[R] diff in a dataframe

[Don MacQueen]

So, what's the easiest way to add a column to a dataframe? Just do it.

Here is a really simple example to illustrate:

>  foo <- data.frame(x=1:4,y=2:5)
>  foo
   x y
1 1 2
2 2 3
3 3 4
4 4 5
>  foo$z <- c(NA,diff(foo$x))
>  foo
   x y  z
1 1 2 NA
2 2 3  1
3 3 4  1
4 4 5  1


Solutions using apply and sapply are overly complicated for what you requested.
If you had to do this for many, many columns a looping solution would 
be worth it, but for just two columns, it's not.

-Don



At 12:46 PM +0530 1/10/08, Vishal Belsare wrote:
>I have a dataframe say:
>
>date   price_g   price_s
>          0.34        0.56
>          0.36        0.76
>            .              .
>            .              .
>            .              .
>
>and so on. say, 1000 rows.
>
>Is it possible to add two columns to this dataframe, by computing say
>diff(log(price_g) and diff(log(price_s)) ?
>
>The elements in the first row of these columns cannot be computed, but
>can I coerce this to happen and assign a missing value there? It would
>be really great if I could do that, because in this case I don't have
>to re-index my transformed series to the dates again in a new
>dataframe.
>
>Thanks in anticipation.
>
>
>Vishal Belsare
>
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-- 
--------------------------------------
Don MacQueen
Environmental Protection Department
Lawrence Livermore National Laboratory
Livermore, CA, USA
925-423-1062