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MathGroup Archive 2005

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Re: a conflicting StringReplace

  • To: mathgroup at smc.vnet.net
  • Subject: [mg56367] Re: [mg56306] a conflicting StringReplace
  • From: Michael_Harre at resimac.com.au
  • Date: Fri, 22 Apr 2005 06:25:50 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Hi Hui,

Mathematica looks at the first element of the initial string and decides
whether or not it is the beginning of a string that matches the
replacement rule, so 

In[1]    StringReplace["abc",{"ab"->"AB"}]
Out[1]   ABc

found "ab" in the initial string matches "ab" in the replacement rule
and so replaces "ab" with "AB", then Mathematica looks at the next
letter after the string that has just been substituted in. So in your
students example, after the first string is replaced the next element to
be looked at is "c", not "b". An example might help here:

In[2]	 StringReplace["abc", {"ab" -> "Q", "bc" -> "ED"}]
Out[2] Qc

because "bc" does not match anything in the remainder of the string
(which is just "c"), However:

In[3]	 StringReplace["abc", {"ab" -> "Q", "c" -> "ED"}]
Out[3] QED

Mike.

-----Original Message-----
From: Hui Fang [mailto:fangh73 at xmu.edu.cn] 
To: mathgroup at smc.vnet.net
Subject: [mg56367] [mg56306] a conflicting StringReplace

I was teaching Mathematica in a college. In the class I was showing them

some built-in functions about strings. Since this is not a very 
important issue, I didn't spend much time on each function. When I show 
them StringReplace, I gave them the following examples:
In[1]    StringReplace["abc",{"ab"->"AB"}]
Out[1]   ABc

In[2]   StringReplace["abc", {"bc"->"BC"}]
Out[2]   aBC

No problem on those. Now a student tried the following:
In[3]   StringReplace["abc", {"ab"->"AB", "bc"->"BC"}]
Out[3]   ABc

Now he asked me why only "ab" is replaced. I said this is because there 
is a conflict because both "ab" and "bc" contains "b". So Mathematica 
will choose the first replacement. I also told him if he changes the 
order, he will get aBC. Now:
In[4]    StringReplace["abc", {"bc"->"BC","ab"->"AB"}]
Out[4]    ABc

This is the part I don't understand. Does Mathematica treat those rules 
in their canonical order (since "ab" is before "bc" in canonical 
order.), or in their written order?

Thanks a lot!

Hui Fang      


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