I have following data for which I need to calculate the weighted aggregate value of the parameter at each time.

Date,Parameter,Weight
2012-01-31,90,200
2012-01-31,80,400
2012-01-31,70,500
2012-01-31,60,800
2012-02-29,120,220
2012-02-29,110,410
2012-02-29,75,520
2012-02-29,65,840
2012-03-31,115,210
2012-03-31,100,405
2012-03-31,70,500
2012-03-31,60,800

So for the above sample the solution looks like:

Date,Weighted Parameter
2012-01-31,70
2012-02-29,82.96482412
2012-03-31,77.10182768

Could I potentially use tapply / aggregate for this?  Would like to avoid a for loop if possible.

Thank you!




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