Best practice of for loops and vectorisation, also maybe cumprod.

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Marshall 2014-9-11

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评论： Marshall 2014-9-12

采纳的回答： David Young

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Hello all,

Like many, I am entering Matlab from a C (embedded) background, so am struggling with the adjustment to vectors.

Simple cases such as the following are beginning to be understandable to me.

for i = 1:length(myVector)
   dim(i)= i;
end

is equivalent to

dim = 1:length(myVector);

However I'm stuck on a slightly more complex case relating to nested arrays. I have a nested cell array (someArray) and I wish to know the cumulative product of all those elements.

someArray = {cell(1,4); cell(1,13); cell(1,3); cell(1,2); cell(1,5)}
someArray = 
    {1x4  cell}
    {1x13 cell}
    {1x3  cell}
    {1x2  cell}
    {1x5 cell}

The following works, but it feels clunky.

product = 1;
for i = 1:length(someArray) 
    dim(i) = length(someArray{i});
    product = product*dim(i);
end
count =
        1560

I feel like this is hacky and that there's a better way, but my thinking is stuck. Is there a way to use cumprod()? Any suggestions are welcome. I'm using ML ed. R2012b

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Adam 2014-9-11

You say you wish to know "the cumulative product of all those elements".

I assume that is just a slight mistake in the description since you seem happy with your result, but I was assuming you meant you want the product of all the elements in each cell array rather than simply the product of the lengths of each cell array. The former would be a little more complex, but if you do just want the product of the lengths then either your code or David Young's below will do fine.

Beware though that while cellfun looks neat it is, unfortunately rarely as performant as simply doing a for loop.

Marshall 2014-9-11

Thanks for the response. Yep, typo there. I just want the product.

The code here works to my satisfaction, I was just hoping there would be a leaner way to perform the same task and at the same time learn a new vectors method. I guess I'll leave it as it is.

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采纳的回答

David Young 2014-9-11

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https://ww2.mathworks.cn/matlabcentral/answers/154379-best-practice-of-for-loops-and-vectorisation-also-maybe-cumprod#answer_151353

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prod(cellfun(@length, someArray))

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David Young 2014-9-11

Marshall - yes, using {} rather than () is like doing an extra pointer dereference in C.

Marshall 2014-9-12

@Stephen, ""but more recent advice seems to be less concerned about them": implying therefore that "I don't have to bother learning it"?"

Not at all, I'm very interested in learning more about it! I was just interested in why there seems to have been a shift in advice over time when I've been reading forums.

Great advice and information from everyone here, thanks for the great welcome!

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