How to find first element of a group that fulfill a condition

Question

structure(list(group = c(17L, 17L, 17L, 18L, 18L, 18L, 18L, 19L, 
19L, 19L, 20L, 20L, 20L, 21L, 21L, 22L, 23L, 24L, 25L, 25L, 25L, 
26L, 27L, 27L, 27L, 28L), var = c         


        

Answer 1

For ungrouped data, one solution is

first_equal_to = function(x, value)
    (x == value) & (cumsum(x == value) == 1)

so

tbl %>% group_by(group) %>% mutate(first = first_equal_to(var, 1))

(it seems appropriate to keep this as a logical vector, since that is what the column represents).

Another implementation is

first_equal_to2 = function(x, value) {
    result = logical(length(x))
    result[match(value, x)] = TRUE
    result
}