Best practice of for loops and vectorisation, also maybe cumprod.
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Marshall
el 11 de Sept. de 2014
Comentada: Marshall
el 12 de Sept. de 2014
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
2 comentarios
Adam
el 11 de Sept. de 2014
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.
Respuesta aceptada
David Young
el 11 de Sept. de 2014
prod(cellfun(@length, someArray))
14 comentarios
David Young
el 11 de Sept. de 2014
Marshall - yes, using {} rather than () is like doing an extra pointer dereference in C.
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