Matlab：if條件的嵌套循環

Question

我正在嘗試通過迭代大小為（50000，21）的主矩陣中最后一列的行來構建子矩陣，並且如果內部的值大於1000，則整個對應的y（i，:)行將被添加到新的子矩陣，並從主矩陣中添加33個連續的行。

我在下面編寫了此代碼，但它僅返回33x21的大小，我想我正在遍歷j索引，但沒有遍歷i索引：

for i=1:50000
  if y(i,21)>1000
  for j=1:33
    n(j,:)=y(j+i,:)
    j=j+1
  end
  i=i+1
end
end

我究竟做錯了什么？

Answer 1

嘗試這個：

k=1;
for i=1:50000
    if y(i,21)>1000
        for j=0:33 %watch this out i am not sure how many rows you want to keep and if these involve the one with the last element greater than 1000 or not..%
            n(k,:)=y(j+i,:);
            j=j+1;
            k=k+1;
        end
    i=i+1;
    end
end

問題是每33條新記錄都將覆蓋前一行。我認為上述代碼可以解決問題。

Answer 2

這是不需要循環的矢量化版本：

%find elements that are greater than 1000
lo = find(y(:,21) > 1000);
%indeices of 33th row after the lo,
%since it may take a value that is greater than the size of matrix we limit this value by min
up = min(size(y,1), lo + 33);
% create indices based on ranges
%see http://stackoverflow.com/a/39434045/6579744
index=cumsum(accumarray(cumsum([1;up(:)-lo(:)+1]),[lo(:);0]-[0;up(:)]-1)+1);
index= index(1:end-1);
n = y(index, :)