For loop going beyond my stated limit, did I find a glitch?

Hi so I have the following for loop that I'm using to create an approximation of the cantor set.

x=0.5;
n=5;
a = cell(1,n);
a{1}=[0,1];
count=1;
for i = 2:n
      a{i} = linspace(0, 1, 3^(i-1)+1);
      clear j;
      for j=1:(length(a{i})-1)
      if a{i}(j)>1/3 
          if a{i}(j)<2/3
          a{i}(j)=[];
          else 
              count=1+count;
          end 
      else
          count=1+count;
      end
      
      end 
      
end

However, matlab keeps pushing j past its normal limits and frankly I have no idea why. For example when i is 4, it'll stop at j=25, but that's impossible, because j has to stop after 23 if you just run the length when i is 4.

Answer 1

[Shubham]

Hi @John Ruf

The issue you're experiencing with the loop is caused by modifying the size of the array a{i} within the loop. When you remove elements from a{i} using a{i}(j) = [], the loop counter j becomes out of sync with the updated size of a{i}.

To fix this issue, you can iterate the loop in reverse order, starting from the last index and moving towards the first index. This way, removing elements won't affect the subsequent iterations. Here's the modified code:

x = 0.5;

n = 5;

a = cell(1, n);

a{1} = [0, 1];

count = 1;

for i = 2:n

a{i} = linspace(0, 1, 3^(i-1) + 1);

for j = length(a{i}):-1:2

if a{i}(j) > 1/3 && a{i}(j) < 2/3

a{i}(j) = [];

else

count = count + 1;

end

end

end

Hope this helps.