For loop on dates in R

R-users, I have this dataframe:

        X.ID_punto   MM.GG.AA  Rad_SWD 
2945377          1 0001-01-06  19.918  
2945378          2 0001-01-06  19.911   
2945379          1 0001-02-06  19.903  
2945380          2 0001-02-06  19.893   
2945381          1 0001-03-06  19.875 
2945382          2 0001-03-06  19.858  

What I need to do is to obtain different subsets for every dates (MM.GG.AA):

subset(M2006, M2006$MM.GG.AA=="0001-10-06" )

or, in other words, different subsets for every sites (X.ID_punto):

subset(M2006, M2006$X.ID_punto==1)

Is it possible to loop this on sites (X.ID_punto) or dates (MM.GG.AA)? I have tried in this way:

 output<- data.frame(ID=rep(1:365))
 for  (p in as.factor(M2006[,1]))  { 
             sub<-  subset(M2006, M2006$X.ID_punto==p )
             output[,p] <- sub$Rad_SWD

the code run, but without looping on every ID. If I can't loop, I have to write down subset(M2006, M2006$X.ID_punto==xxx) for a thousand times... Thank you in advance! Fra


I think from your description of input and desired output you an acheive this pretty simply using the reshape package and the cast function:

cast( M2006 , MM.GG.AA ~ X.ID_punto , value = .(Rad_SWD) )
#   MM.GG.AA      1      2
#1 0001-01-06 19.918 19.911
#2 0001-02-06 19.903 19.893
#3 0001-03-06 19.875 19.858

It will certainly be quicker than using loops ( it isn't going to be the absolute quickest solution but I imagine < 1-2 seconds).

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