在Matlab preallocating阵列？ [英] Preallocating arrays in Matlab?

问题描述

我使用的是简单的为循环裁剪大量图像，然后将它们存储在一个单元阵列。我不断收到消息：

I am using a simple for loop to crop a large amount of images and then storing them in a cell array. I keep getting the message:

变量 croppedSag 出现在每次循环迭代改变大小。考虑速度preallocating。

The variable croppedSag appears to change size on every loop iteration. Consider preallocating for speed.

我而在MATLAB编码之前见过几次。我一直忽略了它，并很好奇多少preallocating将如何增加运行时间，如果我有，比如说，10,000张或更大的数字？

I have seen this several times before while coding in MATLAB. I have always ignored it and am curious how much preallocating will increase the runtime if I have, say, 10,000 images or a larger number?

另外，我看了一下文档中的preallocating，它说，使用零（）用于这一目的。我将如何使用，对于低于code？

Also, I have read about preallocating in the documentation and it says to use zeros() for that purpose. How would I use that for the code below?

croppedSag = {};
for i = 1:sagNum
    croppedSag{end+1} = imcrop(SagArray{i},rect);
end

我不太下面的示例中的文件中。

I didn't quite follow the examples in the documentation.

推荐答案

pre-分配的数组始终是在Matlab是一个好主意。另一种方法是有它通过循环每次迭代中生长的数组。每一个元素被添加到阵列的结束时间，Matlab的必须产生一个完全新的数组，旧数组的内容复制到新的一个，然后，最后，在结束添加新元素。 pre-清分无需分配一个新的阵列，并花时间阵列的现有内容复制到新的内存。

Pre-allocating an array is always a good idea in Matlab. The alternative is to have an array which grows during each iteration through a loop. Each time an element is added to the end of the array, Matlab must produce a totally new array, copy the contents of the old array into the new one, and then, finally, add the new element at the end. Pre-allocating eliminates the need to allocate a new array and spend time copying the existing contents of the array into the new memory.

不过，在你的情况，你可能不会看到那么多好处，你可能期望。当复制单元阵列到一个新的，放大单元阵列，MATLAB实际上并没有复制单元阵列的的内容的（图像数据），但只的指针数据。

However, in your case, you might not see as much benefit as you might expect. When copying the cell array to a new, enlarged cell array, Matlab doesn't actually have to copy the contents of the cell array (the image data), but only pointers to that data.

不过，没有理由不为pre-分配（除非你居然不知道事先的最终大小）。这是你的循环的pre-分配的版本：

Nonetheless, there is no reason not to pre-allocate (unless you actually don't know the final size in advance). Here's a pre-allocated version of your loop:

croppedSag = cell(1, sagNum);
for ii = 1:sagNum
    croppedSag{ii} = imcrop(SagArray{ii}, rect);
end

我也改变了指数变量i到二，所以它不会覆写虚数单位。

I also changed the index variable "i" to "ii" so that it doesn't over-write the imaginary unit.

您也可以在一行中使用cellfun功能重新写这个循环：

You can also re-write this loop in one line using the cellfun function:

croppedSag = cellfun(@(im) imcrop(im, rect), SagArray);

下面是一个博客条目，可能是信息：

Here's a blog entry that might be informative:

• Matlab - 由preallocating阵列，细胞和结构
的规模加速您的code

• Matlab - Speed up your Code by Preallocating the size of Arrays, Cells, and Structures

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