8
4 5 6 7 8 9
>> o = i(end,:)+1
o =
5 6 7 8 9 10