Collapse data frame, adding values for factor [duplicate]

Answer 1

In the spirit of sharing, you can also use the sqldf and data.table packages quite easily:

Your data:

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)

The sqldf alternative:

library(sqldf)
sqldf("select sample, sum(value) `value` from df group by sample")
#   sample value
# 1      a     3
# 2      b    12
# 3      c     6

The data.table alternative:

library(data.table)
DT <- data.table(df, key="sample")
DT[, list(value = sum(value)), by=key(DT)]
#    sample value
# 1:      a     3
# 2:      b    12
# 3:      c     6

Answer 2

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

Answer 3

One solution using R base function is

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

You can also do it with ddply from plyr package

ddply(df,.(sample),numcolwise(sum))

Here df is your data.frame

Answer 4

The "classic" R command is tapply

n <- 17; fac <- factor(rep(1:3, length = n), levels = 1:5)
df=data.frame(target=1:n, factor=fac)
with(df, tapply(target, fac, sum))

by and aggregate both work but the input output as lists or by objects is daft. Also ddply from plyr will work but is somewhat less clear syntactically although it comes into its own for more complicated examples.

Answer 5

I like cast, for these types of problems because it's quick and intuitive:

library(reshape2)
dcast(your_df, sample ~ ., sum) # or just cast with the original reshape

I also like summarize, for this type of question.

library(plyr) 
ddply(df,.(sample),summarise, sum=sum(value))