This question already has answers here:
A similar function to R's rep in Matlab [duplicate]
(4 answers)
Closed 7 years ago.
A = [1 4 5 2 1 2]
How could I concisely replicate each element n times whilst maintaining the overall order e.g. if n = 3, the desired result would be:
[1 1 1 4 4 4 5 5 5 2 2 2 1 1 1 2 2 2]
For Matlab R2015a or higher use repelem
n = 3;
u = repelem(A,n)
For older versions use bsxfun
n = 3;
u = bsxfun(#mtimes ,A(:).',ones(n,1))
u = u(:)
You could do the following:
reshape(repmat(a',[3 1]),[],1)
Related
This question already has answers here:
Octave / Matlab: Extend a vector making it repeat itself?
(3 answers)
Closed 6 years ago.
I'm trying to take:
a = [1 2 3]
and repeat it 5 times to get:
b = [1 2 3 1 2 3 1 2 3 1 2 3 1 2 3]
but when I try:
b = repmat(a, 5, 1)
instead I get:
b =
1 2 3
1 2 3
1 2 3
1 2 3
1 2 3
I could probably do it with a for loop but I'd like to do it correctly if possible. Any suggestions? Thanks in advance
Use the following code:
b = repmat(a,1,5)
The numbers '1' and '5' refer to the amount of rows and columns that you want to repeat the matrix a. The order is important.
This question already has answers here:
Construct a matrix from a set of coordinates
(5 answers)
Closed 6 years ago.
For example, I have a matrix like A=[1,2,3,4;5,6,7,8;9,10,11,12]. And two array x=[1,1,3,2] and y = [2,4,3,1], which represent X- and Y-coordinate.
And I want to get 4 elements in the matrix [A(1,2);A(1,4);A(3,3);A(2,1)]. I use this code: result = diag(A(x,y)); Although I get what I want, but if I deal with a large matrix, such code runs too slow for me. Dose someone have a better way?
thanks!
Probably not the faster ones, but following are some approaches to this:
A = [1 2 3 4;
5 6 7 8;
9 10 11 12];
x = [1 1 3 2];
y = [2 4 3 1];
%Approach-1 (Yours approach)
diagonal = diag(A(x,y))
%Approach-2
A1=A(x,y); LowUp=A1(tril(triu(A1))~=0)
%Approach-3
EYE= A1((eye(4,4).*A1)~=0)
%Approach-4
findeye=A1(find(eye(size(A1))))
%Approach-5
subind=A(sub2ind(size(A),x,y)).'
%Approach-6
for i=1:4
loop(i)=A(x(i),y(i));
end
loop=loop.'
You need sub2ind
A = [1,2,3,4;
5,6,7,8;
9,10,11,12];
x = [1,1,3,2];
y = [2,4,3,1];
id = sub2ind(size(A),x,y)
id =
4 10 9 2
A(id)
ans =
2 4 11 5
This question already has answers here:
Octave / Matlab: Extend a vector making it repeat itself?
(3 answers)
Closed 8 years ago.
If I have a small array like a=[1 2 3 4 5], and want to build a large array from it with repeating it, like b=[1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 ....1 2 3 4 5], how can I do that in simplest way and lowest calculations?
repmat is what you are looking for
n = 5
b = repmat(a,1,n)
This question already has an answer here:
MATLAB: Duplicate each element of a vector? [closed]
(1 answer)
Closed 8 years ago.
It's hard to explain so I will show an example of what I would like to do:
x = [1 2 3 4 5]
I would like the outcome to be:
x = [1 1 2 2 3 3 4 4 5 5]
Preferably without the use of a for loop, but either method would be appreciative.
Thanks.
You can also use the Kronecker tensor product (kron function) which is pretty neat:
x = kron(x,ones(1,2))
x =
1 1 2 2 3 3 4 4 5 5
If you want it sorted as you have here, you could do:
y = sort([x x]);
alternatively if the order matters:
y = reshape([x;x],[1,2*length(x)])
This question already has answers here:
Element-wise array replication in Matlab
(7 answers)
A similar function to R's rep in Matlab [duplicate]
(4 answers)
Closed 8 years ago.
Let's say, I have:
A=[1 2; 3 4];
I want to use repmat that return:
B = [1 1 2 2; 1 1 2 2; 3 3 4 4; 3 3 4 4]
Kindly need your help. Thank you
I do not know a method using repmat but here is a method using kron
kron([1 2 ; 3 4],[1 1;1 1])
ans =
1 1 2 2
1 1 2 2
3 3 4 4
3 3 4 4
An alternative which uses repmat is
A=[1 2; 3 4];
cell2mat(arrayfun(#(x)repmat(x,2,2),A,'UniformOutput',false))
ans =
1 1 2 2
1 1 2 2
3 3 4 4
3 3 4 4
arrayfun is used to evaluate each element in A using the anonymous function #(x)repmat(x,2,2) which replicates that single element into a 2x2 matrix.
The result of arrayfun is a 2x2 cell array where each element is a 2x2 matrix. We then convert this cell array into a matrix via cell2mat.
Let the data be defined as
A = [1 2; 3 4];
R = 2; %// number of repetitions of each row
C = 2; %// number of repetitions of each column. May be different from R
Two possible approaches are as follows:
The simplest method is to use indexing:
B = A(ceil(1/R:1/R:size(A,1)), ceil(1/C:1/C:size(A,2)));
If you really want to do it with repmat, you need to play with dimensions using permute and reshape: move original dimensions 1, 2 to dimensions 2, 4 (permute); do the repetition along new dimensions 1, 3 (repmat); collapse dimensions 1, 2 into one dimension and 3, 4 into another dimension (reshape):
[r c] = size(A);
B = reshape(repmat(permute(A, [3 1 4 2]), [R 1 C 1]), [r*R c*C]);
Example result for R=2, C=3 (obtained with any of the two approaches):
B =
1 1 1 2 2 2
1 1 1 2 2 2
3 3 3 4 4 4
3 3 3 4 4 4