Modify elements in arrays stored in cells at specified rows/columns

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Hi, I have a 2d cell array, where each cell contains a 1x2 double,
E.g. X = { [1,0], [1,0], [1,0], [1,0]; [1,0], [1,0], [1,0], [1,0] };
I want to assign the value 1 to the second element in each cell at row r=2, columns c=1:2. Because my actual data is large I want to do this without a for loop. In concept something like:
cellfun( @(c) (X{r,c}(2) = 1), 1:2 );
Thanks in advance

Accepted Answer

Michael Van de Graaff
Michael Van de Graaff on 22 Mar 2022
I modified X to make it a bit clearer where each element is going, but I believe this does what you want:
X = { [1,0.1], [2,0.2], [3,0.3], [4,0.4]; [11,0.11], [12,0.12], [13,0.13], [14,0.14] };
szeX = size(X);
y = cell2mat(X);
y =reshape(y',[],2);
y(2,:) = 1;
y = reshape(y,[],2)';
szy = size(y);
cellsize = size(X{1});
d1 = cellsize(1)*ones(szeX(1),1);
d2 = cellsize(2)*ones(1,szeX(2));
newX = mat2cell(y,d1,d2)
cell2mat(newX)
cell2mat(X)
The last two lines just for comparison

More Answers (1)

Image Analyst
Image Analyst on 22 Mar 2022
Your cells do not contain arrays with two rows. Look
X = { [1,0], [1,0], [1,0], [1,0]; [1,0], [1,0], [1,0], [1,0] }
X = 2×4 cell array
{[1 0]} {[1 0]} {[1 0]} {[1 0]} {[1 0]} {[1 0]} {[1 0]} {[1 0]}
Your cells contain a 1-row row vector with 2 columns.
But let's say you did have two ross in the arrays in the cells. I think the best way is to use a for loop:
X = { randi(9, 4, 3), randi(9, 4, 3), randi(9, 4, 3), randi(9, 4, 3);
randi(9, 4, 3), randi(9, 4, 3), randi(9, 4, 3), randi(9, 4, 3)}
X = 2×4 cell array
{4×3 double} {4×3 double} {4×3 double} {4×3 double} {4×3 double} {4×3 double} {4×3 double} {4×3 double}
% Show the starting values:
celldisp(X)
X{1,1} = 7 2 6 2 9 1 5 5 3 4 3 3 X{2,1} = 6 4 1 9 2 6 9 1 5 7 6 8 X{1,2} = 6 6 9 6 5 5 2 8 1 6 8 4 X{2,2} = 7 9 5 2 5 1 1 4 1 6 5 1 X{1,3} = 5 9 9 5 9 8 1 9 5 5 4 3 X{2,3} = 3 6 7 2 6 6 9 9 2 9 1 7 X{1,4} = 7 1 6 8 4 2 8 3 8 5 9 6 X{2,4} = 7 9 7 7 3 3 7 6 8 4 4 5
for k = 1 : numel(X)
% Get cell contents.
thisCellContents = X{k};
% Change it.
thisCellContents(2, 1:2) = [1, 2];
% Put it back into the cell.
X{k} = thisCellContents;
end
% See the result:
celldisp(X)
X{1,1} = 7 2 6 1 2 1 5 5 3 4 3 3 X{2,1} = 6 4 1 1 2 6 9 1 5 7 6 8 X{1,2} = 6 6 9 1 2 5 2 8 1 6 8 4 X{2,2} = 7 9 5 1 2 1 1 4 1 6 5 1 X{1,3} = 5 9 9 1 2 8 1 9 5 5 4 3 X{2,3} = 3 6 7 1 2 6 9 9 2 9 1 7 X{1,4} = 7 1 6 1 2 2 8 3 8 5 9 6 X{2,4} = 7 9 7 1 2 3 7 6 8 4 4 5
See? the first and second columns of the second row of each array get changed to 1 and 2 respectively.
Now I know you didn't want a for loop, probably for speed and efficiency, but let's face it - you threw away all possibility of getting speed and efficiency when you decided to use a cell array in the first place.
And it's a myth that cell arrays are horribly slow. Many years ago they've been speeded up a great deal. How many elements are in your cell array anyway? Hundred of millions or billions? For a few million I doubt you'd notice any difference. Any slowness would be caused not by the for loop but by your decision to use a cell array. Look, here is 10 million for loop iterations where it basically does nothing but iterate the for loop:
tic
for k = 1 : 1e7
;
end
toc
Elapsed time is 0.010463 seconds.
How many iterations would you do? See? The for loop alone is not the problem. Takes only a hundredth of a second. It's the data structure and what operations you do that is what eats up the time, not the for loop.

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