On 14 December 2012 13:46, Simon Urbanek <simon.urba...@r-project.org> wrote: > You may be a bit misinformed about with tail recursion means - it still needs > to evaluate the function for each recursion step, the only difference is that > in > such special case there is no additional information that needs to be stored > -- and you have also proven why it's not as simple as you think:
Thank you Simon for your time and patient reply. I was thinking calling another function over again is called indirect recursion. > f <- function(x) if(x >0) f(x-1) > (f(100)) NULL > f1<-function(x) x-1 > f <- function(x) f1(x); > (f(100)) [1] 99 >> Your latter version doesn't actually do anything anywhere close to the >> recursion you defined. I see what you mean. I was confused. My intention was having 100 calls in the second one with f1 as well. Maybe I can make the point with using another example via mutual recursion, 'fibRping/pong' below. But I was thinking 'fibRping' version would need less memory then the plain recursion, which I now understand that I was mistaken. Every call counts then. Even though I had an impression that calling the same function more then once like fibRping/pong would not create additional memory requirement ... (Probably the issue is noting to do with Rs internals though.. Using Dirk's blog entry (http://dirk.eddelbuettel.com/blog/2011/09/08/) : ## R implementation of recursive Fibonacci sequence fibR <- function(n) { if (n == 0) return(0) if (n == 1) return(1) return (fibR(n - 1) + fibR(n - 2)) } # Now Mutual recursion fibRping <- function(n) { if (n == 0) return(0) if (n == 1) return(1) return (fibRpong(n - 1) + fibRpong(n - 2)) } fibRpong <- function(n) { if (n == 0) return(0) if (n == 1) return(1) return (fibRping(n - 1) + fibRping(n - 2)) } options(expressions=500000) fibR(50000)> options(expressions=500000) > fibR(50000) Error: C stack usage is too close to the limit > fibRping(50000) Error: C stack usage is too close to the limit ______________________________________________ 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.
