[R] Help summarizing R data frame

[chris99]

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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