Understanding string reversal via slicing

Question

I was looking for a way to print a string backwards  and after a quick search on google  I found this method   Suppose a is a string variable  This will return the a string backwards   a   -1    Can anyone explain how that works

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It s using extended slicing - a string is a sequence in Python  and shares some methods with other sequences  namely lists and tuples   There are three parts to slicing - start  stop and step  All of them have default values - start defaults to 0  stop defaults to len sequence   and step defaults to 1  By specifying    -1  you re saying  all the elements in sequence a  starting from the beginning  to the end going backward one at a time   This feature was introduced in Python 2 3 5  and you can read more in the What s New docs

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It s called Slice Notation in Python and you can read a bit more of how it works here   Explain Python  39 s slice notation

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I think the following makes a bit more sense for print strings in reverse  but maybe that s just me   for char in reversed  myString        print  char  end

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It s the extended slice notation   sequence start end step    In this case  step -1 means backwards  and omitting the start and end means you want the whole string

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Consider the list below  l  12 23 345 456 67 7 945 467    Another trick for reversing a list may be    l len l  -len l -1 -1   467  945  7  67  456  345  23  12   l  -len l -1 -1   467  945  7  67  456  345  23  12   l len l   -1   467  945  7  67  456  345  23  12

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a string is essentially a sequence of characters and so the slicing operation works on it   What you are doing is in fact   -  get an slice of  a  from start to end in steps of 1 backward

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I would do it like this   variable    string  message      for b in variable      message   b message print  message    and it prints  gnirts

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Without using reversed or    -1   here is a simple version based on recursion i would consider to be the most readable   def reverse s      if len s   2      return s -1    s 0     if len s   1      return s 0     return s -1    reverse s 1 len s -1     s 0

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The  -1  part represents the  step  part of the slicing   in this case  it goes through the string 1 character at a time  but backwards  a negative step means start from the end of the string   If you specify the step to be 2  for instance  you would get every other character of the string  starting with the first one  If you specify a step of -2  then you d get every other character of the string  starting with the last character and working backwards   So  in a nutshell  if a    12345     a   2  becomes 135 a   -1  becomes 54321 a   -2  becomes 531

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Using extended slice syntax  word   input   Type a word which you want to reverse     def reverse word     word   word   -1    return word print  reverse word

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Sure  the      is the extended slice operator   It allows you to take substrings   Basically  it works by specifying which elements you want as  begin end step   and it works for all sequences   Two neat things about it    You can omit one or more of the elements and it does  the right thing  Negative numbers for begin  end  and step have meaning   For begin and end  if you give a negative number  it means to count from the end of the sequence   For instance  if I have a list   l    1 2 3    Then l -1  is 3  l -2  is 2  and l -3  is 1     For the step argument  a negative number means to work backwards through the sequence   So for a list    l    1 2 3 4 5 6 7 8 9 10    You could write l   -1  which basically means to use a step size of -1 while reading through the list   Python will  do the right thing  when filling in the start and stop so it iterates through the list backwards and gives you  10 9 8 7 6 5 4 3 2 1      I ve given the examples with lists  but strings are just another sequence and work the same way   So a   -1  means to build a string by joining the characters you get by walking backwards through the string

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It s basic step notation  consider the functionality of   a 2 4 2   What happens is the index is sliced between position 2 and 4  what the third variable does is it sets the step size starting from the first value  In this case it would return a 2   since a 4  is an upper bounds only two values are return and no second step takes place  The  -  minus operator simply reverses the step output

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-1  gives a slice of the string a  the full syntax is a begin end step  which gives a begin   a begin step       a end-1   WHen step is negative  you start at end and move to begin   Finally  begin defaults to the beginning of the sequence  end to the end  and step to -1

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the first two bounds default to 0 and the length of the sequence  as before  and a stride of -1 indicates that the slice should go from right to left instead of the usual left to right  The effect  therefore  is to reverse the sequence   name  ravi  print name   -1    ivar

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we can use append and pop to do it  def rev s       i   list s      o   list       while len i   gt  0          o append t pop         return    join o

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You can use reversed   function   For example   x    abcd  for i in reversed x           print i  end     print   n   L    1 2 3  for i in reversed L           print i  end       prints dcba and 321

[python] Understanding string reversal via slicing

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