Extract a specific combination from all possible combinations in R

Question

Given the following vector of length n=10:

# [1] 0 0 0 0 0 1 0 0 1 0
vec <- c(0L, 0L, 0L, 0L, 0L, 1L, 0L, 0L, 1L, 0L)

Suppose we want to apply the operator & to all possible combinations of the vector elements. That is:

res <- combn(n, 2, function(v) vec[v[1]]&vec[v[2]])
# total number of possible combinations is n(n-1)/2 = 45

However, we know that the result of & operator is true only when both values are 1 and hence, we can do it in a more clever way. In fact, the result of combn is a vector of length 45 with only one value of 1 and the rest are just 0.

Now the question is that how we can extract the position of the only 1 without applying the combn itself? Once we can get it, we generate the result simply as follow:

res <- rep(0, 45)
res[index] <- 1

where index is the questioned position.

How is `n` in the `combn` function defined? – Jaap Sep 15 '16 at 09:20 — Jaap, Sep 15 '16 at 09:20
@Procrastinatus Maximus: `n = 10` in the first line. – J_F Sep 15 '16 at 09:21 — J_F, Sep 15 '16 at 09:21

Sandipan Dey · Answer 1 · 2016-09-15T09:32:34.467

0

How about this (assuming that only two positions of the vector are 1, rest are zeros):

vec <- c(0L, 0L, 0L, 0L, 0L, 1L, 0L, 0L, 1L, 0L)

n <- length(vec)
pos <- which(vec==1)
index = ifelse(pos[1]==1, 0, sum((n-1):(n-pos[1]+1))) + (pos[2] - pos[1])
index
#[1] 38

res <- combn(n, 2, function(v) vec[v[1]]&vec[v[2]])
which(res==TRUE)
#[1] 38

edited Sep 15 '16 at 09:32

answered Sep 15 '16 at 09:26

Sandipan Dey

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Thanks @ Ronak, missed a line – Sandipan Dey Sep 15 '16 at 09:32
Thanks but this solution only works for `vec`. Isn't it? – 989 Sep 15 '16 at 09:46
It should work for any vec of any length containing exactly 2 ones at any positions and rest all zeros, I thought you wanted this. – Sandipan Dey Sep 15 '16 at 11:01
containing exactly 2 ones, strong assumption. I considered this example for the purpose of simplicity. – 989 Sep 15 '16 at 11:15

Extract a specific combination from all possible combinations in R

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