[R] nested if/else very slow, more efficient ways?

[Duncan Murdoch]

On 10/23/2006 6:03 PM, Kim Milferstedt wrote:
> Hello,
> 
> in the data.frame "resultsfuzzy" I would like to replace the 
> characters in the second column ("5a", "5b", ... "5e") with numbers 
> from 1 to 5. The data.frame has 39150 entries. I seems to work on 
> samples that are << nrow(resultsfuzzy) but it takes suspicously long.
> 
> Do you have any suggestions how to make the character replacing more efficient?

I don't know if nested if/else is inefficient, but indexing of 
data.frames certainly is. You would do a lot better with ifelse() on the 
whole column.

For example, if column 2 is named B, this would be much faster:

resultsfuzzy$B <- ifelse(resultsfuzzy$B == "5a", 1,
                   ifelse(resultsfuzzy$B == "5b", 2, ... ))

(where you'll have to fill in the ... yourself in the obvious way.

But an even faster way for your particular problem would be

resultsfuzzy$B <- match(resultsfuzzy$B, c("5a", "5b", "5c", "5d", "5e"))

Duncan Murdoch
> 
> Code:
> 
> for (i in 1:nrow(resultsfuzzy))
> {
> if (resultsfuzzy[i,2] == "5a"){resultsfuzzy[i,2] <- 1} else
>      if (resultsfuzzy[i,2] == "5b"){resultsfuzzy[i,2] <- 2} else
>          if (resultsfuzzy[i,2] == "5c"){resultsfuzzy[i,2] <- 3} else
>              if (resultsfuzzy[i,2] == "5d"){resultsfuzzy[i,2] <- 4} else
>                  resultsfuzzy[i,2] <- 5
> }
> 
> Thanks,
> 
> Kim
> 
> version
> 
> platform i386-pc-mingw32
> arch     i386
> os       mingw32
> system   i386, mingw32
> status
> major    2
> minor    2.1
> year     2005
> month    12
> day      20
> svn rev  36812
> language R
> 
> __________________________________________
> 
> Kim Milferstedt
> University of Illinois at Urbana-Champaign
> Department of Civil and Environmental Engineering
> 4125 Newmark Civil Engineering Laboratory
> 205 North Mathews Avenue MC-250
> Urbana, IL 61801
> USA
> phone: (001) 217 333-9663
> fax: (001) 217 333-6968
> email: milferst at uiuc.edu
> http://cee.uiuc.edu/research/morgenroth
> 
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