On 27-Jan-2012 Adel ESSAFI wrote: > dear list > > I use runif to generate a ramdom number between min and max > > runif(n, min=0, max=1) > > however , the syntaxe of rexp does not allow that > > rexp(n, rate = 1) > > and it generate a number with the corresponding rate. > The question is: how to generate a number between min and max > using rexp(). > > Regards > -- > PhD candidate in Computer Science > Address > 3 avenue lamine, cité ezzahra, Sousse 4000 > Tunisia > tel: +216 97 246 706 (+33640302046 jusqu'au 15/6) > fax: +216 71 391 166
Your question need to give more detail in order to be well defined! Since the range of an exponentially distributed random number is [0,Inf], in the first instance it makes no sense to seek one distributed over a finite range (min,max) -- unlike a uniformly distributed random number which does have a finite range. However, one possible interpretation of your question would be a *truncated* exponential distribution, i.e. the distribution of an exponentially distributed number *conditional on being between min and max*. For an exponentially distributed random variable X with rate r, exp(-r*X) is uniformaly distributed on (0,1). Conditional on being between min=A and max=B, exp(-r*X) is uniformaly distributed on (exp(-r*B), exp(-r*A). Let C = exp(-r*B), D = exp(r*A) Hence sample U according to runif(n,min=C,max=D) and then X <- (1/r)*log(1/U) will have the exponential distribution truncated to (A,B). For example: A <- 2 ; B <- 11 r <- 0.25 C <- exp(-r*B); D <- exp(-r*A) n <- 10000 U <- runif(n,min=C,max=D) X <- (1/r)*log(1/U) hist(X,breaks=100,xlim=c(0,12)) However, you may have some other meaning in mind for "variable exponentially distributed with min=A and mex=B". We would need to knpow! Ted. ------------------------------------------------- E-Mail: (Ted Harding) <ted.hard...@wlandres.net> Date: 27-Jan-2012 Time: 11:48:14 This message was sent by XFMail ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.