Re: [R] dictionary lookup

From: Duncan Murdoch <murdoch_at_stats.uwo.ca>
Date: Thu, 06 Mar 2008 19:48:35 -0500

On 06/03/2008 6:45 PM, Thomas Manke wrote:
> Hi,
>
> I have a character-valued vector (old_names) and want to translate
> its entries whenever possible, using a dictionary (dict=data.frame).
> The translation direction is dict$V3 --> dict$V2, but
> some values may be undefined (NA). I suppose this is a very basic
> task, but I tried in vain to make it more efficient than below.
> In particular I would like to avoid the explicit (and slow) loop
> Any help is very much welcome.
> Thank you, TM
> ============================================
> new_names = old_names
> m = match(old_names, dict$V3)
> N = length(old_names)
> for (i in 1:N) {
> if (is.na(m[i])) { next ; }
>
> nn = as.vector(dict$V2)[m[i]];
> if (nn == "" ) { next; }
>
> new_names[i] = nn
> }

You can vectorize this and it should be fast. Here's a straightforward replacement for the loop. It keeps the first 2 lines, and replaces the rest with two more:

new_names <- old_names
m <- match(old_names, dict$V3)

change <- !is.na(m)
new_names[change] <- dict$V2[m[change]]

Duncan Murdoch



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