[R] Sampling according to type

[Thomas]

I have a matrix where each entry represents a data subject's type, 1  
or 0:

n <- 10
ntype <- rbinom(n, 1, 0.5)

and I'd like to sample say 3 subjects from ntype where those subjects  
who are Type 1 are selected with probability say 0.75, and Type 0 with  
(1-0.75). (So the sample would produce a list with three indices each  
referring to a position within ntype.)

Can anyone suggest a way to do this please?

Thank you,

Thomas Chesney
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