[R] Running something without a loop when the result from the previous iteration is require for the current iteration

Greg Snow Greg.Snow at imail.org
Wed Aug 11 20:59:13 CEST 2010


What is wrong with using a loop?

It used to be that loops were much slower than some of the alternatives, but now days a well crafted loop runs almost as fast (sometime faster) than the apply functions.  So if the loop is working for you, use it and don't worry about it (though there may be ways to speed up the loop).

-- 
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
801.408.8111


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Adrienne Wootten
> Sent: Wednesday, August 11, 2010 10:36 AM
> To: r-help at r-project.org
> Subject: [R] Running something without a loop when the result from the
> previous iteration is require for the current iteration
> 
> Hello Everyone!
> 
> Here's what I'm trying to do.  I'm working on generating occurrences of
> precipitation based upon precipitation occurrence for a station during
> the
> previous day and two stations that have already been generated by joint
> probablities and 1st order Markov chains or by the same generation
> process.
> This has to be done for each remaining stations for each month.
> 
> > genmat # 7 stations in this example, line_before is the climatology
> of the
> last day of the previous month. Stations 4 and 6 have been generated
> already
> in this example
>             [,1] [,2] [,3] [,4] [,5] [,6] [,7]
> line_before    1    1    1    0    1    1    1
>               NA   NA   NA    1   NA    0   NA
>               NA   NA   NA    0   NA    0   NA
>               NA   NA   NA    0   NA    0   NA
>               NA   NA   NA    0   NA    0   NA
>               NA   NA   NA    0   NA    0   NA
>               NA   NA   NA    0   NA    0   NA
>               NA   NA   NA    0   NA    0   NA
>               NA   NA   NA    1   NA    0   NA
>               NA   NA   NA    1   NA    1   NA
>               NA   NA   NA    1   NA    1   NA
>               NA   NA   NA    0   NA    0   NA
>               NA   NA   NA    0   NA    0   NA
>               NA   NA   NA    1   NA    1   NA
>               NA   NA   NA    0   NA    0   NA
>               NA   NA   NA    0   NA    0   NA
>               NA   NA   NA    0   NA    0   NA
>               NA   NA   NA    0   NA    0   NA
>               NA   NA   NA    0   NA    0   NA
>               NA   NA   NA    1   NA    1   NA
>               NA   NA   NA    0   NA    0   NA
>               NA   NA   NA    1   NA    1   NA
>               NA   NA   NA    1   NA    1   NA
>               NA   NA   NA    1   NA    1   NA
>               NA   NA   NA    0   NA    0   NA
>               NA   NA   NA    0   NA    1   NA
>               NA   NA   NA    0   NA    0   NA
>               NA   NA   NA    0   NA    0   NA
>               NA   NA   NA    1   NA    1   NA
>               NA   NA   NA    1   NA    1   NA
>               NA   NA   NA    1   NA    1   NA
>               NA   NA   NA    0   NA    0   NA
> > num # station to generate
> [1] 2
> > use1 # 1st station to use in generation
> [1] 6
> > use2 # 2nd station to use in generation
> [1] 4
> 
> > genmat = event.gen2(genmat,use1,use2,num,ortho_obs_used) # Generation
> function shown below
> > genmat # genmat - after it has gone through station 2
>             [,1] [,2] [,3] [,4] [,5] [,6] [,7]
> line_before    1    1    1    0    1    1    1
>               NA    0   NA    1   NA    0   NA
>               NA    0   NA    0   NA    0   NA
>               NA    0   NA    0   NA    0   NA
>               NA    0   NA    0   NA    0   NA
>               NA    0   NA    0   NA    0   NA
>               NA    0   NA    0   NA    0   NA
>               NA    0   NA    0   NA    0   NA
>               NA    0   NA    1   NA    0   NA
>               NA    1   NA    1   NA    1   NA
>               NA    1   NA    1   NA    1   NA
>               NA    1   NA    0   NA    0   NA
>               NA    0   NA    0   NA    0   NA
>               NA    1   NA    1   NA    1   NA
>               NA    0   NA    0   NA    0   NA
>               NA    0   NA    0   NA    0   NA
>               NA    0   NA    0   NA    0   NA
>               NA    0   NA    0   NA    0   NA
>               NA    0   NA    0   NA    0   NA
>               NA    1   NA    1   NA    1   NA
>               NA    0   NA    0   NA    0   NA
>               NA    1   NA    1   NA    1   NA
>               NA    0   NA    1   NA    1   NA
>               NA    1   NA    1   NA    1   NA
>               NA    0   NA    0   NA    0   NA
>               NA    1   NA    0   NA    1   NA
>               NA    0   NA    0   NA    0   NA
>               NA    0   NA    0   NA    0   NA
>               NA    1   NA    1   NA    1   NA
>               NA    1   NA    1   NA    1   NA
>               NA    1   NA    1   NA    1   NA
>               NA    0   NA    0   NA    0   NA
> 
> Where event.gen2 is this function:
> 
> event.gen2 = function(genmat,use1,use2,num,ortho_obs_used){
> 
> for(r in 2:nrow(genmat)){
> 
> ran = runif(1,0,1)
> 
> if(genmat[r,use1]==0 & genmat[r,use2]==0){
> genmat[r,num]<-ifelse(genmat[r-
> 1,num]==0,ifelse(ran<ortho_obs_used$Pr[1],1,0),ifelse(ran<ortho_obs_use
> d$Pr[4],1,0))
> }
> 
> if(genmat[r,use1]==0 & genmat[r,use2]==1){
> genmat[r,num]<-ifelse(genmat[r-
> 1,num]==0,ifelse(ran<ortho_obs_used$Pr[2],1,0),ifelse(ran<ortho_obs_use
> d$Pr[5],1,0))
> }
> 
> if(genmat[r,use1]==1 & genmat[r,use2]==0){
> genmat[r,num]<-ifelse(genmat[r-
> 1,num]==0,ifelse(ran<ortho_obs_used$Pr[3],1,0),ifelse(ran<ortho_obs_use
> d$Pr[7],1,0))
> }
> 
> if(genmat[r,use1]==1 & genmat[r,use2]==1){
> genmat[r,num]<-ifelse(genmat[r-
> 1,num]==0,ifelse(ran<ortho_obs_used$Pr[6],1,0),ifelse(ran<ortho_obs_use
> d$Pr[8],1,0))
> }
> 
> gc()
> }
> 
> genmat
> 
> }
> 
> ####
> 
> ortho_obs_used is a data frame that contains the probablity of
> precipitation
> occurring on a given day for a specific set of condtions.
> For instance ortho_obs_used$Pr[1] is the probablity of rain at station
> s for
> day d, given that there was no rain at station s for day d-1 and no
> rain at
> either of the other two stations for day d.
> 
> The event.gen2 function handles the generation, and it runs quickly for
> the
> 5 remaining stations and one month, but I have to run this for 317
> stations
> over 48 months or more, and it becomes a really bad bottleneck.  So
> what I'd
> like to know is if there is anyway that I can re-write this function to
> work
> without a loop.  I couldn't find anything from previous posts about
> getting
> out of loops where the previous iteration is required to determine the
> next
> calculation.
> 
> Sorry for the length of the post, but I thought it best to try to
> explain
> what I was doing first, before diving into my question
> 
> Thanks in advance!
> 
> Adrienne Wootten
> Graduate Research Assistant/Environmental Meteorologist
> M.S. Atmospheric Science
> NC State University
> State Climate Office of North Carolina
> Raleigh, NC 27695
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at 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.



More information about the R-help mailing list