# [R] array manipulation simple questions

andrew andrewjohnroyal at gmail.com
Tue Feb 24 06:53:53 CET 2009

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On Feb 24, 3:57 pm, jdeisenberg <catc... at catcode.com> wrote:
> Λεωνίδας Μπαντής wrote:
>
> > 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 think you mean >=5, not > 5.  In that case, try this:
>
> b <- ifelse( a < 5, 0, 1)

you could also use

b <- 1*(x >= 5) #the logical elements get converted to numeric

>
> Λεωνίδας Μπαντής wrote:
>
> > 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]
>
> > Also I want to find where the 2's in array "a" (if it was an array) are
> > located i.e. positions 3,4.
>
> Not sure how to insert the 4,5, but this will find the where the 2's are:
> which(a == 2)
>

you could use

b <- rep(a,times = 2*(a==2) + 1)
replace(b, which(b==2), c(4,5,2))

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