dplyr broadcasting single value per group in mutate

Do this:

data %>% group_by(category) %>%
  mutate(value2 = value[year == 2000])

You could also do it this way:

data %>% group_by(category) %>%
  arrange(year) %>%
  mutate(value2 = value[1])

or

data %>% group_by(category) %>%
  arrange(year) %>%
  mutate(value2 = first(value))

or

data %>% group_by(category) %>%
  mutate(value2 = nth(value, n = 1, order_by = "year"))

or probably several other ways.

Your attempt with mutate(value = filter(data, year==2002)) doesn’t make sense for a few reasons.

1. When you explicitly pass in data again, it’s not part of the chain that got grouped earlier, so it doesn’t know about the grouping.

2. All dplyr verbs take a data frame as first argument and return a data frame, including filter. When you do value = filter(...) you’re trying to assign a full data frame to the single column value.