> -----Original Message----- > From: r-help-boun...@r-project.org > [mailto:r-help-boun...@r-project.org] On Behalf Of SJ Robson-Davis > Sent: Friday, November 27, 2009 8:52 AM > To: r-help@r-project.org > Subject: [R] Modular inverses > > I want to find the inverse of an integer k mod p (prime.) Is there a > function that can do this for me? I know i could simply write > (k^(p-2)) %% > p, but i need to do this for large primes (above 100) and > this gives the > warning message: > Warning message: > probable complete loss of accuracy in modulus > so this method does not work. Any ideas?
Brute force works: > f<-function(k, p) which(((1:(p-1))*k)%%p == 1) > f(10,p=13) [1] 4 You can precompute the mapping from k to 1/k mod p if you will be working with the same p a lot. Use sapply() or mapply() if you will be working on a vector of k and p. Bill Dunlap Spotfire, TIBCO Software wdunlap tibco.com > > Thanks, > > Samuel > > -- > > ______________________________________________ > 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. > ______________________________________________ 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.