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Multiple replace in a string (Python recipe) by Anand
ActiveState Code (http://code.activestate.com/recipes/194373/)

A hack to quickly replace multiple characters/strings in a string with characters from another string.

Python, 23 lines
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# mrepalce.py - quick string replacement
import string
import array

def mreplace(s, chararray, newchararray):
    """ Replace occurences of chars in
    chararray in string 's' with elements in
    newchararray """

    slist=list(s)
    dlist=list(chararray)
    rlist=list(newchararray)

    for x in range(0, len(slist)):
        charitem=slist[x]
        if charitem in dlist:
            index=dlist.index(charitem)
            try:
                slist[x]=rlist[index]
            except ValueError:
                pass

    return array.array('c', slist).tostring()

Quick and nifty hack. It is better than many string.replace() cycles.

Tags: text

3 comments

Hannu Kankaanpää 20 years, 7 months ago  # | flag

hew. How is that better than multiple string.replaces? I tried it against this:

def mreplace2(s, chararray, newchararray):
    for a, b in zip(chararray, newchararray):
        s = s.replace(a, b)
    return s

And it was a lot slower :P

Josiah Carlson 20 years, 3 months ago  # | flag

Something that is included AND better. There's this magical little thing call string.translate that does EXACTLY what both of you want.

import string
def mreplace3(s, chars, newchars):
    return s.translate(string.maketrans(chars, newchars))

Running this on 512k of random data, translating all 256 characters (chr(0) becomes chr(255), chr(1) becomes chr(254), etc.), the times to execute on my PII-400 are:

mreplace: 39.2030000687

mreplace2: 8.39099991322

mreplace3: 0.0149999856949

I believe translate is a C function call. Could be why it beats the other two by factors of almost 3000 and 600 respectively.

Thane Plummer 20 years, 2 months ago  # | flag

Not the same. There are differences in these algorithms. The maketrans() function requires that the characters mapped are the same length. The replace() function has no such requirement.