Help summarizing R data frame

Nice thing about R is there are a number of ways to do things:
x
identifier quantity
1          1       10
2          1       20
3          2       30
4          1       15
5          2       10
6          3       20
require(sqldf)
sqldf('select identifier, sum(quantity) as quantity from x group by identifier')
identifier quantity
1          1       45
2          2       40
3          3       20

or using 'data.table'
require(data.table)
Loading required package: data.table
x <- data.table(x)
x[, sum(quantity), by = identifier]
identifier V1
[1,]          1 45
[2,]          2 40
[3,]          3 20
I am trying to aggregate data in column 2 to identifiers in col 1

eg..

take this>

identifier ? ? ? quantity
1 ? ? ? ? ? ? ? ? ? ? 10
1 ? ? ? ? ? ? ? ? ? ? 20
2 ? ? ? ? ? ? ? ? ? ? 30
1 ? ? ? ? ? ? ? ? ? ? 15
2 ? ? ? ? ? ? ? ? ? ? 10
3 ? ? ? ? ? ? ? ? ? ? 20

and make this>

identifier ? ? ? ? quantity
1 ? ? ? ? ? ? ? ? ? ?45
2 ? ? ? ? ? ? ? ? ? ?40
3 ? ? ? ? ? ? ? ? ? ?20

Thanks in advance for your help!
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Help summarizing R data frame

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