考虑预先分配速度 [英] Consider preallocating for speed

问题描述

我正在做以下事情

for i = 1:m,
    index = 0;
    for j = 1:n,
        index = index+values(i,j)*2^(j-1);
        if (j==1)
            symbol_chip = chip_values(index+1,:);
        else
            symbol_chip = [symbol_chip chip_values(index+1,:)];
        end
    end
end

它告诉我以下内容:

symbol_chip可能在循环内部增长.考虑预先分配速度.

symbol_chip might be growing inside the loop. Consider preallocating for speed.

有什么想法吗?

推荐答案

是.每次走动时，您的elseif块都会调整symbol_chip的大小，这很昂贵.而是，重写代码，以便在循环之前使用(例如)symbol_chip = zeros(max_size, 1);.然后，更改内容，但不要更改symbol_chip的大小.

Yes. Each time you go around, your elseif block is resizing symbol_chip, which is expensive. Instead, rewrite your code so that you have (say) symbol_chip = zeros(max_size, 1); before the loop. Then, change the contents but not the size of symbol_chip.

您需要稍作更改，但是这样做会更快.如果您对当前的速度不满意，请不要进行任何更改！

You'll need to slightly change your approach, but it will be much faster if you do. If you're not annoyed by the current speed, don't change anything!

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