如何在MATLAB中预分配非数值向量? [英] How can I preallocate a non-numeric vector in MATLAB?

问题描述

我经常发现自己在做这样的事情:

I've often found myself doing something like this:

unprocessedData = fetchData();  % returns a vector of structs or objects
processedData = [];             % will be full of structs or objects

for dataIdx = 1 : length(unprocessedData) 
    processedDatum = process(unprocessedData(dataIdx));
    processedData = [processedData; processedDatum];
end

虽然功能强大，但并不是最佳选择-processedData向量在循环内不断增长.甚至mlint都警告我，应该考虑预先分配速度.

Which, whilst functional, isn't optimal - the processedData vector is growing inside the loop. Even mlint warns me that I should consider preallocating for speed.

数据为int8的向量，我可以这样做:

Were data a vector of int8, I could do this:

% preallocate processed data array to prevent growth in loop
processedData = zeros(length(unprocessedData), 1, 'int8');

并修改循环以填充向量槽，而不是串联.

and modify the loop to fill vector slots rather than concatenate.

是否可以预分配矢量，以便随后可以容纳结构或对象?

更新:受 Azim的回答的启发，我只是颠倒了循环顺序.首先处理最后一个元素将在第一次命中时强制整个向量进行预分配，因为调试器确认:

Update: inspired by Azim's answer, I've simply reversed the loop order. Processing the last element first forces preallocation of the entire vector in the first hit, as the debugger confirms:

unprocessedData = fetchData();

% note that processedData isn't declared outside the loop - this breaks 
% it if it'll later hold non-numeric data. Instead we exploit matlab's 
% odd scope rules which mean that processedData will outlive the loop
% inside which it is first referenced: 

for dataIdx = length(unprocessedData) : -1 : 1 
    processedData(dataIdx) = process(unprocessedData(dataIdx));
end

这要求process()返回的所有对象都必须具有有效的零参数构造函数，因为MATLAB在第一次使用真实对象对其进行写入时会初始化processedData.

This requires that any objects returned by process() have a valid zero-args constructor since MATLAB initialises processedData on the first write to it with real objects.

mlint仍然抱怨数组可能增长，但是我认为这是因为它无法识别反向循环迭代...

mlint still complains about possible array growth, but I think that's because it can't recognise the reversed loop iteration...

推荐答案

由于您知道结构processedData的字段并且知道其长度，因此一种方法如下:

Since you know the fields of the structure processedData and you know its length, one way would be the following:

unprocessedData = fetchData();
processedData = struct('field1', [], ...
                       'field2', []) % create the processed data struct
processedData(length(unprocessedData)) = processedData(1); % create an array with the required length
for dataIdx = 1:length(unprocessedData)
    processedData(dataIdx) = process(unprocessedData(dataIdx));
end

这假定process函数返回的结构与processedData相同的字段.

This assumes that the process function returns a struct with the same fields as processedData.

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