Beginner tips on using plyr to calculate year-over-year change across groups

问题:

I am new to plyr (and R) and looking for a little help to get started. Using the baseball dataset as an exaple, how could I calculate the year-over-year (yoy) change in "at batts" by league and team (lg and team)?

library(plyr) df1 

After doing a little aggregating to simplify the data fame, the data looks like this:

head(df1)    year lg team   ab   1884 UA  ALT  108   1997 AL  ANA 1703   1998 AL  ANA 1502   1999 AL  ANA  660   2000 AL  ANA   85   2001 AL  ANA  219 

I would like to end up with someting like this

  year lg team   ab yoy   1997 AL  ANA 1703  NA   1998 AL  ANA 1502  -201   1999 AL  ANA  660  -842   2000 AL  ANA   85  -575   2001 AL  ANA  219  134 

I started by writign the following function, which I think is wrong:

yoy.func 

Without sucess, I used the following code to attempt return the yoy change.

df2 

Any guidance woud be appreciated.

Thanks

回答1:

I know you asked for a "plyr"-specific solution, but for the sake of sharing, here is an alternative approach in base R. In my opinion, I find the base R approach just as "readable". And, at least in this particular case, it's a lot faster!

output 

Update: data.table

If your data are very large, check out data.table. Even with this example, you'll find a good speedup in relative terms. Plus the syntax is super compact and, in my opinion, easily readable.

library(plyr) df1 

Now, look at this concise solution:

DT[, yoy := c(NA, diff(ab)), by = "team,lg"] DT #       year lg team   ab  yoy #    1: 1884 UA  ALT  108   NA #    2: 1997 AL  ANA 1703   NA #    3: 1998 AL  ANA 1502 -201 #    4: 1999 AL  ANA  660 -842 #    5: 2000 AL  ANA   85 -575 #   ---                        # 2523: 1895 NL  WSN  839  290 # 2524: 1896 NL  WSN  982  143 # 2525: 1897 NL  WSN 1426  444 # 2526: 1898 NL  WSN 1736  310 # 2527: 1899 NL  WSN  787 -949 

回答2:

How about using diff():

df 

文章来源: Beginner tips on using plyr to calculate year-over-year change across groups