call function for each combination of rows in A and columns in B

1 view (last 30 days)
Xtrain = [1 2; 3 4; 5 6];
Xtest = [7 8 ; 9 10];
kernel = @(x1,x2) norm(x1 - x2);
m = size(Xtrain, 1);
n = size(Xtest,1);
kernels = kernel(Xtest, Xtrain);
I want "kernels" to be the following matrix:
kernels = [ kernel( [7 8] ,[ 1 2] ) kernel([7 8] ,[3 4] ) kernel([7 8] ,[5 6] )
kernel([9 10] , [1 2] ) kernel([9 10] ,[3 4]) kernel( [9 10] ,[5 6] )]
How can I call the kernel function properly so that I will get back this matrix?
Thank you.

Accepted Answer

Ameer Hamza
Ameer Hamza on 30 Dec 2020
Try this
Xtrain = [1 2; 3 4; 5 6];
Xtest = [7 8 ; 9 10];
kernel = @(x1,x2) norm(x1 - x2);
m = size(Xtrain, 1);
n = size(Xtest, 1);
XtrainC = mat2cell(Xtrain, ones(m,1), 2);
XtestC = mat2cell(Xtest, ones(n,1), 2);
[R, C] = ndgrid(1:n, 1:m);
kernels = arrayfun(@(r, c) kernel(XtestC{r}, XtrainC{c}), R, C)

More Answers (1)

Bruno Luong
Bruno Luong on 30 Dec 2020
Edited: Bruno Luong on 30 Dec 2020
Simple for-loop is 3-4 times faster than fancy MATLAB pseudo vectorization, and much more readable
Xtrain = rand(1000,2);
Xtest = rand(1000,2);
kernel = @(x1,x2) norm(x1 - x2);
m = size(Xtest,1);
n = size(Xtrain,1);
tic
kernels = zeros(m,n);
for i=1:m
for j=1:n
kernels(i,j) = kernel(Xtest(i,:),Xtrain(j,:));
end
end
toc % Elapsed time is 1.389989 seconds.
tic
XtrainC = mat2cell(Xtrain, ones(m,1), 2);
XtestC = mat2cell(Xtest, ones(n,1), 2);
[R, C] = ndgrid(1:n, 1:m);
kernels = arrayfun(@(r, c) kernel(XtestC{r}, XtrainC{c}), R, C);
toc % Elapsed time is 5.821724 seconds.

Categories

Find more on Matrices and Arrays in Help Center and File Exchange

Tags

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!