def isRotation(s1, s2):
return len(s1) == len(s2) and s2 in s1*2
isRotation("ABCD", "CDAB")
>> True
The code above was given as one of many ways to check if two strings are a rotation of each other. However, I don't understand why String1(s1) has to be multiplied by 2 in the code.
In python, multiplying strings is repeating the string, for example:
>>> 'abc'*2
'abcabc'
>>> 'abc'*4
'abcabcabcabc'
So in order to know if a string is a rotation of another string, you would need to multiplie it by 2, for example:
>>> 'bca' in 'abc'
False
>>> 'bca' in 'abc' * 2
True
Multiplying a string by 2 will double the original string. And because s1 wraps around, regardless of its starting position the whole string can be found somewhere within twice its length
ABCDABCD
|||| DABC
||| CDAB
|| BCDA
| ABCD
Rotating a string is as good as chopping it at point of rotation and putting that chopped part at the end of string.
ABCDEFGH rotated 4 places to left is:
Python str*x gives you str concatenated to itself x times.
>>> a = 'String'
>>> a*2
'StringString'
>>>
Putting these two points together with the fact that any string can be rotated by maximum amount equal to its length (when it becomes the original string) gives you the logic to check the rotated string's presence in str*2.
Understanding of your question is- To compare a string with the rotation string.
My approach to address the same would be.
1- To get the rotation string.
def rotate_str(strg, n):
return strg[n:] + strg[:n]
length = 2 #can change this to whatever value suits to you or even pass this as arg.
print(rotate('SAMPLE', length))
2- compare strings.
str1 = 'SAMPLE'
str2 = rotate(str1, length)
def compare_str(str1, str2):
return str1 == str2
Here when we multiply s1 with 2. It returns s1s1 like if s1="ABCD", it will return "ABCDABCD", and in if s2 in s1*2 we are checking that if "CRAB" is in "ABCDABCD" which is true.
str * 2will just add a second copy of the string so"ABC" * 2 -> "ABCABC". From there, it can check if the original string is a substring of this string.