I am trying to create two vectors out of one,x, where one will contain all the positive elements and the other all the negative elements.

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Frank_m 2015-4-2

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评论： Star Strider 2015-4-2

%this program creates vectors N and P out of x
%N will contain negative elements and P the positive elements
x=[-3.5 -5 6.2 11 0 8.1 -9 0 3 -1 3 2.5];
for k=length(x);
    if x>=0
        k=x
        P(k)=k
    else x<0
        N(k)=k
    end
end
P
N

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Star Strider 2015-4-2

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Just use ‘logical indexing’:

x=[-3.5 -5 6.2 11 0 8.1 -9 0 3 -1 3 2.5];
P = x(x >= 0);
N = x(x < 0);

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Frank_m 2015-4-2

Thanks again, this was very helpful.

Star Strider 2015-4-2

Again, my pleasure!

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James Tursa 2015-4-2

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编辑：James Tursa 2015-4-2

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You are using k as a loop variable but only doing one iteration, k = 12. Maybe you want this result:

P = x(x>0);
N = x(x<0);

Or if you want them to retain their original spots, something like this

P = x;
P(~(x>0)) = 0;
N = x;
N(~(x<0)) = 0;

EDIT:

If you have to use loops, then change your looping index to the following:

for k=1:length(x)

And inside your loop, don't use x by itself in the test since that is the entire vector x. Instead, use the k'th element of x (i.e., use your k index). E.g.,

if x(k) >= 0

Finally, Don't change the value of k inside your loop! That will mess up the looping. So get rid of this line:

k=x

And then use x(k) on the right hand side of your assignments instead of k.

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Frank_m 2015-4-2

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It seemed to solve the problem, P is now a vector but vector N is not coming out.

%this program creates vectors N and P out of x
%N will contain negative elements and P the positive elements
  x=[-3.5 -5 6.2 11 0 8.1 -9 0 3 -1 3 2.5];
  for k=length(x);
      if x(k)>=0
          P = x(x>0);
      else x(k)<0
          N=x(x<=0)
      end
  end
  P
  N

P =

6.2000 11.0000 8.1000 3.0000 3.0000 2.5000

N =

1 2 3 4 0 0 0 0 0 0 0 12

James Tursa 2015-4-2

编辑：James Tursa 2015-4-2

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LOL. Well, if you just change your else statement to this (i.e., drop the x(k) < 0 part) you will have something:

else

However, your "loop" is basically solving the entire problem in a vectorized manner at each iteration! So, this is not really in the spirit of the looping requirement.

I would take a look at Star Strider's comment since he has it basically laid out for you in detail (although it looks like he needs to fix the else line as well).

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