R why "==" operation doesn't return the same as %in%

Question

I have something strange when comparing two vectors:

individuals=c("A","C","X","Z")
m=c("A", "B", "C", "D", "E", "F", "X", "Z")
individuals == m

it return: TRUE FALSE FALSE FALSE FALSE FALSE TRUE TRUE

At the end is for:

which(individuals == m)

which return: 1 7 8

What I am missing?

What would be your expected outcome, perhaps you want `%in%` — user20650, Feb 20 '17 at 11:45
sorry I should have precise, is for which(individuals == m) test. with %in% it's work. — Sosa, Feb 20 '17 at 11:50

score 4 · Accepted Answer · answered Feb 20 '17 at 11:49

The result you get is indeed correct.
R compares each value pairwise:

individuals |   m   |  res
     A      |   A   |  T
     B      |   C   |  F
     C      |   X   |  F
     D      |   Z   |  F

it then recycle m:

individuals |   m   |  res
     E      |   A   |  F
     F      |   C   |  F
     X      |   X   |  T
     Z      |   Z   |  T

If you want to check if an element of m is in individuals use %in%:

m %in% individuals

# TRUE FALSE  TRUE FALSE FALSE FALSE  TRUE  TRUE

akrun · Answer 2 · 2017-02-20T11:52:55.147

2

We can use match

!is.na(match(m, individuals))
#[1]  TRUE FALSE  TRUE FALSE FALSE FALSE  TRUE  TRUE

The reason for the unexpected result is how the == compares the recycled elements i.e. the values in 'individuals' are replicated to the length of the 'm' vector

rep(individuals, length.out = length(m))
#[1] "A" "C" "X" "Z" "A" "C" "X" "Z"

m==rep(individuals, length.out = length(m))
#[1]  TRUE FALSE FALSE FALSE FALSE FALSE  TRUE  TRUE

edited Feb 20 '17 at 11:52

answered Feb 20 '17 at 11:48

akrun

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`m %in% individuals` maybe – user20650 Feb 20 '17 at 11:50
1

@user20650 Thanks, but if found your comment so I left it in case you wanted to answer. Also, somebody else posted – akrun Feb 20 '17 at 11:51
1

fair dos........although, from comment op ^^, perhaps after all the answer is `match(individuals, m)` – user20650 Feb 20 '17 at 11:52

R why "==" operation doesn't return the same as %in%

2 Answers2