R中的条件最大值（dplyr）？

Question

This seems like a simple/common question, but I've had difficulty finding relevant answers online.

I want to generate conditional max values from an existing data frame.

A simplistic version of the dataframe I'm working with looks like this:

ID    Phase   Session    Trial
A1    Train1     1         1
A1    Train1     1         2
A1    Train1     2         1
A1    Train1     2         2
A1    Train2     3         1
A1    Train2     3         2
B1    Train1     1         1
B1    Train2     2         1
B2    Train2     2         2
B1    Train2     3         1
B1    Train2     3         2

What I want to know is, for each subject, what was the max "Session" value for each "Phase". Ideally, I'd like to generate another dataframe that looks like this:

ID    Phase   MaxSession
A1    Train1      2         
A1    Train2      3         
B1    Train1      1         
B1    Train2      3      

I've tried the below code in dplyr, but it doesn't generate the output I'm looking for.

MaxSessions <- DataFrame %>%
   count(ID,Phase,Session)%>%
   mutate(MaxSession = max(Session))

Does anyone have ideas on what I'm doing wrong? Or what code might resolve my issue? Solutions with or without dplyr are fine.

Thanks!

Answer 1

You need to use group_by and summarise rather than count and mutate :

MaxSessions <- DataFrame %>%
   group_by(ID,Phase)%>%
   summarise(MaxSession = max(Session))


# A tibble: 5 x 3
# Groups:   ID [?]
  ID    Phase  MaxSession
  <fct> <fct>       <dbl>
1 A1    Train1       2.00
2 A1    Train2       3.00
3 B1    Train1       1.00
4 B1    Train2       3.00
5 B2    Train2       2.00