Loop through a vector with changing legth

Asked by Alex De C on 22 Mar 2018

Latest activity Edited by Stephen Cobeldick on 22 Mar 2018

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Hello everyone. I want to loop through a vector, say z, and at each iteration remove some elements from it, using something like

z = z(abs(z-z(i)>=1))

My problem is, I don't know how to form the loop. I tried using

for member = z

but then again, I need to do calculations with each element so that doesn't work. Can anyone help me?

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2 Answers

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Answer by KL on 22 Mar 2018

Accepted Answer

why not use a while loop?

 idx = 1;
 while idx<=numel(z)
    %do something
    z = z(abs(z-z(idx)>=1));
    idx = idx+1;
 end

2 Comments

Alex De C on 22 Mar 2018

Ok yes I feel so stupid now :D

Stephen Cobeldick on 22 Mar 2018

"I need to do calculations with each element so that doesn't work"

Neither does this answer. This answer does not "do calculations with each element" as the question requests, it actually misses calculating with elements that follow any removed element that happens to be one position more than the current index (because the element removal and the index increment both conspire against its author).

Compare with this simple example, following the same concept, to remove all even values from z:

z = [1,1,2,2,3,3,4,4,4,5];
idx = 1;
while idx<=numel(z)
  if mod(z(idx),2)==0
    z(idx)=[];
  end
  idx = idx+1;
end

but at the end there are still even values in the vector!:

>> z
z =
   1   1   2   3   3   4   5

Actually it did not test all elements to check if they are even! In general not all elements get tested by this algorithm although in some special cases they might be.

See Jan Simon's answer for a correct analysis of this problem.

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Answer by Jan on 22 Mar 2018

A loop over the elements of a vector cannot work, if you remove elements of the vector, except if you process the elements from the end to the start and remove only elements, which are after the current element.

z = rand(1, 10)
for k = 9:-1:1
  if z(k) < 0.5
    z(k+1) = [];
  end
end

This cannot work, if you run it in the opposite direction from 1 to 9.

So what does "loop through a vector" exactly mean, if you remove elements? It cannot be an "element by element". What should happen exactly with the vector? You have to define this clearly, before it can be implemented.

1 Comment

Alex De C on 22 Mar 2018

Thanks for the reply. The thing is, I calculate something for each element and depending on the result, i remove it and its ajdustents. I will try it with a while loop, I think it will work

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