MATLAB: Extract a portion of vector

vector

Hi, I have a problem that I can not solve.
I need to extract a portion of my vector, this vector has 2 columns (first column is time and second is wave height) and 2500 rows, the values of the second column are number and NaN, the data are sampled every 3 hours. The problem is: I need to create a new vector that contains only the registration longer than 20 hours, and for the rest I must to replace with NaN.
Here an example:
 Initial vector= [0  0.2
  3  0.35
  6  NaN
  9  NaN
  12  NaN
  15  NaN
  18  NaN
  21  NaN
  24  0.29
  27  0
  30  0
  33  0
  36  0
  39  0
  42  0
  45  0.18
  48  0.33
  51  NaN
  54  NaN
  57  NaN]
The result should be 
[0  NaN
3  NaN
6  NaN
9  NaN
12  NaN
15  NaN
18  NaN
21  NaN
24  0.29
27  0
30  0
33  0
36  0
39  0
42  0
45  0.18
48  0.33
51  NaN
54  NaN
57  NaN]
Can anyone suggest an approach to solve this problem?
Thanking you in advance
Alessandro Antonini

Best Answer

I solve the problem with the help of my friend, I needed two while cicle.
to solution was:
C=impuct vector and e1=C(:,2), the code is:
i=1;
while i<length(e1)
      j=0;
      i
      while isnan(e1(i+j))==0 && (i+j)<length(e1)
          j=j+1;
      end
      if j<7
          e1(i:(i+j-1))=NaN;
      end
      i=i+j+1;
  end
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