[R] avoiding too many loops - reshaping data

[Dimitri Liakhovitski]

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