concatenating vectors generated with loop

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alpedhuez
alpedhuez 2018-3-27
评论： Stephen23 2018-4-2
I would like to run a for loop like
for i=1:N
output=[1 i]
output2 = [output2 output]
end
But in this example it seems that I need to define output2 at the beginning. Is there any compact way to write this operation?
  1 个评论
Stephen23
Stephen23 2018-4-2
Expanding the array on each iteration will (in general) slow down your code. As David Goodmanson wrote, it is good practice preallocate the output array before the loop. Read this to know more:
https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html

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采纳的回答

David Goodmanson
David Goodmanson 2018-3-27
编辑：David Goodmanson 2018-3-27
Hi alpedhuez, define output2 by putting
output2 = [];
just before the do loop.
Unless you are using the intermediate results somehow, concatenation in this manner is not recommended. For N = 10000 or less it's all right in terms of execution time. But with execution time growing rapidly, eventually it makes more sense (and is better practice anyway) to define an array by output2 = ones(1,2*N) and then fill all the even entries with output2(2:2:2*N) = 1:N

更多回答（1 个）

David Fletcher
David Fletcher 2018-3-27
编辑：David Fletcher 2018-3-27
Not really more compact, but it does away with the loop
N=10
output=reshape([ones(1,N); 1:N],1,2*N)

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