Hi Rens,

One way would be

x$difference <- do.call(c, with(x, tapply(amount, customer, function(x) c(0,
diff(x)))))
x

Take a look at ?tapply and ?aggregate for more information.

HTH,
Jorge


On Wed, Mar 9, 2011 at 10:27 AM, rens_1112 <> wrote:

> Dear all,
>
> Probably a rather stupid question, but I couldn't find the answer..
>
> I currently have a dataframe with customers' id's and purchase amount, what
> I would like to do is to show the difference between amount purchased
> compared to the month before per customer.
>
> I have made some attempts myself using diff, but what i can not get to work
> is that each different customer starts with a 0.
>
> This is an example of what I have:
>
> >purchase_amount
> customer        amount
> 123             6
> 123             5
> 123             9
> 123             8
> 123             11
> 123             7
> 230             19
> 230             18
> 230             12
> 230             17
> 380             7
> 380             9
> 380             2
> 380             7
> 380             8
> 380             8
> 380             6
> 380             8
>
> How I would like it to look like:
>
> customer        amount  difference
> 123             6               0
> 123             5               -1
> 123             9               4
> 123             8               -1
> 123             11              3
> 123             7               -4
> 230             19              0
> 230             18              -1
> 230             12              -6
> 230             17              5
> 380             7               0
> 380             9               2
> 380             2               -7
> 380             7               5
> 380             8               1
> 380             8               0
> 380             6               -2
> 380             8               2
>
> I hope my question is clear like this.
> Many thanks in advance
> Rens
>
> --
> View this message in context:
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> Sent from the R help mailing list archive at Nabble.com.
>
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