How to make certain indexes of a cell array empty?

Question

Ethan 2022-11-30

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1867863-how-to-make-certain-indexes-of-a-cell-array-empty

评论： Voss 2022-11-30

I'm currently trying to write an image compression algorithm (I know this already exists in Matlab but this is for a course), and wondering if Matlab has functionality to input certain elements of a cell array as "empty". In Python and C# what I would do is set it to None/null, but I found that Matlab has no equivalent. Here is my current code where I would like to implement this:

if (y + by > imgHeight) || (x + bx > imgWidth)
    pixelBlock{by + 1, bx + 1} = [-1, -1, -1]; % empty values here instead of -1
else
    R = img(x + bx, y + by, 1);
    G = img(x + bx, y + by, 2);
    B = img(x + bx, y + by, 3);
    pixelBlock{by + 1, bx + 1} = [R, G, B];
end

I'm blocking the image into 4x4 squares, but if a 4x4 square runs out of the bounds of the image I would like to put the pixel values as some sort of empty, where it can later be decoded and the program can ask (if pixel empty, skip).

Is this possible?

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Voss 2022-11-30

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1867863-how-to-make-certain-indexes-of-a-cell-array-empty#answer_1116633

在 MATLAB Online 中打开

pixelBlock{by + 1, bx + 1} = [];

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

Voss 2022-11-30

在 MATLAB Online 中打开

or, equivalently:

pixelBlock(by + 1, bx + 1) = {[]};

But NOT:

pixelBlock(by + 1, bx + 1) = [];

because that attempts to remove the element pixelBlock(by + 1, bx + 1).

Example:

C = {1 2; 3 4} % 2-by-2 cell array
C = 2×2 cell array
    {[1]}    {[2]}
    {[3]}    {[4]}
% set element 1,2 to the empty array, using {} indexing
C{1,2} = []
C = 2×2 cell array
    {[1]}    {0×0 double}
    {[3]}    {[       4]}
% set element 2,1 to the empty array, using () indexing
C(2,1) = {[]}
C = 2×2 cell array
    {[       1]}    {0×0 double}
    {0×0 double}    {[       4]}

Another example (of the wrong thing, in this case):

C = {1 2 3 4} % 1-by-4 cell array
C = 1×4 cell array
    {[1]}    {[2]}    {[3]}    {[4]}
% remove element 1 (not what you want)
C(1) = []
C = 1×3 cell array
    {[2]}    {[3]}    {[4]}

请先登录，再进行评论。

Answer 2

the cyclist 2022-11-30

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1867863-how-to-make-certain-indexes-of-a-cell-array-empty#answer_1116638

在 MATLAB Online 中打开

Here are a couple ways:

% Make up a cell array of pixelBlock
pixelBlock = {'2','3','5',7; 11, 13, '17', '19'}
% Replace a few cells with empty
pixelBlock(1,2:4) = cell(1,3)
% Replace a couple more, in a different way
pixelBlock(2,1:2) = {[],[]}