Reverse a string in Python
There is no built in reverse function for Python's str object. What is the best way of implementing this method?
If supplying a very concise answer, please elaborate on its efficiency. For example, whether the str object is converted to a different object, etc.
转载于:https://*.com/questions/931092/reverse-a-string-in-python
How about:
>>> 'hello world'[::-1]
'dlrow olleh'
This is extended slice syntax. It works by doing [begin:end:step] - by leaving begin and end off and specifying a step of -1, it reverses a string.
@Paolo's s[::-1] is fastest; a slower approach (maybe more readable, but that's debatable) is ''.join(reversed(s)).
Reverse a string in python without using reversed() or [::-1]
def reverse(test):
n = len(test)
x=""
for i in range(n-1,-1,-1):
x += test[i]
return x
What is the best way of implementing a reverse function for strings?
My own experience with this question is academic. However, if you're a pro looking for the quick answer, use a slice that steps by -1:
>>> 'a string'[::-1]
'gnirts a'
or more readably (but slower due to the method name lookups and the fact that join forms a list when given an iterator), str.join:
>>> ''.join(reversed('a string'))
'gnirts a'
or for readability and reusability, put the slice in a function
def reversed_string(a_string):
return a_string[::-1]
and then:
>>> reversed_string('a_string')
'gnirts_a'
Longer explanation
If you're interested in the academic exposition, please keep reading.
There is no built-in reverse function in Python's str object.
Here is a couple of things about Python's strings you should know:
In Python, strings are immutable. Changing a string does not modify the string. It creates a new one.
-
Strings are sliceable. Slicing a string gives you a new string from one point in the string, backwards or forwards, to another point, by given increments. They take slice notation or a slice object in a subscript:
string[subscript]
The subscript creates a slice by including a colon within the braces:
string[start:stop:step]
To create a slice outside of the braces, you'll need to create a slice object:
slice_obj = slice(start, stop, step)
string[slice_obj]
A readable approach:
While ''.join(reversed('foo')) is readable, it requires calling a string method, str.join, on another called function, which can be rather relatively slow. Let's put this in a function - we'll come back to it:
def reverse_string_readable_answer(string):
return ''.join(reversed(string))
Most performant approach:
Much faster is using a reverse slice:
'foo'[::-1]
But how can we make this more readable and understandable to someone less familiar with slices or the intent of the original author? Let's create a slice object outside of the subscript notation, give it a descriptive name, and pass it to the subscript notation.
start = stop = None
step = -1
reverse_slice = slice(start, stop, step)
'foo'[reverse_slice]
Implement as Function
To actually implement this as a function, I think it is semantically clear enough to simply use a descriptive name:
def reversed_string(a_string):
return a_string[::-1]
And usage is simply:
reversed_string('foo')
What your teacher probably wants:
If you have an instructor, they probably want you to start with an empty string, and build up a new string from the old one. You can do this with pure syntax and literals using a while loop:
def reverse_a_string_slowly(a_string):
new_string = ''
index = len(a_string)
while index:
index -= 1 # index = index - 1
new_string += a_string[index] # new_string = new_string + character
return new_string
This is theoretically bad because, remember, strings are immutable - so every time where it looks like you're appending a character onto your new_string, it's theoretically creating a new string every time! However, CPython knows how to optimize this in certain cases, of which this trivial case is one.
Best Practice
Theoretically better is to collect your substrings in a list, and join them later:
def reverse_a_string_more_slowly(a_string):
new_strings = []
index = len(a_string)
while index:
index -= 1
new_strings.append(a_string[index])
return ''.join(new_strings)
However, as we will see in the timings below for CPython, this actually takes longer, because CPython can optimize the string concatenation.
Timings
Here are the timings:
>>> a_string = 'amanaplanacanalpanama' * 10
>>> min(timeit.repeat(lambda: reverse_string_readable_answer(a_string)))
10.38789987564087
>>> min(timeit.repeat(lambda: reversed_string(a_string)))
0.6622700691223145
>>> min(timeit.repeat(lambda: reverse_a_string_slowly(a_string)))
25.756799936294556
>>> min(timeit.repeat(lambda: reverse_a_string_more_slowly(a_string)))
38.73570013046265
CPython optimizes string concatenation, whereas other implementations may not:
... do not rely on CPython's efficient implementation of in-place string concatenation for statements in the form a += b or a = a + b . This optimization is fragile even in CPython (it only works for some types) and isn't present at all in implementations that don't use refcounting. In performance sensitive parts of the library, the ''.join() form should be used instead. This will ensure that concatenation occurs in linear time across various implementations.
Here is a no fancy one:
def reverse(text):
r_text = ''
index = len(text) - 1
while index >= 0:
r_text += text[index] #string canbe concatenated
index -= 1
return r_text
print reverse("hello, world!")
def reverse(input):
return reduce(lambda x,y : y+x, input)
Quick Answer (TL;DR)
Example
### example01 -------------------
mystring = 'coup_ate_grouping'
backwards = mystring[::-1]
print backwards
### ... or even ...
mystring = 'coup_ate_grouping'[::-1]
print mystring
### result01 -------------------
'''
gnipuorg_eta_puoc
'''
Detailed Answer
Background
This answer is provided to address the following concern from @odigity:
Wow. I was horrified at first by the solution Paolo proposed, but that took a back seat to the horror I felt upon reading the first comment: "That's very pythonic. Good job!" I'm so disturbed that such a bright community thinks using such cryptic methods for something so basic is a good idea. Why isn't it just s.reverse()?
Problem
-
Context
- Python 2.x
- Python 3.x
-
Scenario:
- Developer wants to transform a string
- Transformation is to reverse order of all the characters
Solution
- example01 produces the desired result, using extended slice notation.
Pitfalls
- Developer might expect something like
string.reverse() - The native idiomatic (aka "pythonic") solution may not be readable to newer developers
- Developer may be tempted to implement his or her own version of
string.reverse()to avoid slice notation. - The output of slice notation may be counter-intuitive in some cases:
- see e.g., example02
print 'coup_ate_grouping'[-4:] ## => 'ping'- compared to
print 'coup_ate_grouping'[-4:-1] ## => 'pin'- compared to
print 'coup_ate_grouping'[-1] ## => 'g'
- the different outcomes of indexing on
[-1]may throw some developers off
- see e.g., example02
Rationale
Python has a special circumstance to be aware of: a string is an iterable type.
One rationale for excluding a string.reverse() method is to give python developers incentive to leverage the power of this special circumstance.
In simplified terms, this simply means each individual character in a string can be easily operated on as a part of a sequential arrangement of elements, just like arrays in other programming languages.
To understand how this works, reviewing example02 can provide a good overview.
Example02
### example02 -------------------
## start (with positive integers)
print 'coup_ate_grouping'[0] ## => 'c'
print 'coup_ate_grouping'[1] ## => 'o'
print 'coup_ate_grouping'[2] ## => 'u'
## start (with negative integers)
print 'coup_ate_grouping'[-1] ## => 'g'
print 'coup_ate_grouping'[-2] ## => 'n'
print 'coup_ate_grouping'[-3] ## => 'i'
## start:end
print 'coup_ate_grouping'[0:4] ## => 'coup'
print 'coup_ate_grouping'[4:8] ## => '_ate'
print 'coup_ate_grouping'[8:12] ## => '_gro'
## start:end
print 'coup_ate_grouping'[-4:] ## => 'ping' (counter-intuitive)
print 'coup_ate_grouping'[-4:-1] ## => 'pin'
print 'coup_ate_grouping'[-4:-2] ## => 'pi'
print 'coup_ate_grouping'[-4:-3] ## => 'p'
print 'coup_ate_grouping'[-4:-4] ## => ''
print 'coup_ate_grouping'[0:-1] ## => 'coup_ate_groupin'
print 'coup_ate_grouping'[0:] ## => 'coup_ate_grouping' (counter-intuitive)
## start:end:step (or start:end:stride)
print 'coup_ate_grouping'[-1::1] ## => 'g'
print 'coup_ate_grouping'[-1::-1] ## => 'gnipuorg_eta_puoc'
## combinations
print 'coup_ate_grouping'[-1::-1][-4:] ## => 'puoc'
Conclusion
The cognitive load associated with understanding how slice notation works in python may indeed be too much for some adopters and developers who do not wish to invest much time in learning the language.
Nevertheless, once the basic principles are understood, the power of this approach over fixed string manipulation methods can be quite favorable.
For those who think otherwise, there are alternate approaches, such as lambda functions, iterators, or simple one-off function declarations.
If desired, a developer can implement her own string.reverse() method, however it is good to understand the rationale behind this aspect of python.
See also
Here is one without [::-1] or reversed (for learning purposes):
def reverse(text):
new_string = []
n = len(text)
while (n > 0):
new_string.append(text[n-1])
n -= 1
return ''.join(new_string)
print reverse("abcd")
you can use += to concatenate strings but join() is faster.
Sure, in Python you can do very fancy 1-line stuff. :)
Here's a simple, all rounder solution that could work in any programming language.
def reverse_string(phrase):
reversed = ""
length = len(phrase)
for i in range(length):
reversed += phrase[length-1-i]
return reversed
phrase = raw_input("Provide a string: ")
print reverse_string(phrase)
s = 'hello'
ln = len(s)
i = 1
while True:
rev = s[ln-i]
print rev,
i = i + 1
if i == ln + 1 :
break
OUTPUT :
o l l e h
A lesser perplexing way to look at it would be:
string = 'happy'
print(string)
'happy'
string_reversed = string[-1::-1]
print(string_reversed)
'yppah'
In English [-1::-1] reads as:
"Starting at -1, go all the way, taking steps of -1"
You can use the reversed function with a list comprehesive. But I don't understand why this method was eliminated in python 3, was unnecessarily.
string = [ char for char in reversed(string)]
Another alternative, (not efficient! just to show the diversity of Python with so many possible solutions!): convert the string to a list using the list() function. A list value is a mutable data type. Therefore we can use the method reverse() which reverses the objects of a list in place. And then we convert the list back to a string using the list join method with an empty separator:
>>> s = 'hello world'
>>> s
'hello world'
>>> t = list(s) # convert to list
>>> t
['h', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd']
>>> t.reverse() # reverse method of list
>>> t
['d', 'l', 'r', 'o', 'w', ' ', 'o', 'l', 'l', 'e', 'h']
>>> s = ''.join(t) # convert to string
>>> s
'dlrow olleh'
Here is simply:
print "loremipsum"[-1::-1]
and some logically:
def str_reverse_fun():
empty_list = []
new_str = 'loremipsum'
index = len(new_str)
while index:
index = index - 1
empty_list.append(new_str[index])
return ''.join(empty_list)
print str_reverse_fun()
output:
muspimerol
Recursive method:
def reverse(s): return s[0] if len(s)==1 else s[len(s)-1] + reverse(s[0:len(s)-1])
example:
print(reverse("Hello!")) #!olleH
s = 'Hello world'
s[::-1]
in the above example label s or variable s is holding string which contain Hello world string and on second step i m printing reverse of Hello world string by taking starting from everything to everything in reverse step order with -1.
All of the above solutions are perfect but if we are trying to reverse a string using for loop in python will became a little bit tricky so here is how we can reverse a string using for loop
string ="hello,world"
for i in range(-1,-len(string)-1,-1):
print (string[i],end=(" "))
I hope this one will be helpful for someone.
using slice notation
def rev_string(s):
return s[::-1]
using reversed() function
def rev_string(s):
return ''.join(reversed(s))
using recursion
def rev_string(s):
if len(s) == 1:
return s
return s[-1] + rev_string(s[:-1])
def rev(str1):
print( str1[::-1])
str1="123"
r=rev(str1)
This is simple and meaningful reverse function, easy to understand and code
def reverse_sentence(text):
words = text.split(" ")
reverse =""
for word in reversed(words):
reverse += word+ " "
return reverse
This is also an interesting way:
def reverse_words_1(s):
rev = ''
for i in range(len(s)):
j = ~i # equivalent to j = -(i + 1)
rev += s[j]
return rev
or similar:
def reverse_words_2(s):
rev = ''
for i in reversed(range(len(s)):
rev += s[i]
return rev
Another more 'exotic' way using byterarray which supports .reverse()
b = byterarray('Reverse this!', 'UTF-8')
b.reverse()
b.decode('UTF-8')`
will produce:
'!siht esreveR'
Thats my way:
def reverse_string(string):
character_list = []
for char in string:
character_list.append(char)
reversed_string = ""
for char in reversed(character_list):
reversed_string += char
return reversed_string