Create a Row count by ID in R

Question

I have ID in my table I need to create a Count column based on ID.

ID Count 
123 1
123 2
123 3
456 1
789 1
789 2

Kindly suggest me a code. Thanks in advance!

Answer 1

You can use ave with seq_along to get a row counter per ID.

x$Count <- ave(integer(nrow(x)), x$ID, FUN=seq_along)
x
#   ID Count
#1 123     1
#2 123     2
#3 123     3
#4 456     1
#5 789     1
#6 789     2

Data:

x <- structure(list(ID = c(123L, 123L, 123L, 456L, 789L, 789L), Count = c(1L, 
2L, 3L, 1L, 1L, 2L)), row.names = c(NA, -6L), class = "data.frame")