Collapse data frame, adding values for factor [duplicate]

Answer 1

If you don't want to load a package:

df <- read.table(text="  sample value
1      a     1
2      a     2
3      b     3
4      b     4
5      b     5
6      c     6",header=TRUE)

aggregate(df$value,by=list(sample=df$sample),sum)

  sample  x
1      a  3
2      b 12
3      c  6

Or if you prefer formula syntax:

aggregate(value ~ sample, df, sum)

  sample value
1      a     3
2      b    12
3      c     6