Delete all not allowed characters..

[Steven D'Aprano]

On Thu, 25 Oct 2007 17:42:36 +0200, Michal Bozon wrote:

> the list comprehension does not allow "else", but it can be used in a
> similar form:
> 
> s2 = ""
> for ch in s1:
>     s2 += ch if ch in allowed else " "
> 
> (maybe this could be written more nicely)

Repeatedly adding strings together in this way is about the most 
inefficient, slow way of building up a long string. (Although I'm sure 
somebody can come up with a worse way if they try hard enough.)

Even though recent versions of CPython have a local optimization that 
improves the performance hit of string concatenation somewhat, it is 
better to use ''.join() rather than add many strings together:

s2 = []
for ch in s1:
    s2.append(ch if (ch in allowed) else " ")
s2 = ''.join(s2)

Although even that doesn't come close to the efficiency and speed of 
string.translate() and string.maketrans(). Try to find a way to use them.

Here is one way, for ASCII characters.

allowed = "abcdef"
all = string.maketrans('', '')
not_allowed = ''.join(c for c in all if c not in allowed)
table = string.maketrans(not_allowed, ' '*len(not_allowed))
new_string = string.translate(old_string, table)


-- 
Steven.