[R] simple apply syntax

[Prof Brian Ripley]

On Sun, 11 Jul 2010, Robin Jeffries wrote:

> I know this is a simple question, but I have yet to master the apply
> statements. Any help would be appreciated.

You don't want to use apply() here: rbinom is vectorized. However, you 
cannot use mat$x on a matrix, and the cbind() gave you a matrix 
anyway.  So something like

mat <- data.frame(p=runif(10,0,1), n=rep(1:5))
mat$x <- with(mat, rbinom(10, n, p))

is the idiomatic way to do it.

As an example of using apply and a matrix:

mat <- cbind(p=runif(10,0,1), n=rep(1:5))
x <- apply(mat, 1, function(z) rbinom(1, z['n'], z['p']))
mat <- cbind(mat, x=x)

> I have a column of probabilities and sample sizes, I would like to create a
> column of binomial random variables using those corresponding probabilities.
>
> Eg.
>
> mat = as.matrix(cbind(p=runif(10,0,1), n=rep(1:5)))
>
>
>
>              p n
>
> [1,] 0.5093493 1
>
> [2,] 0.4947375 2
>
> [3,] 0.6753015 3
>
> [4,] 0.8595729 4
>
> [5,] 0.1004739 5
>
> [6,] 0.6292883 1
>
> [7,] 0.3752004 2
>
> [8,] 0.6889157 3
>
> [9,] 0.2435880 4
>
> [10,] 0.9619128 5
>
>
>
>
>
> I want to create mat$x as binomial(n, p)
>
>
>
> Thanks,
>
> Robin
>
>
> 	[[alternative HTML version deleted]]

Please so as we ask, and don't send HTML but rather properly formatted 
plain text (without all these blank lines).

>
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595