how to save values in a matrix from a loop

Question

Adel Hafri 2022-3-4

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编辑： Jan 2022-3-4

Hello, im programming RLE on matlab and i'm still having one issue only and it is to save my values into a matrix instead of a vector so that when i decode, i can go row by row

this is my code

clear all;
x=[2 2 2 3 3 5; 6 6 5 5 5 7]
x = 2×6
     2     2     2     3     3     5
     6     6     5     5     5     7
f=size(x);
c=1;
y=[];
for i=1:f(1)
    for j=1:f(2)-1
            if x(i,j)==x(i,j+1)
            c = c + 1;
            else
            y=[y c x(i,j)];
            c=1;
            end
        
    end
    y =[y c x(i,f(2))]
        end
y = 1×6
     3     2     2     3     1     5
y = 1×12
     3     2     2     3     1     5     2     6     3     5     1     7
    
y               
y = 1×12
     3     2     2     3     1     5     2     6     3     5     1     7
       

My output is coming this way y = 3 2 2 3 1 5 2 6 3 5 1 7 and instead i want it something like y = 3 2 2 3 1 5

2 6 3 5 1 7

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Voss 2022-3-4

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What would you do if the number of unique values on each row of x is not the same? For example, if

x = [2 2 2 3 3 5; 6 6 5 5 5 5]

then what would y be? Maybe this?:

y = [3 2 2 3 1 5; 2 6 4 5 0 0]

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Answer 1

Voss 2022-3-4

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https://ww2.mathworks.cn/matlabcentral/answers/1663809-how-to-save-values-in-a-matrix-from-a-loop#answer_910049

编辑：Voss 2022-3-4

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To handle the case that not all rows of x have the same number of unique values, you can make y a cell array and construct each cell the same as you are already doing:

clear all;
x=[2 2 2 3 3 5; 6 6 5 5 5 5]
x = 2×6
     2     2     2     3     3     5
     6     6     5     5     5     5
f=size(x);
c=1;
y=cell(1,f(1)); % make a cell in y for each row of x
y
y = 1×2 cell array
    {0×0 double}    {0×0 double}
for i=1:f(1)
    for j=1:f(2)-1
        if x(i,j)==x(i,j+1)
            c = c + 1;
        else
            y{i} = [y{i} c x(i,j)];
            c=1;
        end
    end
    y{i} = [y{i} c x(i,f(2))];
end
y
y = 1×2 cell array
    {[3 2 2 3 1 5]}    {[2 6 4 5]}

If all cells of y contain vectors of the same length, as in your example x, then you can make y a matrix like this:

new_y = cell2mat(y(:)) % won't work if not all cells of y are the same length

If not, then you can make a matrix of zeros (or NaNs or whatever) and fill in each row to the appropriate column:

N = max(cellfun(@numel,y));
new_y = zeros(f(1),N);
for i = 1:f(1)
    new_y(i,1:numel(y{i})) = y{i};
end
new_y
new_y = 2×6
     3     2     2     3     1     5
     2     6     4     5     0     0

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Voss 2022-3-4

You're welcome!

Jan 2022-3-4

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A small simplification:

sx = size(x);
y  = cell(1, sx(1));  % pre-allocate the output
for i = 1:sx(1)       % Loop over rows
    c = 1;
    a = [];           % Accumulate
    for j = 1:sx(2) - 1
        if x(i, j) == x(i, j+1)
            c = c + 1;
        else
            a = [a, c, x(i, j)];  % append to the last cell of y
            c = 1;
        end
    end
    y{i} = [a, c, x(i, sx(2))];   % Insert in output
end

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Answer 2

Walter Roberson 2022-3-4

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在 MATLAB Online 中打开

clear all;
x=[2 2 2 3 3 5; 6 6 5 5 5 7]
x = 2×6
     2     2     2     3     3     5
     6     6     5     5     5     7
f=size(x);
c=1;
y = {};
for i=1:f(1)
    y{i} = [];
    for j=1:f(2)-1
            if x(i,j)==x(i,j+1)
            c = c + 1;
            else
            y{i}=[y{i} c x(i,j)];
            c=1;
            end
        
    end
    y{i} =[y{i} c x(i,f(2))]
end
y = 1×1 cell array
    {[3 2 2 3 1 5]}
y = 1×2 cell array
    {[3 2 2 3 1 5]}    {[2 6 3 5 1 7]}
y               
y = 1×2 cell array
    {[3 2 2 3 1 5]}    {[2 6 3 5 1 7]}
 

However you should not proceed to vertcat(y{:}) because you have no reason ahead of time to believe that the encodings will be the same length for each row.

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Answer 3

Jan 2022-3-4

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编辑：Jan 2022-3-4

在 MATLAB Online 中打开

A completely different approach:

x    = [2 2 2 3 3 5; 6 6 5 5 5 7];
nRow = size(x, 2);
y    = cell(1, nRow);  % Pre-allocate: avoid iterative growing of arrays
for k = 1:nRow
  [b, n] = RunLengthEnc(x(k, :));   % Get elements and count them
  y{k}   = reshape([n; b], 1, []);  % Store a row vector
end
% Determine run length parameters without a loop:
function [b, n] = RunLengthEnc(x)
d = [true, diff(x) ~= 0];   % TRUE if values change
b = x(d);                   % Elements without repetitions
n = diff(find([d, true]));  % Number of repetitions
end