find value closest to x by group in dplyr [duplicate]

Answer 1

Not too far from what you had

a %>% group_by(id) %>% summarise(which.min(abs(b-1.43)))
# A tibble: 2 × 2
#      id `which.min(abs(b - 1.43))`
#                         
# 1     A                          2
# 2     B                          3

Or if you need the values, rather than the indices:

a %>% group_by(id) %>% summarise(b[which.min(abs(b-1.43))])
# A tibble: 2 × 2
#      id `b[which.min(abs(b - 1.43))]`
#                            
# 1     A                           1.5
# 2     B                           1.4