Doug's MATLAB Video Tutorials

July 29th, 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.

20:25 UTC | Posted in Format: Video, Level: Basic | Permalink | 6 Comments »

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6 Responses to “How to fix common indexing errors with for loops”

1. Robbie replied on August 1st, 2010 at 08:34 UTC :
   

   Is there anything wrong with using the following?

   c = [];
   for i = 1:0.5:10
   c(end+1) = i*10;
   end
   

2. matt fig replied on August 2nd, 2010 at 14:22 UTC :
   

   @Robbie,

   The code you suggest works, however it is extremely slow for larger problems when compared with constructing a pre-allocated array in a FOR loop. If you want to avoid explicit pre-allocation for some reason, you can often do something like this (which I call dynamic pre-allocation):

   cnt = 1801;
   for ii = 10:-.005:1
   d(cnt) = ii*10; % d is fixed size after first pass.
   cnt = cnt – 1;
   end

3. Daniel Armyr replied on August 12th, 2010 at 09:35 UTC :
   

   @doug:
   Your third case is pretty much what I do on a daily basis. However, I have found myself more comfortable with the following syntax:

   inData = 1:0.5:10;
   c = nan(size(inData));
   for i = 1:length(inData)
      c(i) = inData(i)*10;
   end
   disp ( c );
   

   I also took the liberty of preallocating memory for the sake of it.

4. Kamran replied on September 13th, 2010 at 07:34 UTC :
   

   The output of second part of your code is not the same as the first one.
   The output of the firs part is from 10 to 100 but the output of the second is from 0 to 100.

   Thank you for your great videos.

5. dhull replied on September 20th, 2010 at 17:45 UTC :
   

   @Kamran,

   It is never claimed they have the same results.

   Doug

6. Nike SB replied on September 27th, 2010 at 03:53 UTC :
   

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

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