Question

我是matlab的新手。我想实现KNN算法。我试图阅读fitcknn分类器，但我无法得到它。我有矩阵x，有4个输入向量（每个向量有3个特征）

     1     2     3
     5    19    20
     1     2     4
     8    19    21

我想得出一个输出矩阵Y，它给出了输入矩阵的每个向量的最近邻居（按顺序）。例如：在这种情况下，y将是

      3     2     4
      4     3     1
      1     2     4
      2     3     1

说明：矩阵Y的第一行显示向量1的最接近的向量是：向量3然后是向量2，然后是向量4.

是否有库进行此分类（使用余弦距离作为相似函数）？感谢。

Answer 1

n = size(x,1);
dist = squareform(pdist(x,'cosine')); %// distance matrix
dist(1:n+1:end) = inf; %// self-distance doesn't count
[~, y] = sort(dist,2);
y = y(:,1:n-1);

为了节省内存，您可以使用pdist2代替pdist来处理数据块：

n = size(x,1);
m = 100; %// chunk size. As large as memory allows. Divisor of n
y = NaN(n,n-1); %// pre-allocate results
for ii = 0:m:size(x,1)-1
    ind = ii+(1:m); %// current chunk: these rows
    dist_chunk = pdist2(x(ind,:),x,'cosine'); %// results for this chunk
    [~, y_chunk] = sort(dist_chunk,2);
    y(ind,:) = y_chunk(:,2:end); %// fill results, except self-distance
end

matlab K最近邻

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