Recycling describes the concept of repeating elements of one vector to match the size of another. There are two rules that underlie the "tidyverse" recycling rules:

• Vectors of size 1 will be recycled to the size of any other vector

• Otherwise, all vectors must have the same size

## Examples

Vectors of size 1 are recycled to the size of any other vector:

tibble(x = 1:3, y = 1L)
#> # A tibble: 3 x 2
#>       x     y
#>   <int> <int>
#> 1     1     1
#> 2     2     1
#> 3     3     1

This includes vectors of size 0:

tibble(x = integer(), y = 1L)
#> # A tibble: 0 x 2
#> # ... with 2 variables: x <int>, y <int>

If vectors aren't size 1, they must all be the same size. Otherwise, an error is thrown:

tibble(x = 1:3, y = 4:7)
#> Error:
#> ! Tibble columns must have compatible sizes.
#> * Size 3: Existing data.
#> * Size 4: Column y.
#> i Only values of size one are recycled.

## vctrs backend

Packages in r-lib and the tidyverse generally use vec_size_common() and vec_recycle_common() as the backends for handling recycling rules.

• vec_size_common() returns the common size of multiple vectors, after applying the recycling rules

• vec_recycle_common() goes one step further, and actually recycles the vectors to their common size

vec_size_common(1:3, "x")
#> [1] 3

vec_recycle_common(1:3, "x")
#> [[1]]
#> [1] 1 2 3
#>
#> [[2]]
#> [1] "x" "x" "x"

vec_size_common(1:3, c("x", "y"))
#> Error:
#> ! Can't recycle ..1 (size 3) to match ..2 (size 2).

## Base R recycling rules

The recycling rules described here are stricter than the ones generally used by base R, which are:

• If any vector is length 0, the output will be length 0

• Otherwise, the output will be length max(length_x, length_y), and a warning will be thrown if the length of the longer vector is not an integer multiple of the length of the shorter vector.

We explore the base R rules in detail in vignette("type-size").