清空筛选条件
清空筛选条件

How to vectorize a loop over rows ?

2 次查看（过去 30 天）
Karan
Karan 2014-7-19
评论： Karan 2014-7-20
Hello !
I've got the following code for Kaggle's digit recognizer using KNN, somehow, I unable to replace the following for loop into a vectorized implementation.
The loop is used to loop an entire row from the test data matrix.
knn_mat = zeros(m_test,1);
for i = 1:m_test
fprintf('i is %d \n',i);
compare_mat = repmat(x_test(i,:),m_train,1);
distance_mat = sum(power((compare_mat - x_train),2),2);
[a,b] = min(distance_mat);
knn_mat(i) = y(b);
end
Thank You !
  10 个评论
显示 8更早的评论
Matz Johansson Bergström
Matz Johansson Bergström 2014-7-19
I think that another type of approach or calling compiled code would be of better help. I'm thinking like storing x_train in a spatial data structure to be able to quickly find the closest neighbour of x_test in x_train.
Karan
Karan 2014-7-19
Thank you ! How do we do that ?

请先登录，再进行评论。

请先登录，再回答此问题。

采纳的回答

Jan
Jan 2014-7-19
编辑：Jan 2014-7-19
This is about 25% faster for your small test data file:
knn_mat = zeros(m_test,1);
for i = 1:m_test
distance_mat = sum(bsxfun(@minus, x_test(i,:), x_train).^2, 2);
[a,b] = min(distance_mat);
knn_mat(i) = y(b);
end
The creation of the large intermediate array x_test-x_train might be the bottleneck here for larger arrays. A complete vectorization would increase the problem, most of all, if the data size is larger.
Then a C-Mex would be much faster. Are you familiar with writing C-Mex functions?
  1 个评论
Karan
Karan 2014-7-20
编辑：Karan 2014-7-20
No, this is my first program in matlab. I was using octave but its performance was poor. I'm not familiar with c-mex functions. But even if we did vectorize it, what would the code be ?

请先登录，再进行评论。

更多回答（1 个）

Matz Johansson Bergström
Jan: Nice. That was actually my first thought, but I only used it on x_test (regged and downloaded from Kaggle before Karan uploaded them) and then I only get 5-8% speedup, unfortunately.
Karan: So, no, as I mentioned earlier, compiled code is the way to go here, there is no simple way you can vectorize this code inside of Matlab.
If you wish to call compiled code from Matlab you can, as Jan states, use C-mex. I would write the code directly in C AND/OR process the data a little maybe? For more information about C-mex, see link to documentation.

类别

在 Help Center 和 File Exchange 中查找有关 Matrix Indexing 的更多信息

标签

Community Treasure Hunt

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

Start Hunting!

Translated by