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Re: st: Comparing strings


From   jo la frite <jo_la_frite@yahoo.com>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: Comparing strings
Date   Mon, 26 Mar 2012 09:34:27 -0700 (PDT)

dear
Nick,
 
Thanks
for your reply and sorry for being too cryptic with my question.
 
I am
trying to merge 2 datasets, in which observations (firms) are identified by
their names. The names do not match exactly in the 2 datasets so I am doing a
"fuzzy match". My idea is to match 2 names if a large enough fraction
of a name from dataset 1 (say name1) is in a name from dataset 2 (name 2). For
example, "abcde" could be matched with "abcdtyuk" because
the FIRST 4 letters are in common out of an average of (5+8)/2=6.5. It is
important that the comparison sticks to the ordering of the letters. So
"abcde" is not matched with "edcba" or "bacde",
even though the letters are the same but in a different order. Does that make
any sense? thanks again for your help.
 
Jo


________________________________
From: Nick Cox <njcoxstata@gmail.com>
To: statalist@hsphsun2.harvard.edu 
Sent: Monday, March 26, 2012 11:30 AM
Subject: Re: st: Comparing strings

This is a bit better:

mata :

string scalar strscalarsort(string scalar mystring) {

real scalar len, i
string colvector work
len = strlen(mystring)
work = J(len, 1, "")
for(i = 1; i <= len; i++) work[i] = substr(mystring, i, 1)
_sort(work, 1)
return(invtokens(work', ""))
}

end

I still don't know what the real problem is, so I am just playing. But
if you wanted to compare strings regardless of order of characters
something like this would seem needed as a first step.

On Mon, Mar 26, 2012 at 1:51 AM, Nick Cox <njcoxstata@gmail.com> wrote:
> -indexnot()- is a function, not a command.
>
> It's not clear to me what you want, but you can check for whether the
> same letters occur in two strings, at the cost of some programming.
> For example, a Mata function can be written to sort the characters of
> a string scalar into alphabetical order. Here is one:
>
> mata :
>
> string scalar deorst(string scalar mystring) {
>
> real scalar len
> string vector work
> len = strlen(mystring)
> work = J(len, 1, "")
> for(i = 1; i <= len; i++) work[i] = substr(mystring, i, 1)
> _sort(work, 1)
> mystring = ""
> for(i = 1; i <= len; i++) mystring = mystring + work[i]
> return(mystring)
> }
>
> end
>
> . mata : deorst("sorted")
>  deorst
>
> . mata : deorst("backwards")
>  aabcdkrsw
>
> On Sun, Mar 25, 2012 at 10:20 PM, jo la frite <jo_la_frite@yahoo.com> wrote:
>> thanks Nick and Eric. As far as I understand, the indexnot command compares strings regardless of the ordering of the characters in the string. for example, "frog" and "ogfr" are viewed as identical by indexnot.
>>
>>
>> Is there a way of controling for the ordering of the characters. So for example, "comparing "frog" and "fragro" retuns 3 (position of the first character from "frog" not in "fragro").
>
> From: Nick Cox <njcoxstata@gmail.com>
>
>> Stata naturally does have a concept of alphanumeric order for strings;
>> otherwise it could not -sort- them. Consider
>>
>> . di ("frog" < "toad")
>> 1
>>
>> . di ("frog" < "foo")
>> 0
>>
>> The first statement is true and the second false. Otherwise put, with
>> strings < means "precedes" and > means "follows" in alphanumeric
>> order.
>>
>> This allows one step further forwards:
>>
>> gen compare = cond(str1 > str2, indexnot(str1, str2), -indexnot(str1, str2))
>>
>> If strings are identical, this yields 0. Jo did not make explicit that
>> this is what SAS does too, but either way it seems logical to me.
>>
>> Nick
>>
>> On Sat, Mar 24, 2012 at 10:47 PM, Eric Booth <eric.a.booth@gmail.com> wrote:
>>
>>> Take a look at the string function (-help string_functions-) indexnot() (e.g., "gen x = indexnot(string1, string2)" )  which will give you the leftmost position where the two strings differ.
>>> This Stata string function does not assign the positive/negative sign like the sas function you describe, but you can code those yourself by using other string functions to find how they differ in order/sequence/length.
>>
>> On Mar 24, 2012, at 5:12 PM, jo la frite wrote:
>>
>>>> Is there a Stata function that correspondons to the Sas function "COMPARE". It allows to compare strings. Specifically, in Sas COMPARE(string-1, string-2) returns a numeric value. The sign of the result is negative if string-1 precedes string-2 in a sort sequence, and positive if string-1 follows string-2 in a sort sequence. The magnitude of the result is equal to the position of the leftmost character at which the strings differ.

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