1

I have a dataframe that looks like this:

id                              weekdays              halflife
241732222300860000  Friday, Aug 31, 2012, 22    0.4166666667
241689170123309000  Friday, Aug 31, 2012, 19    0.3833333333
241686878137512000  Friday, Aug 31, 2012, 19    0.4
241651117396738000  Friday, Aug 31, 2012, 16    1.5666666667
241635163505820000  Friday, Aug 31, 2012, 15    0.95
241633401382265000  Friday, Aug 31, 2012, 15    2.3666666667

And I would like to get average half life of items that were created on Monday, then on Tuesday...etc. (My date range spans over 6 months). Please let me know how I can give reproducible code, because I couldn't figure out a way to attach files.

To get the date values I used strptime and difftime. Also, I found the maximum halflife with max(df$halflife), how can I find which id it corresponds to?

Reproducible code: 

structure(list(id = c(241732222300860416, 241689170123309056, 
241686878137511936, 241651117396738048, 241635163505819648, 241633401382264832
), weekdays = c("Friday, Aug 31, 2012, 22", "Friday, Aug 31, 2012, 19", 
"Friday, Aug 31, 2012, 19", "Friday, Aug 31, 2012, 16", "Friday, Aug 31, 2012, 15", 
"Friday, Aug 31, 2012, 15"), halflife = structure(c(0.416666666666667, 
0.383333333333333, 0.4, 1.56666666666667, 0.95, 2.36666666666667
), class = "difftime", units = "mins")), .Names = c("id", 
"weekdays", "halflife"), row.names = c(NA, 6L), class = "data.frame")
d12n
  • 841
  • 2
  • 10
  • 20
  • 3
    To put reproducible data on your post, you can use something like `dput(head(dataframe))` on your data frame. More details here : http://stackoverflow.com/questions/5963269/how-to-make-a-great-r-reproducible-example – juba Feb 15 '13 at 15:18
  • Do you want to calculate this mean for every month and within every month per day or just all Mondays mean, Tuesdays mean etc..? – Arun Feb 15 '13 at 15:37
  • 1
    @demt Whay are u using this date format?..your first example was in another format. – agstudy Feb 15 '13 at 15:38
  • I thought it would be easier if I showed the weekday, since I am looking into aggregating based on the day of the week so I converted the original date format to a date format that has "Weekday, Month day, year, hour". I can change it back. – d12n Feb 15 '13 at 15:40
  • Arun: just for every monday in general. So, the latter. – d12n Feb 15 '13 at 15:41

1 Answers1

4

There may be a better way to get the week days, but you can use tapply like this (here df is the name of your data frame) :

days <- sub(",.*$", "", df$weekdays)
tapply(df$halflife, days, mean)

And to get the ids of your maxium values, use which :

df$id[which(df$halflife==max(df$halflife))]
juba
  • 47,631
  • 14
  • 113
  • 118