Add an index (or counter) to a dataframe by group in R

Question

I have a df like

ProjectID Dist
  1        x
  1        y
  2        z
  2        x
  2        h
  3        k
  ....     ....

I want to add a third column such that we have an incrementing counter for each ProjectID:

ProjectID Dist counter
  1        x     1
  1        y     2
  2        z     1
  2        x     2
  2        h     3
  1        k     3
  ....     ....

I've had a look at seq rank and a couple of other bits particularly looking to see if I could use ddply to help:

df$counter <- ddply(df,.(projectID), function(x).....? )

I think I could adapt this answer How to create a counter/numeration by group? but would prefer something using something like ddply (I can't find an equivalent of cumsum but I think that's the same principle here: Create ascending series of integers by group in Pandas ). That'd let me index occurrences in a list (and e.g. merge on this).

Solution

A dplyr solution is quite simple:

library(dplyr)

df %>% group_by(ProjectID) %>% mutate(counter = row_number(ProjectID))


#  ProjectID Dist counter
#1         1    x       1
#2         1    y       2
#3         2    z       1
#4         2    x       2
#5         2    h       3
#6         1    k       3

Comments

[akrun]

mutate(counter=row_number()) should do it.

[sjgknight]

This is probably a stupid question...what's %>% do? (And slightly tangential, is there a way to effectively search [google] for that type of code?)

[jalapic]

%>% is a pipe or chain operator... it works like this: mydata %>% do_something_with_it %>% do_something_else - it simply enables you to chain together functions.

[sjgknight]

Ah great, very interesting - thanks!