Sort a list of lists with a custom compare function

Question

I know there are several questions named like this  but they don t seem to work for me   I have a list of lists  50 times 5 elements  I want to sort this list by applying a custom compare function to each element  This function calculates the fitness of the list by which the elements shall be sorted  I created two functions  compare and fitness   def compare item1  item2       return  fitness item1   lt  fitness item2     and  def fitness item       return item 0  item 1  item 2  item 3  item 4    Then I tried to call them by   sorted mylist  cmp compare    or  sorted mylist  key fitness    or  sorted mylist  cmp compare  key fitness    or  sorted mylist  cmp lambda x y  compare x y     Also I tried list sort   with the same parameters  But in any case the functions don t get a list as an argument but a None  I have no idea why that is  coming from mostly C   this contradicts any idea of a callback function for me  How can I sort this lists with a custom function   Edit I found my mistake  In the chain that creates the original list one function didn t return anything but the return value was used  Sorry for the bother

User · Answer

You need to slightly modify your compare function and use functools cmp to key to pass it to sorted  Example code   import functools  lst    list range i  i 5   for i in range 5  1  -1    def fitness item       return item 0  item 1  item 2  item 3  item 4  def compare item1  item2       return fitness item1  - fitness item2   sorted lst  key functools cmp to key compare     Output     2  3  4  5  6    3  4  5  6  7    4  5  6  7  8    5  6  7  8  9     Works

User · Answer

Since the OP was asking for using a custom compare function  and this is what led me to this question as well   I want to give a solid answer here   Generally  you want to use the built-in sorted   function which takes a custom comparator as its parameter   We need to pay attention to the fact that in Python 3 the parameter name and semantics have changed   How the custom comparator works  When providing a custom comparator  it should generally return an integer float value that follows the following pattern  as with most other programming languages and frameworks     return a negative value   lt  0  when the left item should be sorted before the right item return a positive value   gt  0  when the left item should be sorted after the right item return 0 when both the left and the right item have the same weight and should be ordered  equally  without precedence   In the particular case of the OP s question  the following custom compare function can be used     def compare item1  item2       return fitness item1  - fitness item2    Using the minus operation is a nifty trick because it yields to positive values when the weight of left item1 is bigger than the weight of the right item2  Hence item1 will be sorted after item2   If you want to reverse the sort order  simply reverse the subtraction  return fitness item2  - fitness item1   Calling sorted   in Python 2  sorted mylist  cmp compare    or   sorted mylist  cmp lambda item1  item2  fitness item1  - fitness item2     Calling sorted   in Python 3  from functools import cmp to key sorted mylist  key cmp to key compare     or   from functools import cmp to key sorted mylist  key cmp to key lambda item1  item2  fitness item1  - fitness item2

User · Answer

Also  your compare function is incorrect   It needs to return -1  0  or 1  not a boolean as you have it   The correct compare function would be  def compare item1  item2       if fitness item1   lt  fitness item2           return -1     elif fitness item1   gt  fitness item2           return 1     else          return 0    Calling list sort key compare

User · Answer

gt  gt  gt  l    list range i  i 4   for i in range 10 1 -1    gt  gt  gt  l   10  11  12  13    9  10  11  12    8  9  10  11    7  8  9  10    6  7  8  9    5  6  7  8    4  5  6  7    3  4  5  6    2  3  4  5    gt  gt  gt  sorted l  key sum    2  3  4  5    3  4  5  6    4  5  6  7    5  6  7  8    6  7  8  9    7  8  9  10    8  9  10  11    9  10  11  12    10  11  12  13     The above works  Are you doing something different   Notice that your key function is just sum  there s no need to write it explicitly

[python] Sort a list of lists with a custom compare function

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