# [R] array manipulation simple questions

Gabor Grothendieck ggrothendieck at gmail.com
Tue Feb 24 06:31:43 CET 2009

```2009/2/23 Λεωνίδας Μπαντής <bleonidas25 at yahoo.gr>:
>
> Hi there,
>
> I am pretty new to R. Actually I started using it yesterday. I have two questions:
>
> 1.   Suppose I have a<-c(1:10)  (or a<-array(c(1:10),dim=c(1,10)))
>
> and I want to end up with vector b=[0 0 0 0 0 1 1 1 1 1]. i.e. I want to substitute alla elements that are <5 with 0 and >5 with 1.
>
> I did this by using a for loop. Is there another function to use so as to avoid the "for"?

(a>5)+0

# or

out <- as.numeric(a > 5)
if (is.matrix(a)) out <- matrix(a, 1)
out

>
>
> 2.  Suppose I have a<-c(1,1,2,2,3,3)  (or array again)
>
> And I want to place a "4,5" before every "2" and end up with a new "bigger" vector "b":
>
> b=[1 1 4 5 2 4 5 2 3 3]

# return c(4, 5, 2) or x
f <- function(x) if (x==2) c(4:5, x) else x

# apply f to each element and
# if a is a matrix make output a matrix too

out <- unlist(sapply(a, f))
if (is.matrix(a)) out <- matrix(out, nrow(a))
out

>
> Also I want to find where the 2's in array "a" (if it was an array) are located i.e. positions 3,4.

which(a == 2)

```