Re: [R] beginner Q: hashtable or dictionary?
Standard R vectors and list elements can given names, and can be accessed by
them. This allows them to be used like the dictionaries or hashes of other
languages.
For example
x = c(a=1, b=2, c=3)
x[a]
a
1
x[foo] = bar
x[foo]
foo
bar
x
a b c foo
1 2 3 bar
-Greg
-Original Message-
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] Behalf Of context grey
Sent: Sunday, January 29, 2006 8:35 PM
To: r-help@stat.math.ethz.ch
Subject: [R] beginner Q: hashtable or dictionary?
Hi,
Is there something like a hashtable or (python)
dictionary in R/Splus?
(If not, is there a reason why it's not needed /
typical way to accomplish the same thing?)
Thank you
__
R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html
--
LEGAL NOTICE\ Unless expressly stated otherwise, this messag...{{dropped}}
__
R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Re: [R] beginner Q: hashtable or dictionary?
On 1/29/06, context grey [EMAIL PROTECTED] wrote: Hi, Is there something like a hashtable or (python) dictionary in R/Splus? On 29 Jan 2006, [EMAIL PROTECTED] wrote: use a 'list': Most of the time, a list will be what you want, but it has some important differences from a Python dictionary. In particular, one can end up with duplicate keys. Here is an example: h - list() h[[foo]] - 1 h[[2]] - 2 names(h) - rep(foo, 2) h $foo [1] 1 $foo [1] 2 h[[foo]] [1] 1 'environments' may be what you are looking for, see help for new.env(). h - new.env(hash=TRUE) An important thing to keep in mind with environments, however, is that they are an exception to the pass by value semantics of the language. Environments are not copied when passed as function args. This has its uses, but can be confusing. + seth __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Re: [R] beginner Q: hashtable or dictionary?
Is a list O(1) for setting and getting? Can you elaborate? R is a vector language, and normally you create a list in one pass, and you can retrieve multiple elements at once. When you use a hash table you expect it to be O(1) (on average) for getting and setting values (conditional on having a good hash function, a large enough table etc...). (and thanks for the pointer to that RNEWS item) Indexing by number is O(1) except where replacement causes the list vector to be copied. There is always the option to use match() to convert to numeric indexing. I read this to mean that setting a value in list will be O(n) (where n is the length of the new list) - If you blindly expect a list to act like a hash table you will be badly surprised. If you are copying an algorithm from another language, you will probably need to rethink the way that updates work. If however, you just want an object that can be indexed by a string, then a list will be fine. Regards, Hadley __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Re: [R] beginner Q: hashtable or dictionary?
On 1/30/2006 9:55 AM, Seth Falcon wrote: On 1/29/06, context grey [EMAIL PROTECTED] wrote: Hi, Is there something like a hashtable or (python) dictionary in R/Splus? On 29 Jan 2006, [EMAIL PROTECTED] wrote: use a 'list': Most of the time, a list will be what you want, but it has some important differences from a Python dictionary. In particular, one can end up with duplicate keys. Here is an example: h - list() h[[foo]] - 1 h[[2]] - 2 names(h) - rep(foo, 2) h $foo [1] 1 $foo [1] 2 h[[foo]] [1] 1 'environments' may be what you are looking for, see help for new.env(). h - new.env(hash=TRUE) An important thing to keep in mind with environments, however, is that they are an exception to the pass by value semantics of the language. Environments are not copied when passed as function args. This has its uses, but can be confusing. Both lists and environments have other oddities, too. For example, mean would be found in your environment above by functions like get(), since the base environment is its parent; f would be found in your list, since partial name matching is used in lists. You can avoid the parent problem in R 2.2.x+ by setting the parent explicitly to emptyenv(). Duncan Murdoch __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Re: [R] beginner Q: hashtable or dictionary?
hadley wickham wrote: I read this to mean that setting a value in list will be O(n) (where n is the length of the new list) - If you blindly expect a list to act like a hash table you will be badly surprised. If you are copying an algorithm from another language, you will probably need to rethink the way that updates work. If however, you just want an object that can be indexed by a string, then a list will be fine. I would think that when translating from another language, it is best to write it in R in the simplest way, which probably means using a list. Then if it turns out to be too slow, try doing something fancy. I suspect that speed improvements are seldom necessary -- I can't believe how fast most computations are these days. I don't see that worrying about O(1) versus O(n) is likely to get you very far. Just try it out on a normal size problem, and try it out on a very large problem and see if it is good enough. Patrick Burns [EMAIL PROTECTED] +44 (0)20 8525 0696 http://www.burns-stat.com (home of S Poetry and A Guide for the Unwilling S User) __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Re: [R] beginner Q: hashtable or dictionary?
I would think that when translating from another language, it is best to write it in R in the simplest way, which probably means using a list. Then if it turns out to be too slow, try doing something fancy. I suspect that speed improvements are seldom necessary -- I can't believe how fast most computations are these days. I totally agree. However, if you are implementing an algorithm that is O(n^2) when using a hash table that has O(1) updates, it is likely to be at least O(n^3) when using a list with O(n) updates. And sometimes speed does matter! (Although not as often as most people think, and, of course, beware premature optimisation.) Hadley __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
[R] beginner Q: hashtable or dictionary?
Hi, Is there something like a hashtable or (python) dictionary in R/Splus? (If not, is there a reason why it's not needed / typical way to accomplish the same thing?) Thank you __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Re: [R] beginner Q: hashtable or dictionary?
use a 'list': x - list() x[['test']] - 64 x[['next one']] - c(1,2,3,4) x $test [1] 64 $next one [1] 1 2 3 4 x[['test']] [1] 64 On 1/29/06, context grey [EMAIL PROTECTED] wrote: Hi, Is there something like a hashtable or (python) dictionary in R/Splus? (If not, is there a reason why it's not needed / typical way to accomplish the same thing?) Thank you __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html -- Jim Holtman Cincinnati, OH +1 513 247 0281 What the problem you are trying to solve? [[alternative HTML version deleted]] __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Re: [R] beginner Q: hashtable or dictionary?
I was wondering if there is an easy way to extend this to implement a 2-D hash, i.e. 2-way indexing? x[[a]][[b]] - something Thanks. Christos Hatzis -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of jim holtman Sent: Sunday, January 29, 2006 8:39 PM To: context grey Cc: r-help@stat.math.ethz.ch Subject: Re: [R] beginner Q: hashtable or dictionary? use a 'list': x - list() x[['test']] - 64 x[['next one']] - c(1,2,3,4) x $test [1] 64 $next one [1] 1 2 3 4 x[['test']] [1] 64 On 1/29/06, context grey [EMAIL PROTECTED] wrote: Hi, Is there something like a hashtable or (python) dictionary in R/Splus? (If not, is there a reason why it's not needed / typical way to accomplish the same thing?) Thank you __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html -- Jim Holtman Cincinnati, OH +1 513 247 0281 What the problem you are trying to solve? [[alternative HTML version deleted]] __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Re: [R] beginner Q: hashtable or dictionary?
Make a list of lists: L - list(a = list(a = 1, b = 2), b = list(a = 3, b = 4)) L[[a]][[b]] L$a$b On 1/29/06, Christos Hatzis [EMAIL PROTECTED] wrote: I was wondering if there is an easy way to extend this to implement a 2-D hash, i.e. 2-way indexing? x[[a]][[b]] - something Thanks. Christos Hatzis -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of jim holtman Sent: Sunday, January 29, 2006 8:39 PM To: context grey Cc: r-help@stat.math.ethz.ch Subject: Re: [R] beginner Q: hashtable or dictionary? use a 'list': x - list() x[['test']] - 64 x[['next one']] - c(1,2,3,4) x $test [1] 64 $next one [1] 1 2 3 4 x[['test']] [1] 64 On 1/29/06, context grey [EMAIL PROTECTED] wrote: Hi, Is there something like a hashtable or (python) dictionary in R/Splus? (If not, is there a reason why it's not needed / typical way to accomplish the same thing?) Thank you __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html -- Jim Holtman Cincinnati, OH +1 513 247 0281 What the problem you are trying to solve? [[alternative HTML version deleted]] __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Re: [R] beginner Q: hashtable or dictionary?
use a 'list': Is a list O(1) for setting and getting? Hadley __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Re: [R] beginner Q: hashtable or dictionary?
One could also use a matrix if appropriate: m - matrix(1:4, 2, dimnames = list(c(a, b), c(a, b))) m[a, b] [1] 3 Also in the 1d case one could do this: v - c(a = 1, b = 2) v[[a]] On 1/29/06, Gabor Grothendieck [EMAIL PROTECTED] wrote: Make a list of lists: L - list(a = list(a = 1, b = 2), b = list(a = 3, b = 4)) L[[a]][[b]] L$a$b On 1/29/06, Christos Hatzis [EMAIL PROTECTED] wrote: I was wondering if there is an easy way to extend this to implement a 2-D hash, i.e. 2-way indexing? x[[a]][[b]] - something Thanks. Christos Hatzis -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of jim holtman Sent: Sunday, January 29, 2006 8:39 PM To: context grey Cc: r-help@stat.math.ethz.ch Subject: Re: [R] beginner Q: hashtable or dictionary? use a 'list': x - list() x[['test']] - 64 x[['next one']] - c(1,2,3,4) x $test [1] 64 $next one [1] 1 2 3 4 x[['test']] [1] 64 On 1/29/06, context grey [EMAIL PROTECTED] wrote: Hi, Is there something like a hashtable or (python) dictionary in R/Splus? (If not, is there a reason why it's not needed / typical way to accomplish the same thing?) Thank you __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html -- Jim Holtman Cincinnati, OH +1 513 247 0281 What the problem you are trying to solve? [[alternative HTML version deleted]] __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Re: [R] beginner Q: hashtable or dictionary?
On Sun, 29 Jan 2006, hadley wickham wrote: use a 'list': Is a list O(1) for setting and getting? Can you elaborate? R is a vector language, and normally you create a list in one pass, and you can retrieve multiple elements at once. Retrieving elements by name from a long vector (including a list) is very fast, as an internal hash table is used. Does the following item from ONEWS answer your question? o Indexing a vector by a character vector was slow if both the vector and index were long (say 10,000). Now hashing is used and the time should be linear in the longer of the lengths (but more memory is used). Indexing by number is O(1) except where replacement causes the list vector to be copied. There is always the option to use match() to convert to numeric indexing. -- Brian D. Ripley, [EMAIL PROTECTED] Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UKFax: +44 1865 272595 __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
