Replace an element of a vector by also redefining the vector

Question

Konstantinos Tsitsilonis 2017-8-3

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https://ww2.mathworks.cn/matlabcentral/answers/351451-replace-an-element-of-a-vector-by-also-redefining-the-vector

回答： Julie Kraus 2017-8-3

Hi all,

I would like to replace some elements of a vector Vec that have indices idx with NaN, to create a new vector VecNaN. However, using the usual code below

Vec(idx) = NaN;

redefines the vector Vec with the NaN elements. Instead I want to find a new vector VecNaN such that it is the same as Vec, but with the NaN elements in the indices idx.

Thanks for your help in advance,

KMT.

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Answer 1

Harry Vancao 2017-8-3

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https://ww2.mathworks.cn/matlabcentral/answers/351451-replace-an-element-of-a-vector-by-also-redefining-the-vector#answer_276671

You could do the following:

vecNaN = vec; vecNaN(idx) = NaN;

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Jan 2017-8-3

编辑：Jan 2017-8-3

在 MATLAB Online 中打开

+1. This is usually the most efficient solution. You could save the copy of the overwritten elements also, which is easier, when idx is a logical index vector:

x       = rand(1, 1000);
idx     = (x > 0.9);
y(idx)  = NaN;
y(~idx) = x(~idx);

This creates the vector y twice if the last element is not NaN. Better:

if idx(end)
  y(idx)  = NaN;
  y(~idx) = x(~idx);
else
  y(~idx) = x(~idx);
  y(idx)  = NaN;
end

Well, in most cases it is not worth the effort and Harry's suggestion is nice and efficient, although it "wastes" some time with copying elements, which are overwritten directly.

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