Reverse a string in Python

Question

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

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Here is one without    -1  or reversed  for learning purposes    def reverse text       new string          n   len text      while  n  gt  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

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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

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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

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def reverse string string       length   len string      temp          for i in range length           temp    string length - i - 1      return temp  print reverse string  foo     prints  oof    This works by looping through a string and assigning its values in reverse order to another string

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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

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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

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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

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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    gt  gt  gt   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    gt  gt  gt     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    gt  gt  gt  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    gt  gt  gt  a string    amanaplanacanalpanama    10  gt  gt  gt  min timeit repeat lambda  reverse string readable answer a string    10 38789987564087  gt  gt  gt  min timeit repeat lambda  reversed string a string    0 6622700691223145  gt  gt  gt  min timeit repeat lambda  reverse a string slowly a string    25 756799936294556  gt  gt  gt  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

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1  using slice notation  def rev string s        return s   -1    2  using reversed   function  def rev string s        return    join reversed s     3  using recursion  def rev string s        if len s     1          return s      return s -1    rev string s  -1

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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

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Reverse a string without python magic    gt  gt  gt  def reversest st       a len st -1     for i in st          print st a  end             a a-1

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The existing answers are only correct if Unicode Modifiers   grapheme clusters are ignored  I ll deal with that later  but first have a look at the speed of some reversal algorithms     list comprehension    min    0 6  s  mean    0 6  s  max     2 2  s reverse func          min    1 9  s  mean    2 0  s  max     7 9  s reverse reduce        min    5 7  s  mean    5 9  s  max    10 2  s reverse loop          min    3 0  s  mean    3 1  s  max     6 8  s     list comprehension    min    4 2  s  mean    4 5  s  max    31 7  s reverse func          min   75 4  s  mean   76 6  s  max   109 5  s reverse reduce        min  749 2  s  mean  882 4  s  max  2310 4  s reverse loop          min  469 7  s  mean  577 2  s  max  1227 6  s   You can see that the time for the list comprehension  reversed   string   -1   is in all cases by far the lowest  even after fixing my typo    String Reversal  If you really want to reverse a string in the common sense  it is WAY more complicated  For example  take the following string  brown finger pointing left  yellow finger pointing up   Those are two graphemes  but 3 unicode code points  The additional one is a skin modifier   example        But if you reverse it with any of the given methods  you get brown finger pointing up  yellow finger pointing left  The reason for this is that the  brown  color modifier is still in the middle and gets applied to whatever is before it  So we have   U  finger pointing up M  brown modifier L  finger pointing left   and  original  LMU reversed  UML  above solutions  reversed  ULM  correct reversal    Unicode Grapheme Clusters are a bit more complicated than just modifier code points  Luckily  there is a library for handling graphemes    gt  gt  gt  import grapheme  gt  gt  gt  g   grapheme graphemes      gt  gt  gt  list g             and hence the correct answer would be  def reverse graphemes string       g   list grapheme graphemes string       return    join g   -1     which also is by far the slowest   list comprehension    min     0 5  s  mean     0 5  s  max     2 1  s reverse func          min    68 9  s  mean    70 3  s  max   111 4  s reverse reduce        min   742 7  s  mean   810 1  s  max  1821 9  s reverse loop          min   513 7  s  mean   552 6  s  max  1125 8  s reverse graphemes     min  3882 4  s  mean  4130 9  s  max  6416 2  s   The Code     usr bin env python  import numpy as np import random import timeit from functools import reduce random seed 0    def main        longstring      join random choices  ABCDEFGHIJKLM   k 2000       functions     list comprehension   list comprehension   longstring                     reverse func   reverse func   longstring                     reverse reduce   reverse reduce   longstring                     reverse loop   reverse loop   longstring                         duration list          for func  name  params in functions          durations   timeit repeat lambda  func params   repeat 100  number 3          duration list name    list np array durations    1000          print   func  lt 20                    min   min 5 1f   s  mean   mean 5 1f   s  max   max 6 1f   s                 format func name                        min min durations    10  6                        mean np mean durations    10  6                        max max durations    10  6                                   create boxplot  Reversing a string of length     format len longstring                           duration list    def list comprehension string       return string   -1    def reverse func string       return    join reversed string     def reverse reduce string       return reduce lambda x  y  y   x  string    def reverse loop string       reversed str          for i in string          reversed str   i   reversed str     return reversed str   def create boxplot title  duration list  showfliers False       import seaborn as sns     import matplotlib pyplot as plt     import operator     plt figure num None  figsize  8  4   dpi 300                 facecolor  w   edgecolor  k       sns set style  whitegrid       sorted keys  sorted vals   zip  sorted duration list items                                               key operator itemgetter 1        flierprops   dict markerfacecolor  0 75   markersize 1                        linestyle  none       ax   sns boxplot data sorted vals  width  3  orient  h                        flierprops flierprops                       showfliers showfliers      ax set xlabel  Time in ms   ylabel         plt yticks plt yticks   0   sorted keys      ax set title title      plt tight layout       plt savefig  output-string png     if   name         main         main

User · Answer

a input    print a   -1    The above code recieves the input from the user and prints an output that is equal to the reverse of the input by adding    -1   OUTPUT   gt  gt  gt  Happy   gt  gt  gt  yppaH  But when it comes to the case of sentences  view the code output below   gt  gt  gt  Have a happy day  gt  gt  gt  yad yppah a evaH  But if you want only the characters of the string to be reversed and not the sequence of string  try this  a input   split    Splits the input on the basis of space   quot   quot   for b in a   declares that var  b  is any value in the list  a      print b   -1   end  quot   quot    End declares to print the character in its quotes   quot   quot   without a new line    In the above code in line 2 in I said that    variable b is any value in the list  a    I said var a to be a list because when you use split in an input the variable of the input becomes a list  Also remember that split can t be used in the case of int input     OUTPUT   gt  gt  gt  Have a happy day  gt  gt  gt  evaH a yppah yad   If we don t add end  quot   quot   in the above code then it will print like the following    gt  gt  gt  Have a happy day  gt  gt  gt  evaH  gt  gt  gt  a  gt  gt  gt  yppah  gt  gt  gt  yad   Below is an example to understand end     CODE  for i in range 1 6        print i   Without end    OUTPUT   gt  gt  gt  1  gt  gt  gt  2  gt  gt  gt  3  gt  gt  gt  4  gt  gt  gt  5   Now code with end     for i in range 1 6       print i  end  quot      quot    OUTPUT   gt  gt  gt  1    2    3    4    5

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Here is a no fancy one   def reverse text       r text          index   len text  - 1      while index  gt   0          r text    text index   string canbe concatenated         index -  1      return r text  print reverse  hello  world

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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

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This class uses python magic functions to reverse a string   class Reverse object           Builds a reverse method using magic methods          def   init   self  data           self data   data         self index   len data        def   iter   self           return self      def   next   self           if self index    0              raise StopIteration          self index   self index - 1         return self data self index    REV INSTANCE   Reverse  hello world    iter REV INSTANCE   rev str      for char in REV INSTANCE      rev str    char  print rev str      Output  dlrow olleh   Reference

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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   bytearray  Reverse this     UTF-8   b reverse   b decode  UTF-8     will produce     siht esreveR

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original    string   rev index   original   -1  rev func   list reversed list original     nsfw  print original  print rev index  print    join rev func

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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

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Paolo s s   -1  is fastest  a slower approach  maybe more readable  but that s debatable  is    join reversed s

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How about    gt  gt  gt   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

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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

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Use a combination of a string array and a for loop  s  Banana-Ana   print s 0    we can access each character of the string this way  where 0 is for the first letter  B in this case  print len s    we can see the length of the string   By combining above with a for loop  we get the reverse printed out  the old fashioned way  for a in range len s    print s len s -a-1

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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           gt   ping  compared to print  coup ate grouping  -4 -1        gt   pin  compared to print  coup ate grouping  -1        gt   g   the different outcomes of indexing on  -1  may throw some developers off    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        gt   c  print  coup ate grouping  1        gt   o   print  coup ate grouping  2        gt   u       start  with negative integers  print  coup ate grouping  -1        gt   g  print  coup ate grouping  -2        gt   n   print  coup ate grouping  -3        gt   i       start end  print  coup ate grouping  0 4          gt   coup      print  coup ate grouping  4 8          gt    ate      print  coup ate grouping  8 12         gt    gro          start end  print  coup ate grouping  -4           gt   ping   counter-intuitive  print  coup ate grouping  -4 -1        gt   pin  print  coup ate grouping  -4 -2        gt   pi  print  coup ate grouping  -4 -3        gt   p  print  coup ate grouping  -4 -4        gt     print  coup ate grouping  0 -1         gt   coup ate groupin  print  coup ate grouping  0            gt   coup ate grouping   counter-intuitive      start end step  or start end stride  print  coup ate grouping  -1  1        gt   g     print  coup ate grouping  -1  -1       gt   gnipuorg eta puoc      combinations print  coup ate grouping  -1  -1  -4        gt   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   alternate simple approach alternate simple approach alternate explanation of slice notation

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def reverse input       return reduce lambda x y   y x  input

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There are a lot of ways to reverse a string but I also created another one just for fun  I think this approach is not that bad   def reverse  str       list char   list  str    Create a hypothetical list  because string is immutable      for i in range len list char  2     just t n 2  to reverse a big string         list char i   list char -i - 1    list char -i - 1   list char i       return    join list char   print reverse  Ehsan

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To solve this in programing way for interview  def reverse a string string  str  - gt  str              This method is used to reverse a string      Args          string  a string to reverse      Returns  a reversed string             if type string     str          raise TypeError   0  This not a string  Please provide a string   format type string        string place holder          start   0     end   len string  - 1     if end  gt   1          while start  lt   end              string place holder   string place holder   string end              end -  1         return string place holder     else          return string   a    hello world  rev   reverse a string a  print rev    Output   dlrow olleh

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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

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With python 3 you can reverse the string in-place meaning it won t get assigned to another variable  First you have to convert the string into a list and then leverage the reverse   function    https   docs python org 3 tutorial datastructures html     def main            my string     h   e   l   l   o           print reverseString my string        def reverseString s         print s        s reverse         return s      if   name         main             main

[python] Reverse a string in Python

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