How do I use a for loop to return a vector

Question

Adam Palmer 2014-7-31

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评论： Ben11 2014-7-31

Hey everyone, I've been using matlab for about a month, so this might be a pretty simple question.

I have a vector x=[-3.5 -5 6.2 11 0 8.1 -9 0 3 -1 3 2.5], and I want to make two other vectors from it using a for loop. The first contains the positive elements of x, and the second contains the negative elements of x. I can get it to return individual elements, but not vectors. Your help is greatly appreciated, I've been trying to figure this out for a couple hours.

Heres my code

x=[-3.5 -5 6.2 11 0 8.1 -9 0 3 -1 3 2.5];

n=length(x);

for k=1:n

if x(k)>0

P=x(k)

 elseif 
x(k)<0
 N=x(k)
    end
end

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Answer 1

Ben11 2014-7-31

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https://ww2.mathworks.cn/matlabcentral/answers/144288-how-do-i-use-a-for-loop-to-return-a-vector#answer_147053

编辑：Ben11 2014-7-31

在 MATLAB Online 中打开

Hi Adam,

you actually don't need a loop:

P = x(x > 0);
N = x(x < 0);
P =
      6.2000   11.0000    8.1000    3.0000    3.0000    2.5000
N =
     -3.5000   -5.0000   -9.0000   -1.0000

If you want to use a loop, you need to assign an index to P and N, otherwise the loop will erase the current value at every iteration. Here's an example;

P = zeros(1,length(x)); % Initialize a vector P of length equal to that of x. After the loop we get rid of the 0 values.

N = P; same thing for N.

for k=1:length(x)
      if x(k)>0 
          P(k) =x(k);
      elseif x(k)<0
          N(k)=x(k);
      end
  end
P(P==0) = []; % Remove values equal to 0
N(N==0) = [];

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Adam Palmer 2014-7-31

Hey It worked!! So what you're doing with the p is creating a vector to store the positive values, and same with N?

Thanks so much Ben11!

Ben11 2014-7-31

Yep. Since normally you would not know beforehand how many values there would be in P and N, it is good practice to initialize the vector with a size large enough to store more values. Anyhow, at each loop iteration you add the current value of x in either P or N depending on its value. Glad to help!

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