Doug's MATLAB Video Tutorials

September 24th, 2010

By Doug Hull

Doug first used MATLAB in 1994, could not figure it out until he got some help in 1995. He is now dedicated to making sure that no one else wastes a year of their life not knowing MATLAB like he did.

17:58 UTC | Posted in Format: Video, Level: Basic | Permalink | 5 Comments »

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5 Responses to “Deleting in a loop”

1. k replied on September 26th, 2010 at 21:02 UTC :
   

   n=10;
   a=rand(1,n);
   a(rand(1,n)<0.1)=[];
   

2. Oliver Woodford replied on September 27th, 2010 at 12:58 UTC :
   

   Is deleting elements in a loop a good idea? Won’t it reallocate memory every iteration, and therefore slow things down?

3. dhull replied on September 27th, 2010 at 14:02 UTC :
   

   @k

   If you can vectorize this kind of operation, then your code is a good idea. The premise of this video, stated at the beginning, was that the operation that decides if something should be deleted only works on scalars. That is why I used a for loop (pretending my silly filter could not be vectorized).

   @Oliver

   For ‘small’ cases the reallocation effect would be negligible. It is a constant balance of simplicity vs optimality.

   -Doug

4. Ed replied on September 27th, 2010 at 15:58 UTC :
   

   counting backwards seems a lot easier than using a while loop, which is what I usually have done in the past.

   i = 1;
   while i <= length(a)
       if isDeleteMe(a(i))
           a(i) = [];
       else
           i = i + 1;
       end
   end
   

5. dhull replied on September 27th, 2010 at 17:30 UTC :
   

   @Ed,

   There are many, many ways of solving this and similar problems in MATLAB. They all have advantages and disadvantages. The more techniques you are aware of, the better. Some, like the flawed one I presented, are never the best (but tempting to try). I can see yours would be good if somewhere in the loop the vector could have more data postpended to it!

   Thanks,
   Doug

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<pre class="code">
a = magic(3);
sum(a)
</pre>

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