# [R] Random number

Sundar Dorai-Raj sundar.dorai-raj at PDF.COM
Wed Oct 27 19:57:17 CEST 2004

```
Kunal Shetty wrote:

> Dear R- User
>
>     I created  two random number sets using rnorm function and calcuted their respective means. However now I would like to randomly
> replace some values with NA. Thus create my own test data.
>       Any help or suggestions on this ?
>
>

Use ?sample:
n <- 100
x <- rnorm(n)
x[sample(n, 4)] <- NA
sum(is.na(x))
#  4

>  Also wanted to confirm when multiplying two random number vectors x am y by matrix..is this how i do it.
> A is the matrix
>
>        z <- c(x,y)    # x and y the two set of vectors
>
>        w <- A%*%Z   # each element in each vector multipled by the matrix .
>

(Be careful: R is case sensitive (z != Z).)

I'm not really clear what you want here. "%*%" is matrix multiplication
and "*" is elementwise multiplication. Also using "c" makes a vector or
length "n = length(x) + length(y)". This implies that "A" above "p x n"
(i.e. p rows, n columns) and the result "w" would be "p x 1". Please
provide an example of what you expect to see by the multiplication.

--sundar

>
> regards
> Kunal
>
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