sum multiple columns by group with tapply

Question

I wanted to sum individual columns by group and my first thought was to use tapply. However, I cannot get tapply to work. Can tapply

Answer 1

I looked at the source code for by, as EDi suggested. That code was substantially more complex than my change to the one line in tapply. I have now found that my.tapply does not work with the more complex scenario below where apples and cherries are summed by state and county. If I get my.tapply to work with this case I can post the code here later:

df.2 <- read.table(text = '

    state   county   apples   cherries   plums
       AA        1        1          2       3
       AA        1        1          2       3
       AA        2       10         20      30
       AA        2       10         20      30
       AA        3      100        200     300
       AA        3      100        200     300

       BB        7       -1         -2      -3
       BB        7       -1         -2      -3
       BB        8      -10        -20     -30
       BB        8      -10        -20     -30
       BB        9     -100       -200    -300
       BB        9     -100       -200    -300

', header = TRUE, stringsAsFactors = FALSE)

# my function works

   tapply(df.2$apples  , list(df.2$state, df.2$county), function(x) {sum(x)})
my.tapply(df.2$apples  , list(df.2$state, df.2$county), function(x) {sum(x)})

# my function works

   tapply(df.2$cherries, list(df.2$state, df.2$county), function(x) {sum(x)})
my.tapply(df.2$cherries, list(df.2$state, df.2$county), function(x) {sum(x)})

# my function does not work

my.tapply(df.2[,3:4], list(df.2$state, df.2$county), function(x) {colSums(x)})