Reverse a string without using reversed or -1

Question

I came across a strange Codecademy exercise that required a function that would take a string as input and return it in reverse order  The only problem was you could not use the reversed method or the common answer here on stackoverflow     -1    Obviously in the real world of programming  one would most likely go with the extended slice method  or even using the reversed function but perhaps there is some case where this would not work   I present a solution below in Q amp A style  in case it is helpful for people in the future

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I created different versions of how to reverse a string in python in my repo: https://github.com/fedmich/Python-Codes/tree/master/Reverse%20a%20String

You can do it by using list-comprehension or lambda technique:

# Reverse a string without using reverse() function
s = 'Federico';
li = list( s )  #convert string to list

ret = [ li[i-1] for i in xrange(len(li),0,-1)  ]    #1 liner lambda
print ( "".join( ret ) )

or by doing a backward for loop

# Reverse a string without using reverse() function
s = 'Federico';
r = []

length = len(s)
for i in xrange(length,0,-1):
    r.append( s[ i - 1] )

print ( "".join(r) )

User · Answer

m   input  Enter string     def rev s       z   -1     x   len s      while x    0          print s z   end             z   z-1         x   x-1 rev m

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You can simply reverse iterate your string starting from the last character  With python you can use list comprehension to construct the list of characters in reverse order and then join them to get the reversed string in a one-liner   def reverse s     return    join  s -i-1  for i in xrange len s       if you are not allowed to even use negative indexing you should replace s -i-1  with s len s -i-1

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You can simply use the pop as suggested  Here are one liners fot that  chaine    qwertyuiop     join  chaine - x   1   for x in range len chaine      poiuytrewq  gg   list chaine     join  gg pop   for   in range len gg      poiuytrewq

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enterString input  Enter a string to be reversed    string List list enterString  revedString     for i in range len string List        revedString append string List pop     Pop last element and append it to the empty list print  reversed string is      join revedString   as the out come is in list convert it into string using  join  Sample Output   gt  gt  gt Enter a string to be reversed  sampleReverse1    gt  gt  gt reversed string is 1esreveRelpmas

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I prefer this as the best way of reversing a string using a for loop   def reverse a string str         result         for i in range len str  1 -1           result  result  str i-1      return result  print reverse a string input

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A golfed version  r lambda x    join x i  for i in range len x-1  -1 -1

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My solution   s   raw input  Enter string    print def reverse text      st        rev        count   len text    print  Lenght of text     len text    print   for c in range len text          count   count - 1       st   st      join text c         rev   rev      join text count         print  count           count       print  print c         c       print  text c          text c        print   print  Original        st   print  Reversed        rev   return rev     reverse s   Result screen    Enter string  joca    Lenght of text   4    count         3 print c       0 text c        j    count         2 print c       1 text c        o    count         1 print c       2 text c        c    count         0 print c       3 text c        a    Original      joca Reversed      acoj None

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def reverse s       return    join s i  for i in range len s -1  -1  -1

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Only been coding Python for a few days  but I feel like this was a fairly clean solution  Create an empty list  loop through each letter in the string and append it to the front of the list  return the joined list as a string   def reverse text   backwardstext      for letter in text      backwardstext insert 0  letter  return    join backwardstext

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Simple Method    gt  gt  gt  a    hi hello   gt  gt  gt     join  a len a -i-1  for i j in enumerate a     olleh ih   gt  gt  gt

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you have got enough answer   Just want to share another way   you can write a two small function for reverse and compare the function output with the given string  var       def reverse data    for i in data      var   i   var return var   if not var    data    print  No palindrome   else    print  Palindrome

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I used this   def reverse text   s    l len text  for i in range l       s  text l-1-i  return s

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You ve received a lot of alternative answers  but just to add another simple solution -- the first thing that came to mind something like this   def reverse text       reversed text              for n in range len text            reversed text    text -1 - n       return reversed text   It s not as fast as some of the other options people have mentioned or built in methods   but easy to follow as we re simply using the length of the text string to concatenate one character at a time by slicing from the end toward the front

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We can first convert string into a list and then reverse the string after that use   join  method to again convert list to single string   text  Youknowwhoiam  lst list text  lst reverse   print    join lst     It will first convert  youknowwhoiam  into   Y    o    u    k    n    o    w    w    h    o    i    a    m      After it will reverse the list into   m    a    i    o    h    w    w    o    n    k    u    o    Y    In last it will convert list into single word  maiohwwonkuoY    For more explanation in brief just run this code -  text  Youknowwhoiam  lst list text  print  lst  lst reverse   print lst  print    join lst

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def reverseThatString theString       reversedString          lenOfString   len theString      for i j in enumerate theString           lenOfString -  1         reversedString    theString lenOfString      return reversedString

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You can also do it with recursion   def reverse text       if len text   lt   1          return text      return reverse text 1      text 0    And a simple example for the string hello      reverse hello     reverse ello    h             The recursive step    reverse llo    e   h    reverse lo    l   e   h    reverse o    l   l   e   h    Base case    o   l   l   e   h    olleh

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Here is one using a list as a stack      def reverse s     rev     t for  t in s    t        while len rev     0      t  rev pop     return t

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Blender s answer is lovely  but for a very long string  it will result in a whopping RuntimeError  maximum recursion depth exceeded   One might refactor the same code into a while loop  as one frequently must do with recursion in python  Obviously still bad due to time and memory issues  but at least will not error   def reverse text       answer          while text          answer   text 0    answer         text   text 1       return answer

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author      Sreedhar   find reverse of the string with out using index or reverse function   def reverse text       revs          for i in text          revs   i   revs     return revs  strig   raw input  enter anything     print reverse strig    index method print strig   -1

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You can do simply like this  def rev str      rev         for i in range 0 len str       rev   rev   str  len str -1 -i     return rev

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Just another option   from collections import deque def reverse iterable       d   deque       d extendleft iterable      return    join d

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All I did to achieve a reverse string is use the xrange function with the length of the string in a for loop and step back per the following   myString    ABC   for index in xrange len myString  -1       print index   My output is  CBA

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Not very clever  but tricky solution  def reverse t       for j in range len t     2           t   t  j    t - j - 1    t j   1 - j - 1    t j    t len t  - j       return t

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Here s my contribution   def rev test         test   list test      i   len test -1     result           print test     while i  gt   0          result append test pop i           i -  1     return    join result

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Use reversed range   def reverse strs       for i in xrange len strs -1  -1  -1           yield strs i                gt  gt  gt     join reverse  hello     olleh    xrange or  range with -1 step would return items in reversed order  so we need to iterate from len string -1 to -1 exclusive  and fetch items from the string one by one    gt  gt  gt  list xrange len strs  -1  -1   -1    4  3  2  1  0    iterate over these indexes and fetch the items from the string   One-liner   def reverse strs       return    join  strs i  for i in xrange len strs -1  -1  -1          gt  gt  gt  reverse  hello    olleh

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reduce lambda x  y   y   x   hello world

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def reverse text       a        l len text      while l gt  1           a  text l-1          l- 1     return a   i just concatenated the string a with highest indexes of text  which keeps on decrementing by 1 each loop

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The way I can think of without using any built-in functions   a    word  count   0 for letter in a      count    1  b      for letter in a      b    a count-1      count -  1   And if you print b   print b drow

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Easiest way to reverse a string       backwards   input  Enter string to reverse         print backwards   -1

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This is my solution using the for i in range loop   def reverse string       tmp          for i in range 1 len string  1           tmp    string len string -i                  return tmp   It s pretty easy to understand  I start from 1 to avoid index out of bound

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Today I was asked this same exercise on pen amp paper  so I come up with this function for lists   def rev s     l   len s    for i j in zip range l-1  0  -1   range l  2        s i   s j    s j   s i    return s   which can be used with strings with    join rev list  hello

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Pointfree   from functools import partial from operator import add  flip   lambda f  lambda x  y  f y  x  rev   partial reduce  flip add     Test    gt  gt  gt  rev  hello    olleh

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Inspired by Jon s answer  how about this one  word    hello  q   deque word     join q pop   for   in range len word

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i just solved this in code academy and was checking my answers and ran across this list  so with a very limited understanding of python i just did this and it seamed to work   def reverse s       i   len s  - 1     sNew          while  i  gt   0          sNew   sNew   str s i           i   i -1     return sNew

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This is the simplest way in python 2 7 syntax def reverse text          rev    quot  quot        final    quot  quot         for a in range 0 len text                rev   text len text -a-1              final   final   rev return final

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I have also just solved the coresponding exercise on codeacademy and wanted to compare my approach to others  I have not found the solution I used so far  so I thought that I sign up here and provide my solution to others  And maybe I get a suggestion or a helpful comment on how to improve the code   Ok here it goes  I did not use any list to store the string  instead I have just accessed the string index  It took me a bit at first to deal with the len   and index number  but in the end it worked       def reverse x   reversestring      for n in range len str x  -1 -1  -1       reversestring    x n  return reversestring    I am still wondering if the reversestring      could be solved in a more elegant way  or if it is  bad style  even  but i couldn t find an answer so far

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EDIT  Recent activity on this question caused me to look back and change my solution to a quick one-liner using a generator   rev      join  text len text  - count  for count in xrange 1 len text  1      Although obviously there are some better answers here like a negative step in the range or xrange function  The following is my original solution     Here is my solution  I ll explain it step by step  def reverse text        lst          count   1      for i in range 0 len text             lst append text len text -count           count    1      lst      join lst      return lst  print reverse  hello     First  we have to pass a parameter to the function  in this case text    Next  I set an empty list  named lst to use later   I actually didn t know I d need the list until I got to the for loop  you ll see why it s necessary in a second    The count variable will make sense once I get into the for loop  So let s take a look at a basic version of what we are trying to accomplish   It makes sense that appending the last character to the list would start the reverse order  For example    gt  gt lst       gt  gt word    foo   gt  gt lst append word 2    gt  gt print lst   o     But in order to continue reversing the order  we need to then append word 1  and then word 0     gt  gt lst append word 2    gt  gt lst append word 1    gt  gt lst append word 0    gt  gt print lst   o   o   f     This is great  we now have a list that has our original word in reverse order and it can be converted back into a string by using  join    But there s a problem  This works for the word foo  it even works for any word that has a length of 3 characters  But what about a word with 5 characters  Or 10 characters  Now it won t work  What if there was a way we could dynamically change the index we append so that any word will be returned in reverse order    Enter for loop   for i in range 0 len text         lst append text len text -count       count    1   First off  it is necessary to use in range   rather than just in  because we need to iterate through the characters in the word  but we also need to pull the index value of the word so that we change the order    The first part of the body of our for loop should look familiar  Its very similar to   gt  gt lst append word   index       In fact  the base concept of it is exactly the same    gt  gt lst append text   index       So what s all the stuff in the middle doing    Well  we need to first append the index of the last letter to our list  which is the length of the word  text  -1  From now on we ll refer to it as l t  -1   gt  gt lst append text len text -1     That alone will always get the last letter of our word  and append it to lst  regardless of the length of the word  But now that we have the last letter  which is l t  - 1  we need the second to last letter  which is l t  - 2  and so on  until there are no more characters to append to the list  Remember our count variable from above  That will come in handy  By using a for loop  we can increment the value of count by 1 through each iteration  so that the value we subtract by increases  until the for loop has iterated through the entire word    gt  gt for i in range 0 len text                       lst append text len text -count           count    1   Now that we have the heart of our function  let s look at what we have so far   def reverse text        lst          count   1      for i in range 0 len text             lst append text len text -count           count    1   We re almost done  Right now  if we were to call our function with the word  hello   we would get a list that looks like     o   l   l   e   h    We don t want a list  we want a string  We can use  join for that   def reverse text        lst          count   1      for i in range 0 len text             lst append text len text -count           count    1      lst      join lst    join the letters together without a space     return lst   And that s it  If we call the word  hello  on reverse    we d get this    gt  gt print reverse  hello   olleh   Obviously  this is way more code than is necessary in a real life situation  Using the reversed function or extended slice would be the optimal way to accomplish this task  but maybe there is some instance when it would not work  and you would need this  Either way  I figured I d share it for anyone who would be interested    If you guys have any other ideas  I d love to hear them

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simply run this but it would print each character in a separate line the second version prints it in one line   def rev str           for i in range 0 len str                print list str  len str -i-1     Print in one line   def rev str       rev   list       for i in range 0 len str            rev append  list str  len str -i-1        print    join rev

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This is a way to do it with a while loop   def reverse s       t   -1     s2          while abs t   lt  len s    1           s2   s2   s t          t    t - 1     return s2

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I have got an example to reverse strings in python from thecrazyprogrammer com   string1     IkNoWwHeReYoUlIvE  string2       i   len string1 -1  while i gt  0     string2   string2   string1 i    i   i-1  print  original       string1  print  reverse        string2

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This is a very interesting question  I will like to offer a simple one liner answer    gt  gt  gt  S  abcdefg   gt  gt  gt     join item 1  for item in sorted enumerate S   reverse True    gfedcba    Brief explanation   enumerate   returns   0   a     1   b     2   c     3   d     4   e     5   f     6   g     The indices and the values  To reverse the values  just reverse sort it by sorted    Finally  just put it together back to a str

[python] Reverse a string without using reversed() or [::-1]?

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