Aggregate multiple rows of the same data.frame in R based on common values in given columns

Question

I have a data.frame that looks like this:

# set example data
df <- read.table(textConnection(\"item\\tsize\         


        

Answer 1

df$value <- ave(df$value,df$item,FUN=mean)
df[!duplicated(df$item),]

  item size weight value
1    A    2      3     5
3    B    1      2     3
4    C    3      2     1

Answer 2

Here is the solution using the ddply from plyr package:

library(plyr)
ddply(df,.(item),colwise(mean))
  item size weight value
1    A    2      3     5
2    B    1      2     3
3    C    3      2     1

Answer 3

Nowadays, this is what I would do:

library(dplyr)

df %>%
  group_by(item, size, weight) %>%
  summarize(value = mean(value)) %>%
  ungroup

This yields the following result:

# A tibble: 3 x 4
   item  size weight value
  <chr> <int>  <int> <dbl>
1     A     2      3     5
2     B     1      2     3
3     C     3      2     1

I will leave the accepted answer as such as I specifically asked for aggregate, but I find the dplyr solution the most readable.

Answer 4

aggregate(value ~ item + size + weight, FUN = mean, data=df)

  item size weight value
1    B    1      2     3
2    C    3      2     1
3    A    2      3     5

Answer 5

The data.table solution...

require(data.table)
DT <- data.table(df)

DT[ , lapply(.SD , mean ) , by = item ]
   item size weight value
1:    A    2      3     5
2:    B    1      2     3
3:    C    3      2     1