I have a a user-inputted polynomial and I only want to use it if it only has characters in the string 1234567890^-+x.
How can I check if it does or not without using external packages? I only want to use built-in Python 2.5 functions.
I am writing a program that runs on any Mac without needing external packages.
Here are some odd ;-) ways to do it:
good = set('1234567890^-+x')
if set(input_string) <= good:
# it's good
else:
# it's bad
or
if input_string.strip('1234567890^-+x'):
# it's bad!
else:
# it's good
timeit potshot at it?
.issuperset() does, exactly, without looking it up :-( But I understand <= at once, and as Martijn says, they're the same thing in the end. .issuperset() is likely slower due to the method lookup expense.<= and which >= without looking it up. So I avoid them. <= and >= are obvious to me.
issuperset would... wait, apparently it doesn't short-circuit. I thought it would do the test without building a set from the input_string, but when I tried {0}.issuperset(xrange(1000000000)) a few seconds ago, it started eating memory in a way that seems to indicate it turns the input into a set. I guess there's no performance advantage.Use a regular expression:
import re
if re.match('^[-0-9^+x]*$', text):
# Valid input
The re module comes with Python 2.5, and is your fastest option.
Demo:
>>> re.match('^[-0-9^+x]*$', '1x2^4-2')
<_sre.SRE_Match object at 0x10f0b6780>
What about just convert both the string into set and checking input_set is subset of good_set as below:
>>> good_set = set('1234567890^-+x')
>>> input_set1 = set('xajfb123')
>>> input_set2 = set('122-32+x')
>>> input_set1.issubset(good_set)
False
>>> input_set2.issubset(good_set)
True
>>>
set, as sets offer faster lookupThen you can use all function like this
valid_chars = set("1234567890^-+x") # Converting to a set
if all(char in valid_chars for char in input_string):
# Do stuff if input is valid
We can convert the input string also a set and check if all characters in the inputstring is in the valid list.
valid_chars = set("1234567890^-+x") # Converting to a set
if set(input_string).issubset(valid_chars):
# Do stuff if input is valid
issuperset method for this.
issubsetset("1234567890^-+x"). (Why did I see the other thing first?)Yet another way to do it, now using string.translate():
>>> import string
>>> all_chars = string.maketrans('', '')
>>> has_only = lambda s, valid_chars: not s.translate(all_chars, valid_chars)
>>> has_only("abc", "1234567890^-+x.")
False
>>> has_only("x^2", "1234567890^-+x.")
True
It is not the most readable way. It should be one of the fastest if you need it.
None as the first argument to translate() instead.whitelist = '1234567890^-+x'
str = 'x^2+2x+1'
min([ch in whitelist for ch in str])
True
str='x**2 + 1'
min([ch in whitelist for ch in str])
False
min() is hardly the best function for this task.