Apply logical mask to every matrix in array

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Marcel345614
Marcel345614 2022-2-24
回答： Loïc Reymond 2023-5-29
I have a 3D array of size 640x640x20 i.e. 20 matrices of size 640x640. In addition I have a logical mask of size 640x640. How can I apply this mask to every matrix in the array? Is this only possible with a for loop?
for jj=1:20
mat_temp=B(:,:,jj);
mat_temp(mask==1)=NaN
B(:,:,jj)=mat_temp;
end
%% I tried the following but it didn't worked (mask was only applied to first matrix)
B(mask==1)=NaN;

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回答（3 个）

Jan
Jan 2022-2-24
编辑：Jan 2022-2-25
Ran in:
B = reshape(1:24, 2,3,4);
mask = logical([1,0,1; 0,1,0]);
sB = size(B);
B = reshape(B, [], sB(3)); % Join the first two dimensions
B(mask, :) = NaN;
B = reshape(B, sB)
D =
D(:,:,1) = NaN 3 NaN 2 NaN 6 D(:,:,2) = NaN 9 NaN 8 NaN 12 D(:,:,3) = NaN 15 NaN 14 NaN 18 D(:,:,4) = NaN 21 NaN 20 NaN 24
% Alternatively:
M = ones(size(mask));
M(mask) = NaN;
B = B .* M;
Note: No need to compare mask with 1: mask==1. Use mask directly, if it is a logical array.
  2 个评论
Jan
Jan 2022-2-25
A speed coparison with R2018b:
X = rand(640, 640, 20);
mask = rand(640, 640) > 0.6;
rep = 1e2;
B = X;
tic;
for k = 1:rep
for jj=1:20
mat_temp = B(:,:,jj);
mat_temp(mask==1) = NaN;
B(:,:,jj) = mat_temp;
end
end
toc % Elapsed time is 8.416893 seconds.
B = X;
tic;
for k = 1:rep
sB = size(B);
B = reshape(B, [], sB(3));
B(mask, :) = NaN;
B = reshape(B, sB);
end
toc % Elapsed time is 0.966218 seconds.
B = X;
tic;
for k = 1:rep
M = ones(size(mask));
M(mask) = NaN;
B = B .* M;
end
toc % Elapsed time is 1.173872 seconds.
B = X;
tic;
for k = 1:rep
s = size(B);
f = find(mask==1) + (0:s(3)-1) .* (s(1)*s(2));
B(f) = nan;
end
toc % Elapsed time is 2.992846 seconds.

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David Hill
David Hill 2022-2-24
s=size(B);
f=find(mask==1)+(0:s(3)-1).*(s(1)*s(2));
B(f)=nan;
Loïc Reymond
Loïc Reymond 2023-5-29
The most compact way would probably be:
bsxfun(@(x,y) x.*y,mat,mask)
Or alternatively (probably slower):
mat.*repmat(mask,1,1,size(mat,3))
Where mat is your 640x640x20 matrix and mask your masking array.

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