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
Frank_m 2015-4-2
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
Star Strider 2015-4-2
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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James Tursa
James Tursa 2015-4-2
编辑：James Tursa 2015-4-2
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
Frank_m 2015-4-2
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
James Tursa 2015-4-2
编辑：James Tursa 2015-4-2
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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