indexing a vector by a power

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?

Answer 1

[W. Owen Brimijoin]

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

Answer 2

[Azzi Abdelmalek]

Why are you using if n = 0:9. what you need is a for loop

[dustin]

I meant for but that doesn't work either.

[Azzi Abdelmalek]

post your code with a for loop

[dustin]

I updated the original post.

[Azzi Abdelmalek]

your for loop woks only for n=9, because you are erasing the previous result each iteration

If you have

n=1
pow=2^n

for the next iteration

n=2
power=2^n

What is the result?

[dustin]

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?

[Azzi Abdelmalek]

For example

k=1
a(k)=2

k=2
a(k)=14

You have to read the getting started with Matlab

Answer 3

[Image Analyst]

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

[dustin]

I need p to be a vector though.

[Jan]

@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.