Question

for a data frame as follow

id<-c(1,1,2,3,3,2)
p<-c(10,0,22,34,0,0)
df<-data.frame(id,p)

I need a summary table like

id   p
1    10
2    22
3    34

which is actually aggregated summation of p for each id.
can I get this with ‘dcast’?

Answer 1

Why do you need dcast? Try aggregate in base R:

df <- data.frame(id = c(1,1,2,3,3,2),
                 p = c(10,0,22,34,0,0))
aggregate(df, by=list(id), sum)
#   Group.1 id  p
# 1       1  2 10
# 2       2  4 22
# 3       3  6 34

You can also trick dcast as below. Your data is not quite in the expected “molten” form for dcast:

library(reshape2)
dcast(df, id ~ ., value.var = "p", sum)
#   id NA
# 1  1 10
# 2  2 22
# 3  3 34