python filter letters in a string

Question

I need a command in python which filters the letters in a string but not deleting them for example:

string =raw_input("Enter the string:")
if string.startswith("a") and string.endswith("aaa"):
    print "String accepted"
else: print "String not accepted"
if "ba" in string:
    print "String accepted"
else :
    print "String is not accepted"

What should I add to ban other letters except a and b in the string

@machineyearning the type is `str`; `string` is a standard library module, so it's probably best not to shadow it, but it's not such a big deal. — jonrsharpe, Oct 08 '15 at 11:08
@machineyearning: as jonrsharpe has already said, no builtin is being shadowed here — Andrea Corbellini, Oct 08 '15 at 11:19
`string` is not a class, nor a "builtin module". Builtins are those names that you can find in `__builtins__`. Perhaps what you meant to say is that string is a module from the standard library. — Andrea Corbellini, Oct 08 '15 at 11:29

score 2 · Answer 1 · answered Oct 08 '15 at 11:05

2

You can simply replace them with the empty string and check whether there's anything left:

string = raw_input("Enter the string:")
if string.replace('a','').replace('b',''):
    print "String not accepted"
else:
    print "String accepted"

The original string string will not be modified.

answered Oct 08 '15 at 11:05

TigerhawkT3

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Which in Python 2.x would be better as `string.translate(None, 'ab')` – Jon Clements Oct 08 '15 at 11:20

Anand S Kumar · Answer 2 · 2015-10-08T11:21:47.060

1

You can use a set , convert your string to a set and check if its a subset of a set with only a and b. Example -

s = raw_input("Enter the string:")
validset = set('ab')
if set(s).issubset(validset):
    print "String accepted"
else: 
    print "String not accepted"

Demo -

>>> s = "abbba"
>>> validset = set(['a','b'])
>>> if set(s).issubset(validset):
...     print "String accepted"
... else: print "String not accepted"
... 
String accepted

>>> s = "abbbac"
>>> if set(s).issubset(validset):
...     print "String accepted"
... else: print "String not accepted"
... 
String not accepted

Or as indicated in the comments you can use set.issuperset() instead . Example -

s = raw_input("Enter the string:")
validset = set('ab')
if validset.issuperset(s):
    print "String accepted"
else: 
    print "String not accepted"

edited Oct 08 '15 at 11:21

answered Oct 08 '15 at 11:05

Anand S Kumar

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You can also use `validset.issuperset(s)`. This way you don't need to convert `s` to a set. – Andrea Corbellini Oct 08 '15 at 11:17
What @AndreaCorbellini said - plus `validset = set('ab')` is also an option – Jon Clements Oct 08 '15 at 11:19
Yes, correct, I have accomodated the comments into the answer. Thanks – Anand S Kumar Oct 08 '15 at 11:19
`{'a', 'b'}` is also another option (though `set('ab')` will always be much shorter and easier to type) – Andrea Corbellini Oct 08 '15 at 11:21
`{'a', 'b'}` wont work in Python 2.6 or lower, so thats why I used something that would work there. – Anand S Kumar Oct 08 '15 at 11:22

machine yearning · Answer 3 · 2015-10-08T11:16:50.000

1

This is a good use of regular expressions:

import re
my_string = raw_input("Enter the string:")
if re.match('^[ab]+$', my_string):
    print "String accepted"
else :
    print "String is not accepted"

This will match strings which contain only the characters a and b, of non-zero length. If you want to match zero-length strings, use * instead of +.

edited Oct 08 '15 at 11:16

answered Oct 08 '15 at 11:09

machine yearning

9,889
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score 0 · Answer 4 · edited Oct 08 '15 at 11:15

0

You could use a regex to search for letters and combinations you want to filter.

^((?![abcd]).)*$ will match things that don't contain a, b, c or d.

edited Oct 08 '15 at 11:15

Andrea Corbellini

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answered Oct 08 '15 at 11:05

Astrogat

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score 0 · Answer 5 · answered Oct 08 '15 at 11:23

0

Try something like this. It lets you set multiple letters, symbols, etc

valid_char = ['a', 'b']

def validate(s):
    for char in s:
        if char not in valid_char:
            return False
    return True

if validate(input("Enter the string:")):
    print('Sting Accepted')
else:
    print('Sting not Accepted')

answered Oct 08 '15 at 11:23

Steven Summers

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I'd recommend using a set for `valid_char` in order to obtain *O(n)* complexity. – Andrea Corbellini Oct 08 '15 at 11:24
Hmm, never used set before, the answer above does look like a better option – Steven Summers Oct 08 '15 at 11:27

python filter letters in a string

5 Answers5