R help - Nov 2010 - avoiding too many loops - reshaping data

Hello!

I have a data frame like this one:

mydf<-data.frame(city=c("a","a","a","a","a","a","a","a","b","b","b","b","b","b","b","b"),
 
brand=c("x","x","y","y","z","z","z","z","x","x","x","y","y","y","z","z"),
  value=c(1,2,11,12,111,112,113,114,3,4,5,13,14,15,115,116))
(mydf)

What I need to get is a data frame like the one below - cities as
rows, brands as columns, and the sums of the "value" within each
city/brand combination in the body of the data frame:

city x   y    z
a    3   23  336
b    7   42  231


I have written a code that involves multiple loops and subindexing -
but it's taking too long.
I am sure there must be a more efficient way of doing it.

Thanks a lot for your hints!


-- 
Dimitri Liakhovitski
Ninah Consulting
www.ninah.com

Hadley's reshape package (google for it) can
do this. There's a nice intro on the site.

 > library(reshape)

 > cast(melt(mydf, measure.vars = "value"), city ~ brand,
       fun.aggregate = sum)

   city  x  y   z
1    a  3 23 450
2    b 12 42 231

Although the numbers differ slightly?

I've heard of the reshape2 package, but have no idea
if that's replaced the reshape package yet.

--Erik


Dimitri Liakhovitski wrote:
> Hello!
> 
> I have a data frame like this one:
> 
>
mydf<-data.frame(city=c("a","a","a","a","a","a","a","a","b","b","b","b","b","b","b","b"),
>  
brand=c("x","x","y","y","z","z","z","z","x","x","x","y","y","y","z","z"),
>   value=c(1,2,11,12,111,112,113,114,3,4,5,13,14,15,115,116))
> (mydf)
> 
> What I need to get is a data frame like the one below - cities as
> rows, brands as columns, and the sums of the "value" within each
> city/brand combination in the body of the data frame:
> 
> city x   y    z
> a    3   23  336
> b    7   42  231
> 
> 
> I have written a code that involves multiple loops and subindexing -
> but it's taking too long.
> I am sure there must be a more efficient way of doing it.
> 
> Thanks a lot for your hints!
> 
>

Try this:

 xtabs(value ~ city + brand, mydf)

On Wed, Nov 3, 2010 at 6:23 PM, Dimitri Liakhovitski <
dimitri.liakhovitski@gmail.com> wrote:
> Hello!
>
> I have a data frame like this one:
>
>
>
mydf<-data.frame(city=c("a","a","a","a","a","a","a","a","b","b","b","b","b","b","b","b"),
> 
brand=c("x","x","y","y","z","z","z","z","x","x","x","y","y","y","z","z"),
>  value=c(1,2,11,12,111,112,113,114,3,4,5,13,14,15,115,116))
> (mydf)
>
> What I need to get is a data frame like the one below - cities as
> rows, brands as columns, and the sums of the "value" within each
> city/brand combination in the body of the data frame:
>
> city x   y    z
> a    3   23  336
> b    7   42  231
>
>
> I have written a code that involves multiple loops and subindexing -
> but it's taking too long.
> I am sure there must be a more efficient way of doing it.
>
> Thanks a lot for your hints!
>
>
> --
> Dimitri Liakhovitski
> Ninah Consulting
> www.ninah.com
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Henrique Dallazuanna
Curitiba-Paraná-Brasil
25° 25' 40" S 49° 16' 22" O

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