changing the first occurrence only of a string in a line

[Alex Tweedly]

I'm not going toanswer your direct question (replace first occurrence) 
because that is a subset of the whole question, which I think is more 
interesting :-)

How would I remove the duplicates as described below ?
(I have assumed that in each field the tag is separated from
the content by a space, so used 'word' to extract the tag)

[I tested this in the msg box]
> put "A aaa" & tab & "B bbb" & tab & "B bb2" & "C ccc" &cr into tData
> put "A a2a" & tab & "B b2b" & tab & "B 2b2" & "C ccc" &tab& "C cc2" 
> &cr after tData
>
> set the itemDel to tab
> repeat for each line L in tData
>   put empty into lastTag
>   put empty into tLine
>   repeat for each item itm in L
>     if word 1 of itm <> lastTag then
>        put itm & tab after tLine
>        put word 1 of itm into lastTag
>    end if
>   end repeat
>   delete the last char of tLine   -- trailing tab character
>   put tLine & cr afer tOutput
> end repeat
> put tOutput
>     
-- Alex.


Peter Alcibiades wrote:
> How would you do the following in Rev?
>
> We have a file consisting of records with tab separated fields.  Each field 
> has a tag followed by contents.  Some tags occur more than once in some 
> records which thus have varying numbers of fields. Duplicates are always 
> consecutive.  I want to eliminate all the occurrences of any tag except 
> the first one.  The duplicate tags can occur any place in the record, but 
> if they are duplicated, will always be consecutive.
>
> Doing this in SED is not particularly difficult, but it does require going 
> out to shell, and so its not cross platform.  You just change the tag 
> using the local scope to something else.  SED then only changes the first 
> occurrence in a record.  Then you use the global scope and change all of 
> them.  Then you go back and change the first one back to what it was.  In 
> fact, if using SED like this, the only thing you need it for is to do the 
> local, first tag, change - once this is done, the rest can be done in Rev.  
> But it would be nice to stay in Rev for the whole thing.  
>
> Is there a way in Rev to pick the first occurence of a string in a record, 
> change it and not subsequent occurences, and then move on to the next 
> record and do the same thing? 
>
> That is, mimic the 'local' editing mode of SED?  
>
> Bet you all thought them dinosaurs like SED had to be extinct by now!  But 
> no, they are still trampling around in the swamps of text manipulation....
>
> For the sake of clarity, a record might look like this:
>
> A aa TAB B bb TAB B bb TAB B bb TAB C cc TAB D dd TAB D dd
>
> and what is wanted is to change the first occurrence of B to, for 
> example !1, and the first occurence of D to, for example !2, or anyway 
> something that will not occur by chance, to allow the subsequent editing 
> to work globally on the file.  This is what SED does in local mode.
>