R- Collapse rows and sum the values in the column

Using data.table

setDT(df1)[,  lapply(.SD, sum) , by = ID, .SDcols = "PSM" ]

In base:

aggregate(PSM ~ ID, data=x, FUN=sum)
##    ID PSM
## 1 ABC   2
## 2 CCC  58
## 3 DDD  56
## 4 EEE  80
## 5 FFF   1
## 6 GGG  90
## 7 KOO  45
## 8 LLL   4
## 9 ZZZ   8

Example using dplyr, the next iteration of plyr:

df2 <- df1 %>% group_by(ID) %>%
     summarize(Sum_PSM = sum(PSM))

When you put the characters %>%, you are "piping." This means you're inputting what is on the left side of that pipe operator and performing the function on the right.

This is super easy using the plyr package:

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