[R] newbie question about index

[z s]

Hi,

  I am trying to convert a variable a = sample(1:3,100,rep = T) represents 
choices into a 3X100 dummy varible b with corresponding element set to 1 
otherwise 0.
eg.

a: 1 3 2 1 2 3 1 1....

b: 1 0 0 1 0 0 1 1..
    0 0 1 0 1 0 0 0...
    0 1 0 0 0 1 0 0...

 Is there something like b[a] =1 existing? I could not figure this out myself.

                
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