How to add a specific number to elements in a vector?

Hi,

I have a 1x600 matrix called 'samples' containing numbers from 0 to 255. I want to add the number 255 to every element in the array that is less than 100. I also want to divide the matrix into two different matrices, one contains the elements 0 to 300 and the other one from 301 to 600. I am able to divide the matrix in 2, but when I add the value 255 to every element less than 100 it adds it to all the elements in the matrix. How can I fix my error? Thanks! The code I have so far is

   %samples is a 1x600 matrix
      frames = 300;

      on = wkeep(samples,frames,[frames+1 1]);
      off = wkeep(samples,frames,[1 1]);

      if any (on < 100)
          on_count = on + 255;
      end 

      if any (off < 100)
            off_count = off +255 ;

      end

[Giacomo Maurelli]

   %samples is a 1x600 matrix
      % Loop over elements
      for i=length(samples)
          % Define if i-element of "samples" is greater of 100 or not
          if samples(1,i)<100
              % Adding 255 to elements that are less than 100
              samples(1,i)=samples(1,i)+255;
          end
      end
      % Definind matrix size
      frames = 300;
      % Creating the two matrices
      matrix1 = samples(1,1:frames);
      matrix2 = samples(1,frames+1:length(samples))

[Ivonne Escorcia]

Hi, thanks for your answer. I was not able to get the right results with this code. The samples stay the same. But I was able to create the 2 matrices, thanks!

Answer 1

[Guillaume]

   if any (on < 100)
      on_count = on + 255;
   end

says: if any element of on is less than 100 (1st), then set on_count to the sum of on (hence all the elements of on) with 255. So, if any element of on is less than 100, you add 255 to all the element of on and put it in a new on_count variable. If none of the elements of on are less than 100, then nothing happens, on_count is not even created.

The correct way:

on_count = on;  %copy everyting
mask = on_count < 100;  %logical vector telling you which elements to change
on_count(mask) = on_count(mask) + 100;  %only change those elements for which the mask is true

Same for off. No if or for needed.

edit: another way to obtain the same result with no comparison at all, just using mathematical operations would be:

on_count = mod(on - 100, 255) + 100;

[Stephen23]

+1 logical indexing is the way to go.

[Ivonne Escorcia]

Thank you for your answer and for explaining why the if statements do not work in this case. I was able to implement your first suggestion. Can I ask how the second suggestion works? "on_count = mod(on - 100, 255) + 100;"

[Guillaume]

"Can I ask how the second suggestion works?"

You're basically constraining your integers to the range 100-355 modulo 255. modulo operations are 0-based, so you need to offset your numbers down to 0 before the modulo operation and back to 100 after.

Answer 2

[Giacomo Maurelli]

%samples is a 1x600 matrix
      % Loop over elements
      for i=length(samples)
          % Define if i-element of "samples" is greater of 100 or not
          if samples(1,i)<100
              % Adding 255 to elements that are less than 100
              samples(1,i)=samples(1,i)+255;
          end
      end
      % Definind matrix size
      frames = 300;
      % Creating the two matrices
      matrix1 = samples(1,1:frames);
      matrix2 = samples(1,frames+1:length(samples))