# How to make certain indexes of a cell array empty?

조회 수: 2(최근 30일)
Ethan 2022년 11월 30일
댓글: Voss 2022년 11월 30일
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?

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### 채택된 답변

Voss 2022년 11월 30일
pixelBlock{by + 1, bx + 1} = [];
##### 댓글 수: 1표시숨기기 없음
Voss 2022년 11월 30일
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
{} {} {} {}
% set element 1,2 to the empty array, using {} indexing
C{1,2} = []
C = 2×2 cell array
{} {0×0 double} {} {[ 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
{} {} {} {}
% remove element 1 (not what you want)
C(1) = []
C = 1×3 cell array
{} {} {}

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### 추가 답변(1개)

the cyclist 2022년 11월 30일
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) = {[],[]}
Note that one needs to be careful about when one uses parentheses vs. curly brackets.

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