[R] to modify a vector

[Mitsuo Igarashi]

Hi All.

I am quite a newbie to R.
This is a basic question.

I like to modify elements of a vector.
For Example:
a1 <- c(1,2,3,4,3,5)

TThe following program sentence does not work but the intention is;
  
  if (a1==3) a1*3 .

3 in the vector should be changed to 9, and
the resulted vector is (1,2,9,4,9,5).

How can I get the result?

Thanks in advance for help.
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mitsu5
mitsu5 at ruby.famille.ne.jp