MATLAB矩陣預分配比動態矩陣擴展慢

Question

在循環的每次迭代中，我正在計算一個MATLAB矩陣。這些矩陣必須連接在一起以創建一個最終矩陣。我在進入循環之前知道這個最終矩陣的維數，所以我通過使用'零'函數預分配矩陣比初始化一個空數組要快，然後在我的循環的每次迭代中簡單地追加子陣列。奇怪的是，當我預先分配時，我的程序運行速度很慢。下面是代碼（只有第一個和最後一個行不同）：

這是慢：

w_cuda = zeros(w_rows, w_cols, f_cols); 

for j=0:num_groups-1 

    % gets # of rows & cols in W. The last group is a special 
    % case because it may have fewer than max_row_size rows 
    if (j == num_groups-1 && mod(w_rows, max_row_size) ~= 0) 
     num_rows_sub = w_rows - (max_row_size * j);  
    else 
     num_rows_sub = max_row_size; 
    end; 

    % calculate correct W and f matrices 
    start_index = (max_row_size * j) + 1; 
    end_index = start_index + num_rows_sub - 1; 

    w_sub = W(start_index:end_index,:); 
    f_sub = filterBank(start_index:end_index,:); 

    % Obtain sub-matrix 
    w_cuda_sub = nopack_cu(w_sub,f_sub); 

    % Incorporate sub-matrix into final matrix 
    w_cuda(start_index:end_index,:,:) = w_cuda_sub; 

end 

這是快：

w_cuda = []; 

for j=0:num_groups-1 

    % gets # of rows & cols in W. The last group is a special 
    % case because it may have fewer than max_row_size rows 
    if (j == num_groups-1 && mod(w_rows, max_row_size) ~= 0) 
     num_rows_sub = w_rows - (max_row_size * j);  
    else 
     num_rows_sub = max_row_size; 
    end; 

    % calculate correct W and f matrices 
    start_index = (max_row_size * j) + 1; 
    end_index = start_index + num_rows_sub - 1; 

    w_sub = W(start_index:end_index,:); 
    f_sub = filterBank(start_index:end_index,:); 

    % Obtain sub-matrix 
    w_cuda_sub = nopack_cu(w_sub,f_sub); 

    % Incorporate sub-matrix into final matrix 
    w_cuda = [w_cuda; w_cuda_sub]; 

end 

至於其他潛在有用的信息 - 我矩陣是3D的，其中的數字很複雜。一如往常，任何見解都被讚賞。

Answer 1

我一直認爲預分配對於任何數組大小都更快，並且從未實際測試過它。所以，我做了一個簡單的測試，通過追加和預分配方法，使用1000次迭代計算不同數組大小的數量，從1x1x3到20x20x3。下面的代碼：

arraySize = 1:20; 
numIteration = 1000; 

timeAppend = zeros(length(arraySize), 1); 
timePreAllocate = zeros(length(arraySize), 1); 

for ii = 1:length(arraySize); 
    w = []; 
    tic; 
    for jj = 1:numIteration 
     w = [w; rand(arraySize(ii), arraySize(ii), 3)]; 
    end 
    timeAppend(ii) = toc; 
end; 

for ii = 1:length(arraySize); 
    w = zeros(arraySize(ii) * numIteration, arraySize(ii), 3); 
    tic; 
    for jj = 1:numIteration 
     indexStart = (jj - 1) * arraySize(ii) + 1; 
     indexStop = indexStart + arraySize(ii) - 1; 
     w(indexStart:indexStop,:,:) = rand(arraySize(ii), arraySize(ii), 3); 
    end 
    timePreAllocate(ii) = toc; 
end; 

figure; 
axes; 
plot(timeAppend); 
hold on; 
plot(timePreAllocate, 'r'); 
legend('Append', 'Preallocate'); 

這裏是（預期）結果：你的代碼的