[R] how can I delete those columes with the same element in every row?

[bbslover]

e.g.  

a=
  a b c d e
1 1 1 3 1 1
2 1 2 3 4 5
3 1 3 3 8 3
4 1 4 3 3 5
5 1 1 3 1 1        I want to delete  colume a  and colume c, because they
have the same values in every row, then ,I want to get this data.frame .

b=
  b d  e
1 1 1  1
2 2 4  5
3 3 8  3
4 4 3  5
5 1 1  1        the following is my code but it's wrong.

rm(list=ls())
a=c(1,1,1,1,1); b=c(1,2,3,4,1); c=c(3,3,3,3,3); d=c(1,4,8,3,1);
e=c(1,5,3,5,1) 
data.f=data.frame(a,b,c,d,e)
origin.data<-data.f
dim.frame=dim(data.f)
rn=dim.frame[1] 
n<-0
for (k in 1:(dim.frame[2]-n)) 
{if (data.f[1,k]==data.f[rn,k]) 
  {data.f<-data.f[,-k]
   n<-n+1
   k<-k-1
   }
}
origin.data
data.f  


how can i modify it and obtain my wanted result.

thank you!
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