indexing a vector by a power

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dustin
dustin 2013-9-25
I would like my vector to go by powers of 2 but everything I tried hasn't worked.
for n = 0:9
pow = (1:2^n:512)
end
but this doesn't return a vector with 1, 2, 4, 16, ...
What can I do to achieve this?

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采纳的回答

W. Owen Brimijoin
W. Owen Brimijoin 2013-9-25
You could accomplish this without using a for loop as follows:
n = 4;
p = 2.^cumprod([1,2+zeros(1,n-1)])
this results in:
p =
2 4 16 256
*Mind you, the start of the sequence you are asking for doesn't follow your own rules. Yes each element is the square of the previous one, except that 1^2 does not equal 2. So if you are intent on breaking that rule and want this sequence to start with a 1 and have n elements, then you would need to do this:
p = [1,2.^cumprod([1,2+zeros(1,n-2)])]
yielding:
p =
1 2 4 16

更多回答（2 个）

Azzi Abdelmalek
Azzi Abdelmalek 2013-9-25
Why are you using if n = 0:9. what you need is a for loop
  6 个评论
显示 4更早的评论
dustin
dustin 2013-9-25
I don't want to erase the previous result each iteration. I want to put each step in a vector. How can I do this?

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Image Analyst
Image Analyst 2013-9-25
I don't know what happened to 2^3=8 - maybe it's missing, but if each element is the square of the previous element, this will do that and give you the output you gave:
p = [1, 2];
for k = 3 : 9
p(k) = p(k-1)^2;
end
% Display p
p
In the command window:
p =
Columns 1 through 7
1 2 4 16 256 65536 4294967296
Columns 8 through 9
1.84467440737096e+19 3.40282366920938e+38
On the other hand, this code seems to be more like what your code does, but does not give you the output you gave because the 8 is in there:
for k = 0 : 9
p(k+1) = 2^k;
end
% Display p
p
In the command window:
p =
1 2 4 8 16 32 64 128 256 512
  2 个评论
dustin
dustin 2013-9-25
I need p to be a vector though.
Jan
Jan 2013-9-25
编辑：Jan 2013-9-25
@dustin: But p is a vector already.
These questions are such basic, that I suggest (as Azzi's did already) to read the Getting Started chapters of the documentation. The forum is not the right place to learn the fundamental usage of Matlab, because this has been done exhaustively by experts in the documentation already. If we re-tell what you can find there, too much time would be wasted.

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