String comparison technique used by Python

Question

I m wondering how Python does string comparison  more specifically how it determines the outcome when a less than   lt   or greater than   gt   operator is used   For instance if I put print  abc   lt   bac   I get True  I understand that it compares corresponding characters in the string  however its unclear as to why there is more  for lack of a better term   weight  placed on the fact that a is less than b  first position  in first string rather than the fact that a is less than b in the second string  second position

User · Answer

Strings are compared lexicographically using the numeric equivalents  the result of the built-in function ord    of their characters  Unicode and 8-bit strings are fully interoperable in this behavior

User · Answer

Python and just about every other computer language use the same principles as  I hope  you would use when finding a word in a printed dictionary    1  Depending on the human language involved  you have a notion of character ordering   a   lt   b   lt   c  etc      2  First character has more weight than second character   az   lt   za   whether the language is written left-to-right or right-to-left or boustrophedon is quite irrelevant       3  If you run out of characters to test  the shorter string is less than the longer string   foo   lt   food   Typically  in a computer language the  notion of character ordering  is rather primitive  each character has a human-language-independent number ord character  and characters are compared and sorted using that number  Often that ordering is not appropriate to the human language of the user  and then you need to get into  collating   a fun topic

User · Answer

From the docs      The comparison uses lexicographical   ordering  first the first two items   are compared  and if they differ this   determines the outcome of the   comparison  if they are equal  the   next two items are compared  and so   on  until either sequence is   exhausted    Also      Lexicographical ordering for strings uses the Unicode code point number to order individual characters    or on Python 2      Lexicographical ordering for strings uses the ASCII ordering for individual characters    As an example    gt  gt  gt   abc   gt   bac  False  gt  gt  gt  ord  a    ord  b    97  98    The result False is returned as soon as a is found to be less than b  The further items are not compared  as you can see for the second items  b   a is True    Be aware of lower and uppercase    gt  gt  gt    x  ord x   for x in abc     a   97     b   98     c   99     d   100     e   101     f   102     g   103     h   104     i   105     j   106     k   107     l   108     m   109     n   110     o   111     p   112     q   113     r   114     s   115     t   116     u   117     v   118     w   119     x   120     y   121     z   122    gt  gt  gt    x  ord x   for x in abc upper       A   65     B   66     C   67     D   68     E   69     F   70     G   71     H   72     I   73     J   74     K   75     L   76     M   77     N   78     O   79     P   80     Q   81     R   82     S   83     T   84     U   85     V   86     W   87     X   88     Y   89     Z   90

User · Answer

Take a look also at How do I sort unicode strings alphabetically in Python  where the discussion is about sorting rules given by the Unicode Collation Algorithm  http   www unicode org reports tr10     To reply to the comment     What  How else can ordering be defined other than left-to-right    by S Lott  there is a famous counter-example when sorting French language  It involves accents  indeed  one could say that  in French  letters are sorted left-to-right and accents right-to-left  Here is the counter-example   we have e  lt     and o  lt      so you would expect the words cote  cot    c  te  c  t   to be sorted as cote  lt  cot    lt  c  te  lt  c  t    Well  this is not what happens  in fact you have  cote  lt  c  te  lt  cot    lt  c  t    i e   if we remove  c  and  t   we get oe  lt    e  lt  o    lt        which is exactly right-to-left ordering   And a last remark  you shouldn t be talking about left-to-right and right-to-left sorting but rather about forward and backward sorting   Indeed there are languages written from right to left and if you think Arabic and Hebrew are sorted right-to-left you may be right from a graphical point of view  but you are wrong on the logical level   Indeed  Unicode considers character strings encoded in logical order  and writing direction is a phenomenon occurring on the glyph level  In other words  even if in the word      the letter shin appears on the right of the lamed  logically it occurs before it  To sort this word one will first consider the shin  then the lamed  then the vav  then the mem  and this is forward ordering  although Hebrew is written right-to-left   while French accents are sorted backwards  although French is written left-to-right

User · Answer

This is a lexicographical ordering  It just puts things in dictionary order

User · Answer

Python string comparison is lexicographic   From Python Docs  http   docs python org reference expressions html     Strings are compared lexicographically using the numeric equivalents  the result of the built-in function ord    of their characters  Unicode and 8-bit strings are fully interoperable in this behavior    Hence in your example   abc   lt   bac    a  comes before  less-than   b  numerically  in ASCII and Unicode representations   so the comparison ends right there

User · Answer

A pure Python equivalent for string comparisons would be  def less string1  string2         Compare character by character     for idx in range min len string1   len string2               Get the  quot value quot  of the character         ordinal1  ordinal2   ord string1 idx    ord string2 idx             If the  quot value quot  is identical check the next characters         if ordinal1    ordinal2              continue           It s not equal so we re finished at this index and can evaluate which is smaller          else              return ordinal1  lt  ordinal2       We re out of characters and all were equal  so the result depends on the length       of the strings      return len string1   lt  len string2   This function does the equivalent of the real method  Python 3 6 and Python 2 7  just a lot slower  Also note that the implementation isn t exactly  quot pythonic quot  and only works for  lt  comparisons  It s just to illustrate how it works  I haven t checked if it works like Pythons comparison for combined unicode characters  A more general variant would be  from operator import lt  gt  def compare string1  string2  less True       op   lt if less else gt     for char1  char2 in zip string1  string2           ordinal1  ordinal2   ord char1   ord char1          if ordinal1    ordinal2              continue         else              return op ordinal1  ordinal2      return op len string1   len string2

[python] String comparison technique used by Python

Examples related to python

Examples related to string

Examples related to comparison