How to iteratively append elements to an array

I = imread(chart_image);
R = I(:,:,1);
G = I(:,:,2);
B = I(:,:,3);
N= zeros(24,3);
final_chart =zeros (24,3);

Vals = [175 175;  425 175; 650 175; 925 175;
        175 420; 425 420; 650 420; 925 420;
        175 670; 425 670; 650 670 ; 925 670 ;
        175 925;  425 925; 650 925; 925 925;
        175 1170; 425 1170; 650 1170; 925 1170;
        175 1425; 425 1425 ; 650 1425 ; 925 1425];

 for i = 1:24
    N(i)=[R(Vals(i,1),Vals(i,2)) G(Vals(i,1),Vals(i,2)) B(Vals(i,1),Vals(i,2))];   
    final_chart % the variable I want to modify to be able to store the vales of all N as it iterates over the 24 values in Vals into a single 24x3 matrix
  end

hello, I wanted to add the value of each N to a final array called final_chart, but I keep getting error "Unable to perform assignment because the left and right sides have a different number of elements.", I've tried other ways to append but for some reason I keep failing all of those. Can someone help me understand where I'm going wrong and help me fix my code. Thank you.

[Guillaume]

I don't see how you can get the error "Unable to perform assignment because the left and right sides have a different number of elements" from the code you've posted, since there's no indexing on the left-hand side of any the assignment lines. This error would only occur if you'd written

N(i, :) = ...
%not N(i) = ...

It's not clear what you're trying to do. Your current code overwrites N at each step, so only the value calculated on the last step is retained.

[Abdul Rahim Mohammad]

Apoligies, let me edit the question to make it more clear.

Answer 1

[Guillaume]

If I understood what you're trying to do, there's no need for the loop:

I = imread(chart_image);
R = I(:,:,1);
G = I(:,:,2);
B = I(:,:,3);
Vals = [175 175;  425 175; 650 175; 925 175;
        175 420; 425 420; 650 420; 925 420;
        175 670; 425 670; 650 670 ; 925 670 ;
        175 925;  425 925; 650 925; 925 925;
        175 1170; 425 1170; 650 1170; 925 1170;
        175 1425; 425 1425 ; 650 1425 ; 925 1425];

locations = sub2ind(size(R), Vals(:, 1), Vals(:, 2));
N = [R(locations), G(locations), B(locations)];

You don't even need to separate the image into colour planes:

I = imread(chart_image);
[width, height, ~] = size(I);
Vals = [175 175;  425 175; 650 175; 925 175;
        175 420; 425 420; 650 420; 925 420;
        175 670; 425 670; 650 670 ; 925 670 ;
        175 925;  425 925; 650 925; 925 925;
        175 1170; 425 1170; 650 1170; 925 1170;
        175 1425; 425 1425 ; 650 1425 ; 925 1425];

locations = sub2ind([width, height], Vals(:, 1), Vals(:, 2));
N = I(locations + (0:2) * width * height);

[Abdul Rahim Mohammad]

Thank you very much for the response, your code works beautifully but I also need to understand where I'm going wrong with code, Can you help me understand why I end up errors?

[Guillaume]

N(i) = [array of 3 elements]

Is indeed going to result in an error since you're trying to put 3 numbers into just one slot.

N(i, :) = ...

would have worked (for N preallocated with 3 columns as you have done).

[Abdul Rahim Mohammad]

Thank you, that was very helpful. I was having a hard time understading what was going wrong but I see now that I've messed up at N(i).