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Matt Munson
Matt Munson

Reputation: 3003

index from one vector to another by closest values

Given two sorted vectors, how can you get the index of the closest values from one onto the other.

For example, given:

a = 1:20
b = seq(from=1, to=20, by=5)

how can I efficiently get the vector

c = (1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4)

which, for each value in a, provides the index of the largest value in b that is less than or equal to it. But the solution needs to work for unpredictable (though sorted) contents of a and b, and needs to be fast when a and b are large.

Upvotes: 2

Views: 102

Answers (2)

Ronak Shah
Ronak Shah

Reputation: 388862

We can use cut

as.integer(cut(a, breaks = unique(c(b-1, Inf)), labels = seq_along(b)))

#[1] 1 1 1 1 1 2 2 2 2 2 3 3 3 3 3 4 4 4 4 4

Upvotes: 1

Joris C.
Joris C.

Reputation: 6234

You can use findInterval, which constructs a sequence of intervals given by breakpoints in b and returns the interval indices in which the elements of a are located (see also ?findInterval for additional arguments, such as behavior at interval boundaries). 

a = 1:20
b = seq(from = 1, to = 20, by = 5)

findInterval(a, b)
#>  [1] 1 1 1 1 1 2 2 2 2 2 3 3 3 3 3 4 4 4 4 4

Upvotes: 4

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