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Proba( Ut+2=1 / ((Ut+1==1) && (Ut==1))) ?

5 messages · vincent, Dimitris Rizopoulos, Tony Plate +1 more

vincent
# 
Dear all,

First I apologize if my question is quite simple,
but i'm very newbie with R.

I have vectors of the form v = c(1,1,-1,-1,-1,1,1,1,1,-1,1)
(longer than this one of course).
The elements are only +1 or -1.

I would like to calculate :
- the frequencies of -1 occurences after 2 consecutives -1
- the frequencies of +1 occurences after 2 consecutives +1

It looks probably something like :
Proba( Ut+2=1 / ((Ut+1==1) && (Ut==1)))

could someone please give me a little hint about how
i should/could begin to proceed ?

Thanks
(Thanks also to the R creators/contributors, this soft
seems really great !)
Dimitris Rizopoulos
# 
maybe something like this:

x <- sample(c(1, -1), 100, TRUE)
y <- rle(x)
##
ind1 <- y$length[y$value == 1]
sum(ind1[ind1 > 2] - 2)
ind2 <- y$length[y$value == -1]
##
sum(ind1[ind1 > 2] - 2)


could be helpful.

Best,
Dimitris

----
Dimitris Rizopoulos
Ph.D. Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/16/336899
Fax: +32/16/337015
Web: http://www.med.kuleuven.ac.be/biostat/
     http://www.student.kuleuven.ac.be/~m0390867/dimitris.htm


----- Original Message ----- 
From: "vincent" <vincent at 7d4.com>
To: <r-help at stat.math.ethz.ch>
Sent: Monday, April 25, 2005 6:03 PM
Subject: [R] Proba( Ut+2=1 / ((Ut+1==1) && (Ut==1))) ?
Tony Plate
# 
table() can return all the n-gram statistics, e.g.:

 > v <- sample(c(-1,1), 1000, rep=TRUE)
 > table("v_{t-2}"=v[-seq(to=length(v), len=2)], 
"v_{t-1}"=v[-c(1,length(v))], "v_t"=v[-(1:2)])
, , v_t = -1

        v_{t-1}
v_{t-2}  -1   1
      -1 136 134
      1  131 112

, , v_t = 1

        v_{t-1}
v_{t-2}  -1   1
      -1 131 113
      1  115 126

 >

This says that there were 136 cases in which a -1 followed two -1's (and 
126 cases in which a 1 followed to 1's).

If you're really only interested in particular contexts, you can do 
something like:

 > table(v[-seq(to=length(v), len=2)]==1 & v[-c(1,length(v))]==1 & 
v[-(1:2)]==1)

FALSE  TRUE
   872   126
 > table(v[-seq(to=length(v), len=2)]==-1 & v[-c(1,length(v))]==-1 & 
v[-(1:2)]==-1)

FALSE  TRUE
   862   136

or

 > sum(v[-seq(to=length(v), len=2)]==-1 & v[-c(1,length(v))]==-1 & 
v[-(1:2)]==-1)
[1] 136
 >
vincent wrote:
vincent
# 
Thank you all for all your answers.
I see than there are many ways to solve my question !
Thank you very much.