Problem assigning multiple values simultaneously to a matrix

Question

Chris 2011-9-7

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https://ww2.mathworks.cn/matlabcentral/answers/15317-problem-assigning-multiple-values-simultaneously-to-a-matrix

Hello,

I have attached some code for reference;

flag=zeros([10 10]);
flag(5,5)=1;
flag(5,9)=1;%Initialize Flag with 2 1's
temp=find(flag);%Find index of where the ones are, gives a 2x1 vector for both indecies
[I,J]=ind2sub(size(flag),temp);%Convert to columns and rows giving 2 2x1 vectors
flag((I-2):I,(J-2):J)=1;
flag((I-2):I,J:(J+2))=1;
flag(I:(I+2),(J-2):J)=1;
flag(I:(I+2),J:(J+2))=1;%Make flag values within 5x5 window 1, but it only does it for one of the flaged values?

My question is what is an efficient way to to this to ALL of the indecies in I and J? right now it only does it for the first one,

Thanks,

Chris

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Fangjun Jiang 2011-9-7

use [I,J,V]=find(flag)

Fangjun Jiang 2011-9-7

Pay attention that +2/-2 might exceed the size of the matrix.

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Answer 1

Oleg Komarov 2011-9-7

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https://ww2.mathworks.cn/matlabcentral/answers/15317-problem-assigning-multiple-values-simultaneously-to-a-matrix#answer_20779

在 MATLAB Online 中打开

double(logical(conv2(flag,ones(5),'same')))

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Chris 2011-9-7

You are very smart ;)

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Answer 2

David Young 2011-9-7

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在 MATLAB Online 中打开

You can use a loop, like this

% data
flag=zeros([10 10]);
flag(5,5)=1;
flag(5,9)=1;%Initialize Flag with 2 1's
%%find indices
temp=find(flag);%Find index of where the ones are, gives a 2x1 vector for both indecies
[I,J]=ind2sub(size(flag),temp);%Convert to columns and rows giving 2 2x1 vectors
% update array
for k = 1:length(I)
    flag(I(k)-2:I(k)+2, J(k)-2:J(k)+2) = 1;
end

Alternatively, if you have the Image Processing Toolbox, you can just do

% data
flag=zeros([10 10]);
flag(5,5)=1;
flag(5,9)=1;%Initialize Flag with 2 1's
% dilate
flag = imdilate(flag, ones(5));

Note that the first method, with an explicit assignment, will grow the array to 11 columns in order to put ones in a 5x5 region round the (5,9) element. The second method will return an array the same size as the original.