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df <- structure(list(x = 1:10, time = c(0.5, 0.5, 1, 2, 3, 0.5, 0.5, 1, 2, 3)), .Names = c("x", "time"), row.names = c(NA, -10L), class = "data.frame") df[df$time %in% c(0.5, 3), ] ## x time ## 1 1 0.5 ## 2 2 0.5 ## 5 5 3.0 ## 6 6 0.5 ## 7 7 0.5 ## 10 10 3.0 df[df$time == c(0.5, 3), ] ## x time ## 1 1 0.5 ## 7 7 0.5 ## 10 10 3.0 What is the difference between %in% and == here?
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What is the Difference Between the == and %in% Operators in R. The %in% operator is used for matching values. “returns a vector of the positions of (first) matches of its first argument in its second”. On the other hand, the == operator, is a logical operator and is used to compare if two elements are exactly equal.
The Equality Operator == Relational operators, or comparators, are operators which help us see how one R object relates to another. For example, you can check whether two objects are equal (equality) by using a double equals sign == .
You can use the following basic syntax to select all elements that are not in a list of values in R: ! (data %in% c(value1, value2, value3, ...))
R operator %in% is handy for working with vectors, but how to use it oppositely? Something like %notin% that will exclude anything that is in a vector. There is no actual %notin% operator in R, but below is the explanation on how to get the desired result.
The problem is vector recycling.
Your first line does exactly what you'd expect. It checks what elements of df$time are in c(0.5, 3) and returns the values which are.
Your second line is trickier. It's actually equivalent to
df[df$time == rep(c(0.5,3), length.out=nrow(df)),] To see this, let's see what happens if use a vector rep(0.5, 10):
rep(0.5, 10) == c(0.5, 3) [1] TRUE FALSE TRUE FALSE TRUE FALSE TRUE FALSE TRUE FALSE See how it returns every odd value. Essentially it's matching 0.5 to the vector c(0.5, 3, 0.5, 3, 0.5...)
You can manipulate a vector to produce no matches this way. Take the vector: rep(c(3, 0.5), 5):
rep(c(3, 0.5), 5) == c(0.5, 3) [1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE They're all FALSE; you are matching every 0.5 with 3 and vice versa.
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In
df$time == c(0.5,3) the c(0.5,3) first gets broadcast to the shape of df$time, i.e. c(0.5,3,0.5,3,0.5,3,0.5,3,0.5,3). Then the two vectors are compared element-by-element.
On the other hand,
df$time %in% c(0.5,3) checks whether each element of df$time belongs to the set {0.5, 3}.
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