Using R to reshape data

Sometimes marker data are sent to us in this format, where each row only contains one genotype for a particular marker and person: 

 
ID      Marker  A1      A2 
1       M1      1       2 
2       M1      2       2 
3       M1      2       2 
1       M2      1       1 
3       M2      1       2 

 
However, in order to put this marker data in LINKAGE-format, we need to reshape the data so that each row contains all the marker data for a specific person. This can easily be done using the ‘reshape’ command in R: 
 

 
> a < - read.table("marker.txt",header=T) 
> a 
  ID Marker A1 A2 
1  1     M1  1  2 
2  2     M1  2  2 
3  3     M1  2  2 
4  1     M2  1  1 
5  3     M2  1  2 
> attach(a) 
> b < - reshape(a,idvar="ID",direction="wide",timevar="Marker") 
> b 
  ID A1.M1 A2.M1 A1.M2 A2.M2 
1  1     1     2     1     1 
2  2     2     2    NA    NA 
3  3     2     2     1     2