A general question about intersect

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David McVea
David McVea 2019-11-10
评论： David McVea 2019-11-10
Hello -
I have a general question. I can provide specifics if needed.
How is it possible for
size(intersect(MyData(i),MyData))
to be not equal to
size(i)
Put another way, how can the intersect of a subset of a vector and the original vector not be the entire subset of the vector? I seem to have a case where it is and I can't understand how that can be. It seems impossible, by defintion.
Thanks.

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采纳的回答

Walter Roberson
Walter Roberson 2019-11-10
  1. Duplicates in the data. An element that occurs more than once among the MyData(i) will only result in one output
  2. Duplicates in the i values have the same effect.
  3. NaN == NaN is never true, so the results of intersect() remove all NaN

更多回答（1 个）

Image Analyst
Image Analyst 2019-11-10
Because one element, MyData(i), does not necessarily match every single element of MyData. For example 2 will match only 1 element of this three element vector [1, 2, 3]. It matches only element #2 and does not match element #1 or element #3 so the length will be only 1, NOT 3.
  1 个评论
David McVea
David McVea 2019-11-10
Thanks for your reply - I'm not sure this is exactly what I'm asking. from your example,
length(2) is 1, and length(intersect(2,[1 2 3]) is 1. So length(MyData(i)) equals length(i)
I think you are answering how
length(intersect(MyData(i),MyData))
can be different from
length (MyData)
I'm asking how
length(intersect(MyData(i),MyData))
can be different from
length(i)
Thanks.

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