Lead and lag.

Find the "next" or "previous" values in a vector. Useful for comparing values ahead of or behind the current values.

lead(x, n = 1L, default = NA, order_by = NULL, ...)

lag(x, n = 1L, default = NA, order_by = NULL, ...)

Arguments

x	a vector of values
n	a positive integer of length 1, giving the number of positions to lead or lag by
default	value used for non-existent rows. Defaults to NA.
order_by	override the default ordering to use another vector
...	Needed for compatibility with lag generic.

Examples

lead(1:10, 1)
#>  [1]  2  3  4  5  6  7  8  9 10 NA
lead(1:10, 2)
#>  [1]  3  4  5  6  7  8  9 10 NA NA

lag(1:10, 1)
#>  [1] NA  1  2  3  4  5  6  7  8  9
lead(1:10, 1)
#>  [1]  2  3  4  5  6  7  8  9 10 NA

x <- runif(5)
cbind(ahead = lead(x), x, behind = lag(x))
#>          ahead         x    behind
#> [1,] 0.3789607 0.6390610        NA
#> [2,] 0.3595307 0.3789607 0.6390610
#> [3,] 0.8442314 0.3595307 0.3789607
#> [4,] 0.4567492 0.8442314 0.3595307
#> [5,]        NA 0.4567492 0.8442314

# Use order_by if data not already ordered
df <- data.frame(year = 2000:2005, value = (0:5) ^ 2)
scrambled <- df[sample(nrow(df)), ]

wrong <- mutate(scrambled, prev = lag(value))
arrange(wrong, year)
#>   year value prev
#> 1 2000     0    4
#> 2 2001     1   NA
#> 3 2002     4   16
#> 4 2003     9   25
#> 5 2004    16    1
#> 6 2005    25    0

right <- mutate(scrambled, prev = lag(value, order_by = year))
arrange(right, year)
#>   year value prev
#> 1 2000     0   NA
#> 2 2001     1    0
#> 3 2002     4    1
#> 4 2003     9    4
#> 5 2004    16    9
#> 6 2005    25   16