R help - Jun 2013 - change factor to mtrix

i wish to change a column of factor variable to multiple columns of
zero-ones

for example, my factor could be

ff=c('a','a','b','b','c','c')

then I want to have two columns (for three levels) that are

0 0
0 0
1 0
1 0
0 1
0 1

how can i do this fast?

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On 06/12/2013 05:47 PM, Gallon Li wrote:
> i wish to change a column of factor variable to multiple columns of
> zero-ones
>
> for example, my factor could be
>
>
ff=c('a','a','b','b','c','c')
>
> then I want to have two columns (for three levels) that are
>
> 0 0
> 0 0
> 1 0
> 1 0
> 0 1
> 0 1
>
> how can i do this fast?
Hi Gallon,
If you want exactly the output shown above, it is not trivial. You could 
convert ff to a factor, then use as.numeric to get:

a = 1
b = 2
c = 3

If you subtract one and display the numbers in two digits of binary:

a = 00
b = 01
c = 10

Then if you apply as.character and strsplit, you can get:

a = "0" "0"
b = "0" "1"
c = "1" "0"

Finally, as.numeric will give you numbers. This does not produce the 
numbers above, but it might give you an idea of what to do.

Jim

On Jun 12, 2013, at 09:47 , Gallon Li wrote:
> i wish to change a column of factor variable to multiple columns of
> zero-ones
> 
> for example, my factor could be
> 
>
ff=c('a','a','b','b','c','c')
> 
> then I want to have two columns (for three levels) that are
> 
> 0 0
> 0 0
> 1 0
> 1 0
> 0 1
> 0 1
> 
> how can i do this fast?
Maybe not fast, but quick:
> fff <- factor(ff)
> model.matrix(~fff)[,-1]
  fffb fffc
1    0    0
2    0    0
3    1    0
4    1    0
5    0    1
6    0    1

Possibly faster, skipping some "red tape":
> CC <- contrasts(fff)
> CC
  b c
a 0 0
b 1 0
c 0 1
> CC[fff,]
  b c
a 0 0
a 0 0
b 1 0
b 1 0
c 0 1
c 0 1


> 
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> 
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