r - how to select previous rows in data frame? -

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let's have data frame:

df <- data.frame(a=1:5, b=4:8)

how can subset previous rows? example, if @ row 3, want values of row 1 , row 2, if @ row 4, want values row 2 , row 3. how can in r?

using zoo library (be careful, base lag works differently):

library(zoo) df <- zoo(df) df[lag(df$b, -1) == 4, ]    b 2 2 5  df[lag(df$b, -2) == 4, ]    b 3 3 6

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