R-Common elements on all pairwise comparisons of elements of a vector

Question

I have a vector in R, say c(2, 2, 3, 2, 3, 4, 4), and I want to build a square matrix of size n (the number of elements of the vector) that has a 1 if the element i of the vector has the same value of the element j , and 0 otherwise. In this example , the element [1,2] and [1,4] of the matrix must have a 1 because the first, second and fourth elements of the vector are the same. Is there a way to do this ? A command or function to build ? Something with combinations ? I would like to avoid loops like for.

Thank you !

score 4 · Answer 1 · answered Jan 04 '18 at 06:25

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This just came to my mind... Is this what you want?

a <- c(2, 2, 3, 2, 3, 4, 4)
mat <- a%*%t(a)

apply(mat, 2, function(x){as.integer((x/a)==a)})

answered Jan 04 '18 at 06:25

Tino

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Or `(mat/a == a) * 1` – Rich Scriven Jan 04 '18 at 06:40
@RichScriven haha of course, thanks. That makes me feel foolish, but hey, I posted this before my first coffee ;-) – Tino Jan 04 '18 at 06:51

score 3 · Accepted Answer · answered Jan 04 '18 at 06:20

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We can use outer to create a square matrix by comparing each element of the vector with the other elements

+(outer(v1, v1, `==`))

Or use sapply

+(sapply(v1, `==`, v1))

answered Jan 04 '18 at 06:20

akrun

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Ok! Will check it tomorrow :-) – coolsv Jan 04 '18 at 06:26
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Worked perfectly! Thank you!! – coolsv Jan 04 '18 at 16:14

R-Common elements on all pairwise comparisons of elements of a vector

2 Answers2