# [R] how to make this sequence: 1,2,3,4,5,4,3,2,1

(Ted Harding) Ted.Harding at manchester.ac.uk
Fri Mar 5 09:14:58 CET 2010

```On 05-Mar-10 06:04:51, kensuguro wrote:
>
> I'm just beginning R, with book Using R for Introductory Statistics,
> and one of the early questions has me baffled.  The question is,
> create the sequence: 1,2,3,4,5,4,3,2,1 using seq() and rep().
>
> Now, as a programmer, I am punching myself to not be able to figure
> it out..
> I mean, as simple as a for loop, but using seq, I am stumped.
> I would think c(1:5, 4:1) would be the brute force method with very
> non intelligent coding..  there has to be a way to make the "turning
> point" (in this case 5) parametric right?  So you could change it
> later and the sequence will reflect it.
> --
> View this message in context:
> http://n4.nabble.com/how-to-make-this-sequence-1-2-3-4-5-4-3-2-1-tp15792
> 45p1579245.html
> Sent from the R help mailing list archive at Nabble.com.

You can indeed do it using seq(), and with rep() plus a little
help from something else:

seq():
c(seq(1,5,1),seq(4,1,-1))

rep():
cumsum(c(rep(1,5),rep(-1,4)))

Parametrised "1,2,...,n,(n-1),...,2,1":

updown1 <- function(n){ c(seq(1,n,1),seq((n-1),1,-1)) }

updown2 <- function(n){ cumsum(c(rep(1,n),rep(-1,(n-1)))) }

updown1(10)
# [1]  1  2  3  4  5  6  7  8  9 10  9  8  7  6  5  4  3  2  1
updown2(10)
# [1]  1  2  3  4  5  6  7  8  9 10  9  8  7  6  5  4  3  2  1

Ted.

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Date: 05-Mar-10                                       Time: 08:14:53
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