Question

我有一个单元格列表，每个元素包含不同数量的坐标来访问矢量。例如，

C ={ [1 2 3] , [4 5],  [6], [1 8 9 12 20]}

这只是一个例子，在实际情况中，C的大小为10 ^ 4到10 ^ 6，每个元素包含1到1000个元素的向量。我需要使用每个元素作为坐标来访问向量中的相应元素。我正在使用循环来查找由单元格元素

指定的向量元素的平均值

 for n=1:size(C,1)
   x = mean(X(C{n}));
   % put x to somewhere  
 end

这里X是10000个元素的大向量。使用循环是好的，但我想知道是否有任何方法可以做同样的事情，但没有使用循环？我问的原因是上面的代码需要运行这么多次，现在使用lopp很慢。

Answer 1

方法＃1

C_num = char(C{:})-0; %// 2D numeric array from C with cells of lesser elements 
             %// being filled with 32, which is the ascii equivalent of space

mask = C_num==32; %// get mask for the spaces
C_num(mask)=1; %// replace the numbers in those spaces with ones, so that we 
                %// can index into x witout throwing any out-of-extent error

X_array = X(C_num); %// 2D array obtained after indexing into X with C_num
X_array(mask) = nan; %// set the earlier invalid space indices with nans
x = nanmean(X_array,2); %// final output of mean values neglecting the nans

方法＃2

lens = cellfun('length',C); %// Lengths of each cell in C
maxlens = max(lens); %// max of those lengths

%// Create a mask array with no. of rows as maxlens and columns as no. of cells. 
%// In each column, we would put numbers from each cell starting from top until
%// the number of elements in that cell. The ones(true) in this mask would be the 
%// ones where those numbers are to be put and zeros(false) otherwise.
mask = bsxfun(@le,[1:maxlens]',lens) ; %//'

C_num = ones(maxlens,numel(lens)); %// An array where the numbers from C are to be put

C_num(mask) = [C{:}]; %// Put those numbers from C in C_num.
  %// NOTE: For performance you can also try out: double(sprintf('%s',C{:}))
X_array = X(C_num); %// Get the corresponding X elements
X_array(mask==0) = nan; %// Set the invalid locations to be NaNs
x = nanmean(X_array); %// Get the desired output of mean values for each cell

方法＃3

这与方法＃2几乎相同，但最后会有一些更改以避免nanmean。

因此，编辑方法＃2的最后两行到这些 -

X_array(mask1==0) = 0;
x = sum(X_array)./lens;

寻找更快的方法来处理细胞和载体操作

1 个答案: