10.12 Lag Variable Calculations

20220523 A common task in wrangling a dataset is to calculate the difference between a current value and that from the previous row. The function data.table::lag() can be used to do this, in combination with data.table::first() to get the first value.

"gas_consumption.csv" %>%
  read_csv(show_col_types=FALSE) %T>%
  mutate(diff=cost-lag(cost, default=first(cost))) %>%
  mutate(elapsed=timestamp-lag(timestamp, default=first(timestamp))) %>%
  ...

When a dplyr::group_by() operation is in effect the data.table::first() is determined as the first within the group.

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