Replace certain elements in an array

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MK96 2017-2-11

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https://ww2.mathworks.cn/matlabcentral/answers/324504-replace-certain-elements-in-an-array

评论： MK96 2017-2-11

The following code generates a 100x1 array of random 1s and -1s:

N = 100;
States = 2*randi(2,N,1) - 3;

Once generated, is there an easy way I can change some of the -1s to 1s whilst keeping the other elements the same?

e.g. for a 4x1 example if I had [1; -1; -1; -1] and wanted to change any 2 of the -1 elements to 1s: [1; -1; 1; 1], how could I do this?

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Answer 1

Image Analyst 2017-2-11

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https://ww2.mathworks.cn/matlabcentral/answers/324504-replace-certain-elements-in-an-array#answer_254361

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Try this:

N = 10; % Number of elements.
States = 2*randi(2,N,1) - 3
% Get the minus 1 element indexes
minusOneIndexes = find(States == -1)
% Get two random indexes:
indexes = randperm(length(minusOneIndexes), 2);
% Set them to +1
indexesToChange = minusOneIndexes(indexes)
States(indexesToChange) = 1