[R] dictionary lookup

[Thomas Manke]

Duncan Murdoch wrote:
> 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
Thank you all for your responses, it certainly pointed me to the right 
direction.
For my purposes, I only had to slightly modify Duncan's suggestion

new_names <- rownames(E)
m <- match(rownames(E), dict$V3)
change <- ( !is.na(m) & dict$V2[m] != "")      
new_names[change] <- as.vector(dict$V2[m[change]] )    

Best wishes, TM