Match beginning of two strings

[Bengt Richter]

On Sat, 2 Aug 2003 22:23:12 -0500, Jeff Epler <jepler at unpythonic.net> wrote:

>This is a naive implementation of the 'extract' function.
>    def extract(a, b):
>	m = min(len(a), len(b))
>	for i in range(m):
>	    if a[i] != b[i]:
>		return a[:i]
>	return a[:m]
>
>Here's one that uses the new zip() function:
I don't see "zip" ;-)

>    def extract2(a, b):
>	m = min(len(a), len(b))
>	for i, ai, bi in (range(m), a, b):
>	    if ai != bi: return a[:i]
>	return a[:m]

>.. unfortunately, it seems to be slower than the first method.  On my
>machine (800MHz PIII):
>$ python timeit.py -s 'import ravi' \
>    'ravi.extract("abcdefghijklmnopqrstuvwxyz","abcdefghijklmnopBHLHT")'
>10000 loops, best of 3: 32.7 usec per loop
>

My timing harness (I seem to need a new getopt for timeit.py under 2.3)
shows a slight (15-22% less time) improvement for this 2.3 alternative:

def commonprefix(s1, s2): # very little tested!
    try:
        for i, c in enumerate(s1):
            if c != s2[i]: return s1[:i]
    except IndexError:
        return s1[:i]
    return s1


[12:39] C:\pywk\clp>timefuns ravi -c extract -s 'abcdefghijklmnopqrstuvwxyz' -s 'abcdefghijklmno
pBHLHT' -c commonprefix -s 'abcdefghijklmnopqrstuvwxyz' -s 'abcdefghijklmnopBHLHT'
           timing oh:  0.000007  ratio
             extract:  0.000088   1.00
        commonprefix:  0.000074   0.85

[12:39] C:\pywk\clp>timefuns ravi -c extract -s 'abcdefghijklmnopqrstuvwxyz' -s 'abcdefghijklmno
pBHLHT' -c commonprefix -s 'abcdefghijklmnopqrstuvwxyz' -s 'abcdefghijklmnopBHLHT'
           timing oh:  0.000007  ratio
             extract:  0.000091   1.00
        commonprefix:  0.000071   0.78

[12:40] C:\pywk\clp>timefuns ravi -c extract -s 'abcdefghijklmnopqrstuvwxyz' -s 'abcdefghijklmno
pBHLHT' -c commonprefix -s 'abcdefghijklmnopqrstuvwxyz' -s 'abcdefghijklmnopBHLHT'
           timing oh:  0.000007  ratio
             extract:  0.000091   1.00
        commonprefix:  0.000071   0.78

[12:40] C:\pywk\clp>timefuns ravi -c extract -s 'abcdefghijklmnopqrstuvwxyz' -s 'abcdefghijklmno
pBHLHT' -c commonprefix -s 'abcdefghijklmnopqrstuvwxyz' -s 'abcdefghijklmnopBHLHT'
           timing oh:  0.000007  ratio
             extract:  0.000088   1.00
        commonprefix:  0.000071   0.81

Regards,
Bengt Richter