Match beginning of two strings

John Machin sjmachin at
Mon Aug 4 15:55:23 CEST 2003

On Mon, 04 Aug 2003 11:56:04 GMT, Alex Martelli <aleax at>

> I'm not sure where I went wrong in
>the Pyrex coding (it doesn't seem to be performing anywhere
>as well as I thought it might) and I'll be happy for real
>Pyrex expert to show me the way.

I don't call myself an expert, but here's my best shot:

If you look at the generated C code, you'll see lots of conversion
between C and Python types. The trick is to get your args into C, stay
in C as much as possible, and ship back a Python return value.

It's made harder with strings as there is not (yet) any way of hinting
to Pyrex to use the "s#" gadget, you have to DIY, see below.

cdef extern from "Python.h":
   int PyString_Size(object s)        
def exa2(arga, argb):
    cdef int la, lb, lmin, i
    cdef char *a, *b
    a = arga
    b = argb
    la = PyString_Size(arga)
    lb = PyString_Size(argb)
    # living dangerously, not testing for error;
    # Easy to eyeball for correctness in this case,
    # but ...
    if la <= lb:
        lmin = la
        lmin = lb
    i = 0
    while i < lmin:
        if a[i] != b[i]:
            return arga[:i]
        i = i + 1
    if lmin == la:
        return arga
        return argb

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