R- Collapse rows and sum the values in the column

Question

I have the following dataframe (df1):

ID    someText    PSM OtherValues
ABC   c   2   qwe
CCC   v   3   wer
DDD   b   56  ert
EEE   m   78  yu
FFF           


        

Answer 1

This is super easy using the plyr package:

library(plyr)
ddply(df1, .(ID), summarize, Sum=sum(PSM))